Sex Chromosomes

Cytogenet Genome Res 2009;127:249–260 Published online: March 23, 2010 DOI: 10.1159/000300507

Sex Chromosome Evolution in : Independent Origins and Rapid Transitions

a a b b a T. Ezaz S.D. Sarre D. O’Meally J.A. Marshall Graves A. Georges a b Institute for Applied Ecology, University of Canberra, and Research School of Biology, Australian National University, Canberra, A.C.T., Australia

Key Words this paper, we review the current state of knowledge of sex -Cytogenetics ؒ Gene mapping ؒ Genotypic sex chromosomes in lizards and the distribution of sex deter determination ؒ ؒ ؒ mining mechanisms and sex chromosome forms within and Temperature-dependent sex determination among families. We establish for the first time an association between the occurrence of female heterogamety and TSD within families, and propose mechanisms by which fe- Abstract male heterogamety and TSD may have co-evolved. We sug- epitomize the variability of reproductive and sex de- gest that lizard sex determination may be much more the termining modes and mechanisms among amniotes. These result of an interplay between sex chromosomes and tem- modes include gonochorism (separate sexes) and partheno- perature than previously thought, such that the sex determi- genesis, oviparity, viviparity, and ovoviviparity, genotypic nation mode is influenced by the nature of heterogamety as sex determination (GSD) with male (XX/XY) and female (ZZ/ well as temperature sensitivity and the stage of sex chromo- ZW) heterogamety and temperature-dependent sex deter- some degeneration. Copyright © 2010 S. Karger AG, Basel mination (TSD). Lizards (order Squamata, suborder Sauria) are particularly fascinating because the distribution of sex- determining mechanisms shows no clear phylogenetic seg- regation. This implies that there have been multiple transi- Sex chromosomes differ from autosomes in that the tions between TSD and GSD, and between XY and ZW sex two members of the sex chromosome pair typically vary chromosome systems. Approximately 1,000 of liz- in morphology and gene content. They are highly special- ards have been karyotyped and among those, fewer than ized and appear to have evolved independently many 200 species have sex chromosomes, yet they display remark- times in vertebrates [for review see Graves, 2008]. Sex able diversity in morphology and degree of degeneration. chromosomes are thought to evolve from an autosomal The high diversity of sex chromosomes as well as the pres- pair by the acquisition of a male or female-determining ence of species with TSD, imply multiple and independent gene that defines a non-recombining region which is pro- origins of sex chromosomes, and suggest that the mecha- gressively extended, promoting degeneration of the sex- nisms of sex determination are extremely labile in lizards. In specific chromosome [Muller, 1914; Ohno, 1967; Charles-

© 2010 S. Karger AG, Basel Tariq Ezaz 1424–8581/09/1274–0249$26.00/0 Institute for Applied Ecology Fax +41 61 306 12 34 University of Canberra, Building 3 E-Mail [email protected] Accessible online at: Canberra, A.C.T. 2601 (Australia) www.karger.com www.karger.com/cgr Tel. + 61 2 6201 2297, Fax + 61 2 6201 5305, E-Mail Tariq.Ezaz @ canberra.edu.au worth, 1991]. This mode of sex determination is often cinating because the distribution of sex-determining referred to as chromosomal sex determination or ge- mechanisms shows no clear phylogenetic segregation notypic sex determination (GSD). [Janzen and Phillips, 2006; Organ and Janes, 2008; Po- GSD vertebrates typically have either a male heteroga- korná and Kratochvíl, 2009] ( table 1 ). metic (XY male/XX female) or a female heterogametic Lizards with GSD display remarkable diversity in sex (ZZ male/ZW female) sex chromosome system. The XX/ chromosome differentiation, ranging from cryptic or ho- XY sex chromosome system is conserved in therian momorphic to highly differentiated (fig. 1 ). Much of this mammals, and a ZZ/ZW pair is conserved within birds variation occurs within families, often among closely re- and within snakes [Ohno, 1967; Graves and Shetty, 2000; lated species and even within the various races or popula- Matsuda et al., 2005]. The sex chromosome pair (X and tions of the same species. For example, the gekkonid liz- Y, or Z and W) may differ only in a restricted region (even ard, Gehyra purpurascens , displays two Z chromosome a single locus), as is expected given their autosomal ori- and six W chromosome morphs, primarily as the result of gin, or can be highly differentiated as a result of progres- centromeric inversions [Moritz, 1984] ( fig. 1 ). Variation in sive degeneration due to suppression of recombination. the morphology of sex chromosomes among closely-relat- However, vertebrate XY and ZW sex chromosomes are ed taxa, or populations of one taxon, indicate that mor- not homologous, suggesting independent evolution of sex phological evolution of sex chromosomes, and perhaps chromosomes in different lineages from non-homolo- also sex-determining mechanisms in lizards may occur gous ancestral autosomes [Fridolfsson et al., 1998; Nanda relatively easily in comparison to mammals and birds. et al., 2000, 2002]. In some lineages, notably birds and There are substantial gaps in our knowledge of the dis- mammals, sex chromosomes and sex determination are tribution and characteristics of sex chromosomes within very stable. For example, all mammals have GSD with and among lizard families. There are data from all mem- male heterogamety and sex is determined in therian ber species in only two families, one of which is mono- mammals by the presence of a master sex-determining typic and the other bitypic (Rhineuridae and Annielidae, gene (e.g. SRY ), on the Y chromosome [Sinclair et al., respectively), and for many of these, only one has 1990]. All birds also have GSD but with female hetero- been examined, thereby excluding an examination for gamety and sex, at least in chickens, is determined by dos- sex chromosomes (table 1 ). In addition, many lizards have age of the DMRT1 gene located on the Z chromosome sex microchromosomes that are difficult to identify us- [Smith et al., 2009]. ing standard banding and staining techniques. Modern Many reptiles have GSD but, unlike birds or mam- cytogenetic techniques, such as comparative genomic hy- mals, sex can also be determined by incubation tempera- bridization (CGH) that are more sensitive than G- and ture (temperature-dependent sex determination or TSD) C-banding have enabled the characterization of sex chro- independent of specific genes or chromosomes [Bull, mosomes across a much wider range of species. This in- 1983; Charnier, 1966]. In yet others, sex is determined by cludes the identification of sex chromosomes that were an interaction between environmental influences and ge- cryptic because they involve microchromosomes or be- netic factors [Quinn et al., 2007; Radder et al., 2008]. The cause they have undergone little morphological differen- widespread influence of temperature on sex determina- tiation [Traut et al., 1999; Ezaz et al., 2005; Kawai et al., tion may provide a viable state from which novel master 2007]. sex genes and chromosomes in reptiles can evolve [Quinn, Here, we review the current state of knowledge of sex 2008; Georges et al., 2010] and influence directly the chromosomes in lizards and the distribution of sex-deter- manner in which sex chromosomes are acquired and lost. mining mechanisms and sex chromosomal forms within The high diversity of sex determining mechanisms seen and among families. We establish for the first time an as- in reptiles (e.g. XY, XXY, ZW, ZZW, TSD, genetic-envi- sociation between the occurrence of female heterogamety ronment interactions; table 1 ) is testament to the evolu- and TSD within lizard families, and propose mechanisms tionary lability of reptile sex determination. by which female heterogamety and TSD may have co- There are close to 5,000 lizard species [Uetz, 2009] and evolved. We suggest that lizard sex determination may be they exhibit an astonishing array of reproductive and sex much more the result of an interplay between sex chromo- determining modes including gonochorism (separate somes and temperature than previously thought, such sexes) and parthenogenesis, oviparity, viviparity, and that the sex determination mode is influenced by the na- ovoviviparity, GSD and TSD, and male (XX/XY) and fe- ture of heterogamety, temperature sensitivity and the male (ZZ/ZW) heterogamety. They are particularly fas- stage of sex chromosome degeneration.

250 Cytogenet Genome Res 2009;127:249–260 Ezaz /Sarre /O’Mealy /Marshall Graves /

Georges Table 1. C urrent knowledge about the occurrence of sex chromosomes and TSD among lizard families

XY XXY ZW ZZW Cryptic TSD NKD SCH TNS PKD PSC

Amphisbaenidae 0 0 0 0 0 0 26 0 159 16 0 Trogonophidae 0 0 0 0 0 0 2 0 6 33 0 Bipedidae 0 0 1 0 0 0 3 1 3 100 33 Blanidae 0 0 0 0 0 0 2 0 4 50 0 Cadeidae 0 0 0 0 0 0 0 0 1 0 0 Rhineuridae 0 0 0 0 0 0 1 0 1 100 0 Lacertidae 0 0 39 4 0 0 104 43 225 46 41 Teiidae 2 0 0 0 0 0 62 2 83 75 3 Gymnophthalmidae 5 2 0 0 0 0 22 7 175 13 32 Anguidae 0 0 0 0 0 0 1 0 16 6 0 Annielidae 0 0 0 0 0 0 2 0 2 100 0 Diploglossidae 0 0 0 0 0 0 6 0 50 12 0 Helodermatidae 0 0 0 0 0 0 1 0 2 50 0 Xenosauridae 0 0 0 0 0 0 1 0 6 17 0 Lanthanotidae 0 0 0 0 0 0 0 0 1 0 0 Varanidae 0 0 4 0 0 0 23 4 68 34 17 Shinisauridae 0 0 0 0 0 0 0 0 1 0 0 Iguanidae 36 33 0 0 0 0 249 69 700 36 28 Chamaeleonidae 0 0 1 0 1 0 50 1 178 28 2 0 0 5 0 9 13 93 5 380 24 5 Scincidae 30 1 0 0 0 0 118 31 1,200 10 26 Xantusiidae 0 0 0 0 0 0 12 0 29 41 0 Gerrhosauridae 0 0 0 0 0 0 12 0 34 35 0 Cordylidae 0 0 0 0 0 0 11 0 55 20 0 Sphaerodactylidae 1 0 0 0 0 0 5 1 196 3 20 Gekkonidae 2 0 11 0 0 8 75 13 794 9 17 Phyllodactylidae 0 0 1 0 0 1 7 1 109 6 14 Eublepharidae 0 0 0 0 4 5 9 0 29 31 0 Diplodactylidae 0 0 0 0 0 5 4 0 25 16 0 Carphodactylidae 0 0 0 0 0 0 6 0 28 21 0 Pygopodidae 1 1 0 0 0 0 5 2 40 13 40 Dibamidae 1 0 0 0 0 0 1 1 21 5 100

Data compiled from various sources including Donnellan, 1985; Zeng et al., 1997; Yonenaga-Yassuda and Rodrigues, 1999; Harlow, 2004; Andrews, 2005; Ezaz et al., 2005, 2009a; Olmo and Signorino, 2005; Yonenaga- Yassuda et al., 2005; Kawai et al., 2009; Pokorná and KratochvÍl, 2009; Gamble, 2010. TSD = Temperature de- pendent sex determination; NKD = number of species karyotyped; SCH = species with sex chromosomes; TNS = total number of species; PKD = proportion karyotyped; PSC = proportion with sex chromosomes.

An Astonishing Variety of Sex Chromosomes in dae, Varanidae, Chameleonidae, Agamidae and Phyllo- Lizards dactylidae) species with female heterogamety (ZZ/ZW). Only 1 family (Gekkonidae) includes species with both Karyotype information is available for at least one male and female heterogamety. No sex chromosomes species from 29 of the 32 lizard families [Olmo and Si- have been detected in the remaining 18 families (table 1 ). gno rino, 2005; Vidal and Hedges, 2009]. No species At the level of species, approximately 913 have been of Cadeidae, Lanthanotidae or Shinisauridae have karyotyped. Of these, sex chromosomes have been de- been examined. Seven families (Iguanidae, Scincidae, tected in only 181 species of which, about two thirds (115, Sphaerodactylidae, Pygopodidae, Dibamidae, Teiidae 64%) have male heterogamety (XX/XY) and the remain- and Gymnophathalmidae) include species with male het- der (66, 36%) female heterogamety ( table 1 ). erogamety (XX/XY) and 6 families (Bipedidae, Lacerti-

Lizard Sex Chromosomes Cytogenet Genome Res 2009;127:249–260 251 Fig. 1. A snapshot of morphological diversity of sex chromosomes 1984], GHO: Gekko hokouensis [Kawai et al., 2009], GPL: Gym- in lizards. Ch: Chromosome. ACO: Anolis conspersus [Gorman nophthalmus pleei [Cole et al., 1990], GPU: Gehyra purpurascens and Aitkins, 1968], AMO: Anolis monesis [Gorman and Stamm, [Moritz, 1984], LBU: Lialis burtonis [Gorman and Gress, 1970], 1975], BDU: Bassiana duperreyi [Shine et al., 2002], BTR: Bipes LVI: Lacerta vivipera [Odierna et al., 2001], MAL: Micrablepharus tridactylus [Cole and Gans, 1987], CEN: Claireascincus entrecas- allicolus [Yonenaga-Yassuda and Rodrigues, 1999], PLA: Phyllo- teauxii [Hutchinson and Donnellan, 1992], CLE: Calyptommatus dactylus lanei [King, 1981], PSI: Podarchis sicula [Odierna et al., [Yonenaga-Yassuda et al., 2005], CLI: Cnemidophorus lit- 1993], PVI: Pogona vitticeps [Ezaz et al., 2005], PVL: Phrynoce- toralis [Peccinini-Seale et al., 2004], CTI: Cnemidophorus tigris phalus vlangalii [Zeng et al., 1997], SCZ: Saproscincus czechurai [Cole et al., 1969], DIN: Delma inornata [King, 1990], DNO: Di- [Donnellan, 1991], SLA: Scincella lateralis [Wright, 1973], SLU: bamus novaeguineae [Cole and Gans, 1997], GCE: Gonatodes ce- Sceloporus lundelli [Cole, 1970], SMA: Sceloporus maculosus ciliae [McBee et a l., 1987], GGE: Gekko gecko [Solleder and Schmid, [Cole, 1971], VAC: Varanus acanthurus [King et al., 1982].

Although the majority of lizards with sex chromo- table 1 , fig. 1 ) and these are confined to the family Lacer- somes have XY or ZW type systems, approximately 23% tidae (3 species of Lacerta and one species of Podacris have multiple sex chromosomes ( table 1 ). Multiple sex [Olmo and Signorino, 2005] (table 1 ). In contrast 37 (32%) chromosome systems, thought to have evolved via auto- of the 115 species with XY sex chromosomes have mul- some-sex chromosome fusions [Wright, 1973; King, 1977; tiple sex chromosomes (X 1X 1 X2 X2 female and X1 X2 Y Olmo, 1986; Odierna et al., 2001; Leache and Sites, this male), most of which occur in Iguanidae (33/37, table 1 , issue], are common in XY, but not in ZW species. Only 4 fig. 1 ). (6%) of all female heterogametic species have multiple sex In addition, both simple and multiple sex chromo- chromosome systems (Z1 Z 1 Z 2Z 2 male and Z1 Z 2 W female, somes have evolved within closely related species (e.g.

252 Cytogenet Genome Res 2009;127:249–260 Ezaz /Sarre /O’Mealy /Marshall Graves /

Georges Anolis) and even different populations of the same species 1968; King and King, 1975; King et al., 1982; Ezaz et al., have been found to include multiple and simple sex chro- 2005; Ezaz et al., 2009a]. The patterns of differentiation mosome systems. Intra-specific variation involving mul- of these sex microchromosomes are highly variable with- tiple XY sex chromosome systems occurs in populations in and among groups ( fig. 1 ), and have evolved primarily of Scincella lateralis (XY/XXY, fig. 1 ) [Wright, 1973] and via the accumulation and amplification of heterochro- Sceloporus clarkii [Leache and Sites, this issue], while matin. multiple ZW systems occur in Lacerta vivipera (Z 1Z 1Z 2Z 2 / Sex microchromosomes appear to be highly labile in Z 1Z 2 W) [Odierna et al., 2001]. at least one family of lizards, with agamids exhibiting a A number of mechanisms have been proposed to ex- substantial array of forms among closely related species. plain the formation of heteromorphic sex chromosomes Four out of 5 agamid lizards whose sex chromosomes are in lizards [Cole et al., 1967; King, 1981; Bickham, 1984; known (Pogona vitticeps , P. barbata , Amphibolurus nobbi Olmo, 1986; Olmo et al., 1987]. Most of these are derived and Ctenophorus fordi) have ZW sex microchromosomes from observation through classical cytogenetic analysis that are among the smallest in the complement [Ezaz et (mainly differential staining and banding) and include al., 2005, 2009a; Quinn et al., 2009a]. In this group, the peri- or para-centromeric inversion, centric fusion, het- W chromosomes are highly to moderately heterochro- erochromatinization followed by amplification of tan- matic, whereas the Z chromosomes are euchromatic and dem repeats, heterochromatinization followed by degen- can be detected only by mapping sex chromosome spe- eration, and autosome-sex chromosome translocation. cific DNA sequences [Ezaz et al., 2005, 2009a] or by map- Rather than degeneration, as seen in the sex chromo- ping sex chromosome specific BAC clones [Ezaz unpubl.]. somes of mammals and most birds and some snakes, the Both the Z and W chromosomes are DAPI faint in all 4 sex-specific chromosomes (Y or W) in many lizards are species, suggesting they comprise GC rich sequences. larger than their homologs, usually as a result of the tan- The C-banding pattern on the W chromosomes are also dem amplification of repetitive elements. For example, in variable among the 4 dragon species, ranging from fully Varanidae, the W chromosome is substantially larger heterochromatic in P. barbata and P. vitticeps , to highly than the Z chromosome in all 4 species with sex chromo- heterochromatic in A. nobbi and dot-like in C. fordi [Ezaz somes, and is thought to have expanded by tandem am- et al., 2005, 2009a]. These patterns of heterochromatic plification of heterochromatin [King, 1977] (fig. 1 ). variability suggest various stages of sex chromosome dif- Like birds, turtles and snakes, most lizards have a ferentiation within closely related species. karyotype composed of macrochromosomes (ranging Cryptic or homomorphic sex chromosomes are often from 2n = 10–46) and microchromosomes (ranging from considered to be the norm in fish, amphibians and rep- 2n = 0–26) [Olmo and Signorino, 2005]. Microchromo- tiles. While this may be true for fish and amphibians somes have been found to be gene rich in birds with 2–3 where GSD is the prevalent mode of sex determination, times the number of genes than contained in macrochro- the frequency of homomorphic sex chromosomes in rep- mosomes [Smith et al., 2000] and to have higher recom- tiles is under-reported because incubation experiments bination rates [Rodionov et al., 1992]. In addition, micro- have been needed to distinguish GSD species with homo- chromosomes are GC and CpG-rich and contain few re- morphic sex chromosomes from TSD species that lack petitive elements [Hillier et al., 2004] and therefore, are sex chromosomes. These data are available for relatively likely to be important for generating genetic variation few lizard species ( table 1 ). Nevertheless, homomorphic [Organ et al., 2008]. In contrast, very little (primarily sex chromosomes are certainly likely to be common in from classical cytogenetics) is known about the origin, GSD lizards, as they are in fish and amphibians. High evolution and genomics of microchromosomes in lizards turnover of sex chromosomes, where new master sex de- [Olmo, 2008]. termining genes arise on autosomes to regularly reset the Importantly, in some species of lizards, sex chromo- progressive deterioration of the Y or W [Schartl, 2004; somes have been found to be microchromosomes [Gor- Volff et al., 2007], and generation of novel Y and W hap- man and Atkins, 1966; Gorman, 1973; Bull, 1983; Ezaz et lotypes by temperature induced sex reversal [Perrin, al., 2005; Ezaz et al., 2009a]. Sex microchromosomes have 2009], have been suggested as possible causes. Regular been found to be in most species of Iguanidae with mul- transitions between ZW dosage and XY dominance (or tiple sex chromosomes, in all 4 species so far examined in vice versa) involving the same master sex gene and asso- Varanidae and 4 out of the 5 species examined in Agam- ciated chromosomes [Quinn, 2008] may also play a role. idae [Gorman and Atkins, 1966; Gorman and Aitkins,

Lizard Sex Chromosomes Cytogenet Genome Res 2009;127:249–260 253 1964; Ohno, 1967; Solari, 1993]. Chromosome painting Gallus gallus Z152 27 attested to the homology of the Z chromosome, even be- (Chicken: ZZ/ZW) tween the most distantly related birds [Shetty et al., 1999], and the gene content of the bird Z chromosome is also Pelodiscus sinensis 6Zconserved [Fridolfsson et al., 1998; Stiglec et al., 2007]. So (Chinese softshell turtle: ZZ/ZW micro) is the gene content of the snake Z between 3 distantly re- lated species ( Python molurus , Elaphe quadrivirgata , Tri- Gekko hokouensis Z (Hokou gecko: ZZ/ZW) meresurus flavoviridis [Matsubara et al., 2006]. In each lineage, the W chromosome shows homology to the Z, but is degraded by different degrees [Bull, 1980, 1983; Elaphe quadrivirgata 2 Z Ohno, 1967]. (Four-lined ratsnake: ZZ/ZW) The similar sizes of the snake and bird Z chromo- somes were initially thought to reflect ancient homology Leiolepis reevesii rubritaeniata 2 6 [Ohno, 1969]. However, gene mapping showed that the (Butterfly lizard) bird and snake Z chromosomes are non-homologous; the bird Z is completely contained within the short arm of snake chromosome 2, and the snake Z corresponds to the Pogona vitticeps 26 bird chromosomes 2 and 27 [Matsuda et al., 2005; Mat- (Central bearded dragon: ZZ/ZW) subara et al., 2006]. Several recent studies using compar- ative gene mapping in snakes, a species of turtle (Pelodis- cus sinensis ), a species of gecko ( Gekko hokouensis ) and a species of dragon lizard ( Pogona vitticeps ) have shown Fig. 2. Schematic representation showing the non-homology and multiple and independent origins of ZZ/ZW sex chromosome that sex chromosomes, particularly ZW pairs, are not ho- systems in reptiles based on reciprocal mapping of sex chromo- mologous between reptile groups [Matsuda et al., 2005; somal genes from chickens, snakes and turtles. Data are summa- Matsubara et al., 2006; Kawai et al., 2007, 2009; Ezaz et rized after [Matsuda et al., 2005; Matsubara et al., 2006; Kawai et al., 2009b; Kawagoshi et al., 2009]. This is consistent with al., 2007, 2009; Ezaz et al., 2009a, b; Kawagoshi et al., 2009; Sri- the idea that sex chromosomes evolved many times inde- kulnath et al., 2009], and http://www.ensembl.org/index.html. pendently in reptiles ( fig. 2 ). Comparative mapping between snakes, birds, the gecko G. hokouensis and the agamid lizard P. vitticeps confirms the independent origin of their sex chromo- The conclusion we draw from the studies reviewed somes. Four genes ( ATP5A1, GHR, DMRT1, CHD1 ) that above is that the lizard genome is dynamic at the level of are sex linked in the gecko G . hokouensis were found to sex chromosome organization. There is significant varia- be autosomal in the dragon lizard P. vitticeps [Ezaz et al., tion in the mode of sex determination and variation in 2009b; Kawai et al., 2009]. Similarly, 5 snake sex-linked the degree of heteromorphism with little regard for phy- genes ( WAC, KLF6, TAX1BP1, RAB5A and CTNNB1 ), lie logenetic relationship or distance. Such variation occurs on chromosome 6, and 5 chicken Z chromosome-borne between families, species and even populations of one genes are on chromosome 2 in P. vitticeps (fig. 2 ). Thus, species. the ZW pair of agamid lizard P. vitticeps is not homolo- gous either to those of birds or snakes or the gecko G. hokouensis ( fig. 2 ) [Ezaz et al., 2009b]. This suggests in- Multiple Origins of Sex Chromosomes in Lizards dependent origins of sex chromosomes not only between 2 lizard species, but also among 2 species of lizards, 3 spe- The extreme variation in sex-determining mecha- cies of snakes and a species of turtle. The independent nisms within lizards is not typical of all reptile groups. evolution of sex chromosomes is likely to be more com- Sex chromosomes are conserved within several groups. mon in lizards as more species are subjected to compara- For example, chromosome 4 is the Z chromosome in all tive study. snakes studied to date [Becak et al., 1963, 1964; Ohno, Lizards also show much variation within groups al- 1967; Solari, 1993], whereas, chromosome pairs 4 or 5 are though, with only 6 functional sex-linked genes, and 4 Z chromosomes in most birds [Suzuki, 1930; Ohno et al., sex chromosome or sex-linked markers mapped or tested

254 Cytogenet Genome Res 2009;127:249–260 Ezaz /Sarre /O’Mealy /Marshall Graves /

Georges in only 5 species of lizards [Ezaz et al., 2009b; Quinn et An Ancient Reptile ZW Pair? al., 2009a, b], the degree of variation is debatable. W-spe- cific markers from the Komodo dragon were found to be Against this variation, the conservation of the ZW sex-linked in the Australian varanid V. rosenbergi [W. pair between birds and the gecko G. hokouensis is re- Smith, pers. comm.], suggesting the conservation of sex markable, suggesting these 2 distantly-related taxa have chromosome sequences within Varanidae. ZW-linked conserved synteny of 6 functional genes over more than AFLP markers isolated from the dragon lizard, P. vitti- 285 MYA [Rest et al., 2003; Kawai et al., 2009]. A parsi- ceps were found by PCR analysis to be sex-linked in a monious view of this finding is that chickens and gecko closely related species, P. barbata . However, they were G. hokouensis retain the primitive condition of their also found to be autosomal in several other species tested common ancestor, with the remaining squamate clade [Quinn et al., 2009b], suggesting the independent evolu- [Vidal and Hedges, 2009] having more recently derived tion of sex-chromosome-specific sequences within Aus- states, albeit, independently derived [Ezaz et al., 2009b]. tralian agamids. Also, in a preliminary study, a sex-linked However, in the context of the very great evolutionary la- AFLP marker isolated from Bassiana duperreyi was found bility of sex chromosomes in lizards, an alternate expla- not to be sex-linked in a species of Tasmanian skink Ni- nation presents itself. The master sex-determining genes veoscincus ocellatus [Ezaz unpub.], suggesting indepen- of chickens and gecko may well be different but by chance dent origin of sex-specific sequences in skinks despite have come to reside on homologous chromosomal re- their morphologically conserved sex chromosomes [Don- gions, either by the chance capture of sex determination nellan, 1985; Hutchinson and Donnellan, 1992]. by genes in a syntenic region common to the bird-gecko Another recent study on the comparative mapping of ancestor, or through translocation. Translocation is more P. vitticeps sex chromosome-specific marker has provid- likely to happen in lizards, because of the apparent lack ed evidence for the rapid evolution of non-homologous of morphologically differentiated sex chromosomes ZW sex chromosomes within Australian dragon lizards which is likely to represent nascent sex chromosomes, [Ezaz et al., 2009a]. Cross-species chromosome painting and such nascent sex chromosomes are known to accu- using a P. vitticeps sex chromosomal marker [Quinn et mulate transposable elements and have been reported in al., 2009b] revealed that the ZW sex microchromosomes a wide range of plants and [Adams et al., 2000; of 3 Australian species ( P. vitticeps , P. barbata and A. nob- Skaletsky et al., 2003; Charlesworth et al., 2005; Kej- bi) were homologous, but that those of a 4th species, C. novsky et al., 2008; Matsunaga, 2009]. fordi, were not. This suggests at least two independent Support for convergence in the chromosomal location origins of ZW sex microchromosome systems in Austra- of the master sex determining gene comes from the ob- lian agamids [Ezaz et al., 2009a]. No comparative data on servation that it is unlikely that birds and geckos have the the sex chromosomes in Iguanidae and Varanidae are same master sex-determining genes. The male determin- available, so homology between the sex chromosomes of ing gene DMRT1 in chickens is present only on the Z these 3 lizard groups is unknown. chromosome and sex is determined by dosage of the These demonstrations of variation in lizard sex chro- DMRT1 gene product, 2 copies produce males, whereas 1 mosomes, as well as the presence of species with temper- copy produces females [Smith et al., 2009]. In G. hok- ature dependent sex determination, imply multiple and ouensis, this gene is present in both Z and W chromo- independent origins of sex chromosomes, and suggest somes [Kawai et al., 2009], making it unlikely to be sex- that the mechanisms of sex determination are extremely determining in this species unless a paralog of DMRT1 is labile in this group. This lability may indicate frequent present and functions as a dominant master gene (as transitions between modes (TSD and GSD) and mecha- found in Xenopus laevis ; DMW) [Yoshimoto et al., 2008], nisms (XY and ZW) of sex determination in lizards which is likely to function as a suppressor of autosomal through the evolution of novel sex chromosomes, per- DMRT1 dosage. Moreover, closely related species of gecko haps via the acquisition of novel genes. The activation of have both XY and ZW sex chromosome systems as well unlinked sex-determining genes on autosomes could also as TSD, and possibly sex chromosome-temperature inter- be a possible mechanism for the independent origin of sex actions (e.g. Gekko japonicus ) [Tokunaga, 1985; Gamble, chromosomes in different lizard lineages, as proposed for 2010], indicating rapid transitions between modes of sex the evolution of non-homologous sex chromosomes in chromosomes as well as modes of sex determination [for salmonid fishes [Woram et al., 2003]. review see Gamble, 2010]. The emergence of a novel sex- determining gene in geckos in the 285 million years since

Lizard Sex Chromosomes Cytogenet Genome Res 2009;127:249–260 255 their divergence from birds, and their chance residence sex identification techniques employed [see Viets et al., on homologous chromosomes, would seem at least an 1994; Harlow, 2004; Andrews, 2005; Pokorná and Kra- equally plausible explanation for the homology of the sex tochvíl, 2009], so we did not include those species as hav- chromosomes of these taxa, given the lability of sex chro- ing TSD in this review. mosomes and mechanisms in other lizards. The matter is The classical view of transitions between GSD and unlikely to be resolved until the genetic mechanisms of TSD has sex captured by a temperature-sensitive element, their sex determination become better known and with- the production of lethal or suboptimal YY or WW indi- out a comprehensive genomic analysis across the squa- viduals, and the loss of these chromosomal elements from mate phylogeny (e.g. geckos, skinks, varanids, lacertids, the genome through natural selection or drift [Bull, 1980, agamids). 1983]. GSD arises from TSD through the capture of sex Whatever the mechanism of sex determination in the determination by novel genes which come to influence gecko, the homology between the gecko and bird sex sex and the formation of sex chromosomes [Bull, 1980, chromosomes suggests that a bird-like ZW could be an- 1983; Charlesworth, 1991]. However, there is emerging cestral for reptiles. The homology between the bird ZW evidence of interactions between GSD and TSD both in and the XY complex of the platypus [Veyrunes et al., forms regarded as GSD ( Bassiana duperreyi , Pogona vit- 2008] suggests that this homology might extend back to ticeps ) [Quinn et al., 2007; Radder et al., 2008] and forms a common amniote ancestor that lived 310 MYA [Graves, regarded as TSD (Amphibolurus muricatus and Agama

2008]. Alternatively, if ZW is the ancestral state, some of impalearis have 50: 50 sex ratios at intermediate tempera- the fundamental genetic machinery of a ZW-driven sex tures) [Harlow and Taylor, 2000; El Mouden et al., 2001] determination and differentiation network may have and that sex determination should be regarded more as a been retained to varying degrees in different reptile lin- continuum of states than a dichotomy of GSD and TSD eages, leading to a predisposition to evolve similar sex- [Sarre et al., 2004]. determining mechanisms. Thus similar sex chromosome Mapping the distribution of TSD and GSD across the systems may have independent origins, but are con- lizard phylogeny shows an almost haphazard distribution strained in some way by their phylogenetic history. of the traits. However, one trend demands some atten- tion: an association between the occurrence of TSD and the occurrence of female heterogamety within families An Association between TSD and Female ( fig. 3 ). This suggests that ZW sex chromosomes evolved Heterogamety in Lizards only in those families in which species with TSD occur. An exception is the Gekkonidae, which contains species In TSD, males and females are produced differentially with XY, ZW or TSD (fig. 3 ). Is this association an arti- according to the incubation temperatures experienced by fact, arising from limited data on the mode of sex deter- the developing embryos. Although TSD was first de- mination in the majority of the lizard lineages? Or does scribed in an agamid lizard, Agama agama [Charnier, the pattern arise from fundamental underlying processes 1966], it is more frequent in non-squamate reptiles (in we do not yet fully understand? possibly all crocodilians, many turtles, tuatara) than liz- An association might suggest that ZW systems are ards. Since Charnier’s discovery, TSD has been identified more prone to coming under thermal influence, or per- unequivocally in 32 species from 6 families, most of haps female heterogamety is in some way more compat- which are from the Agamidae (13/32) and Gekkonidae ible with selection for TSD when thermal tendencies first (8/32, table 1 ) families. In addition, both GSD (4 species) arise. Alternatively, it might mean that ZW systems are and TSD (5 species, table 1 ) have been detected in the more compatible with the retention of an underlying ge- family Eublepharidae, although no sex chromosomes are netic predisposition (cryptic sex-linked ZW chromo- known. somes) in TSD species [Quinn, 2008]. It has also been Evidence of TSD has been reported in another 9 spe- suggested that ZW systems in lizards act via dosage cies from the families Chamaeleonidae (2 species) Lacer- mechanisms that are more likely to evolve TSD than tidae (1 species), Scincidae (4 species), Anguidae (1 spe- those systems that act via male dominance [Quinn et al., cies) and Varanidae (1 species) [see Harlow, 2004; Po- 2007], and it is gene dosage systems that are more suscep- korná and Kratochvíl, 2009]. However, these data are tible to temperature influence than dominant gene sys- equivocal mainly because of small sample size as well as tems. the nature and design of the incubation experiments and

256 Cytogenet Genome Res 2009;127:249–260 Ezaz /Sarre /O’Mealy /Marshall Graves /

Georges These questions arise from an apparent association between ZW systems and TSD in the lizard phylogeny but our knowledge of the mechanisms of sex determina- tion in lizards, and in particular, the interaction between genotype and temperature in determining sex, is as yet too rudimentary to take the ideas beyond speculation. The fundamental differences to the molecular mecha- nisms of sex differentiation between female homogamet- ic and female heterogametic systems is not fully known. There is evidence that both dominant and dosage mecha- nisms can determine sex in ZW systems (e.g. DMRT1 dosage in birds [Smith et al., 2009]; DMW dominance in Xenopus laevis [Yoshimoto et al., 2008]), however, no XY dosage systems have been observed in any vertebrate. It is likely that among female heterogametic lizards, some will determine sex via dosage mechanisms as in birds, while others via dominant mechanisms, as in X. laevis . The dominance of a master sex-determining gene could be achieved by regulation of one or more of the inf luential genes in the sex differentiation pathway, in the way SRY interacts with SOX3 or SOX9 as hypothesized previously in mammals [Graves, 1998]. Alternatively, it could be achieved by inhibition of what was formerly a dosage sys- tem, in which the dosage is inhibited or knocked down Fig. 3. Phylogeny of reptiles [pruned phylogenetic tree modified by members of the same gene family as is suspected in X. after Vidal and Hedges, 2009] showing apparent association be- leavis (likely DMW inhibition of DMRT1). Thus, regard- tween TSD and female heterogamety in lizards, suggesting fre- less of heterogamety, sex in many lizard clades may be quent transitions between modes and mechanisms of sex deter- determined by gene dosage or gene dosage captured by a mination in lizards via TSD and female heterogamety. MYA: Mil- ‘dominant’ master sex gene and so generally predisposed lion Years Ago. to capture of the genetic pathway of sex differentiation by exogenous cues, such as temperature. Further studies, particularly in gekkonid and agamid Conclusions lizards, could reveal the nature of such co-evolution of ZW sex chromosomes and TSD. For example, Gekkoni- Lizards occupy a key evolutionary position in the am- dae is the only family where XY, ZW and TSD exist niote phylogeny, so understanding the origin, evolution [Gamble, 2010]. If XY and ZW sex chromosomes are and mechanisms of sex determination and sex chromo- found to be homologous (e.g. Rana rugosa) [Ogata et al., somes will help us to better understand the origin and 2003], and if closely related species or populations have evolution of sex chromosomes in other vertebrates in- TSD (e.g. Gekko japonicus , G. gecko , G. hokouensis ) then cluding mammals. In particular, such studies will resolve identifying sex-determining genes in these species and the debate about the ancestry of sex determination, TSD, comparative analyses of gonad differentiation pathways or GSD, XY or ZW, but they will also unravel how GSD in XY, ZW, TSD populations (species) of geckos would and TSD co-exist and how transitions between hetero- reveal the true nature of co-evolution of TSD and ZW sex gamety occur. chromosomes and mechanisms of transitions between Fewer than 200 species of lizards have identifiable sex modes. Evidence for a relationship between TSD and ZW chromosomes, yet they display remarkable diversity in can also be sought in species from Agamidae, particu- morphology and extent of degeneration. The high diver- larly Australian dragon lizards, in which these features sity of sex chromosomes as well as the presence of species have been reliably identified [Harlow, 2004; Ezaz et al., with TSD, imply multiple and independent origins of sex 2005, 2009a, b; Quinn et al., 2007, 2009b] and genomic chromosomes, and suggest that the mechanisms of sex resources are already available. determination are extremely labile in lizards. This appar-

Lizard Sex Chromosomes Cytogenet Genome Res 2009;127:249–260 257 ent lability is likely to have a significant role in rapid Therefore, lizard sex determination may be much transitions among modes and mechanisms perhaps via more the result of an interplay between sex chromosomes female heterogamety and TSD. However, little is known and temperature than previously thought, such that the about the genomics of sex chromosomes and their rela- sex determination mode is influenced by the nature of tionship with the plasticity of sex determination in liz- the heterogamety, temperature sensitivity and the stage ards. of sex chromosome degeneration. Future comparative ge- How sex chromosomes and temperature interact to nomic analysis of sex chromosomes among closely relat- determine sex in many lizard species is not known, al- ed lizard species would appear to be one of the most im- though theoretical models have been proposed [Bull, portant steps to understanding the origin, evolution and 1983; Quinn 2008; Geroges et al., 2010]. Thermal influ- transitions of sex chromosomes and sex determinations. ence on sex reversal in 2 lizard species with highly het- Recent advances in molecular genetics, cytogenetics and eromorphic sex chromosomes [Quinn et al., 2007; Rad- sequencing technologies promise to advance our knowl- der et al., 2008] is an indication of the propensity of sex edge substantially in the near future. determination to be sensitive to temperature among liz- ards. It is quite likely that sex chromosomes will be found in lizard species that have long been regarded as classical Acknowledgements TSD species. Therefore, interactions between sex chro- mosomes and temperature may be more intrinsic in liz- We thank Ettore Olmo for the opportunity to contribute to this special issue. Many thanks to T. Gamble for discussion about ards than previously thought, and may depend on the sex chromosomes and sex determination in lizards and reptiles. extent of differentiation of sex chromosomes as well as This work is supported by the Australian Research Council Dis- heterogamety. The apparent co-occurrence of TSD and covery Grant (DP0881196) awarded to S.D.S., A.G. and Scott female heterogamety may provide such a clue and per- V. Edwards. haps represent a transitional phase between alternate modes and mechanisms.

References

Adams MD, Celniker SE, Holt RA, Evans CA, Charnier M: Action of temperature on the sex Cole CJ, Dessauer HC, Townsend CR, Arnold Gocayne JD, Amanatides PG, et al: The ge- ratio in the Agama agama (Agamidae, Lacer- MG: Unisexual lizards of the genus Gymn-

nome sequence of Drosophila melanogaster . tilia) embryo. C R Seances Soc Biol Fil 160: ophthalmus (Reptilia: Teiidae) in the neo-

Science 287: 2185–2195 (2000). 620–622 (1966). tropics: Genetics, orgin, and systematics.

Andrews RM: Incubation temperature and sex Cole CJ: Karyotypes and evolution of the spino- American Museum Novitates 2994: 2–31 ratio of the veiled chameleon ( Chamaeleo ca- sus group of lizards in the genus Sceloporus . (1990).

lyptratus ). J Herpetol 39: 515–518 (2005). American Museum Novitates 2431: 1–48 Donnellan SC: The evolution of sex chromo- Becak W, Becack ML, Nazareth HRS: Chromo- (1970). somes in scincid lizards: School of Biologi - somes of snakes in short term cultures of leu- Cole CJ: Karyotypes of the five monotypic spe- cal Sciences, p. 218 (Macquarie University,

cocytes. Am Nat 97: 253–256 (1963). cies groups of lizards in the genus Sceloporus . 1985).

Becak W, Becak ML, Nazareth HRS, Ohno S: American Museum Novitates 2450: 1–18 Donnellan SC: Chromosomes of Australian Close karyological kinship between the rep- (1971). lygosomine skinks (Lacertilia: Scincidae) ii.

tilian suborder Serpentes and the class Aves. Cole C, Gans C: Chromosomes of Bipes , Meso- The genus Lampropholis . Genetica 83: 223–

Chromosoma 15: 606–617 (1964). baena, and other amphisbaenians (Reptilia), 234 (1991). Bickham JW: Patterns and Modes of Chromo- with comments on their evolution. Ameri- El Mouden EH, Znair M, Pieau C: Effects of in-

somal Evolution in Reptiles, in Sharma AK, can Museum Novitates 2869: 1–9 (1987). cubation temperature on embryonic devel- Sharma A (eds): Chromosomes in Evolution Cole C, Gans C: The karyotype of Dibamus no- opment and sex determination in the North

of Eukaryotic Group, pp 13–40 (CRC Press, vaeguineae (Squamata). Herpetologica 53: African lizard, Agama impalearis . Herpetol

Boca Raton, Florida 1984). 229–232 (1997). J 11: 72–77 (2001). Bull JJ: Sex determination in reptiles. Quart Rev Cole CJ, Lowe CH, Wright JW: Sex chromo- Ezaz T, Quinn AE, Miura I, Sarre SD, Georges A,

Biol 55: 3–21 (1980). somes in lizards. Science 155: 1028–1029 Marshall Graves JA: The dragon lizard Pog- Bull JJ: Evolution of Sex Determining Mecha- (1967). ona vitticeps has ZZ/ZW micro-sex chromo-

nisms (Benjamin/Cummings, Menlo Park, Cole CJ, Lowe CH, Wright JW: Sex chromo- somes. Chromosome Res 13: 763–776 (2005). California 1983). somes in teiid whiptail lizards (genus Cnemi- Ezaz T, Quinn AE, Sarre SD, O’Meally D, Georg- Charlesworth B: The evolution of sex chromo- dophorus ). American Museum Novitates es A, Graves JA: Molecular marker suggests

somes. Science 251: 1030–1033 (1991). 2395: 1–14 (1969). rapid changes of sex-determining mecha- Charlesworth D, Charlesworth B, Marais G: nisms in Australian dragon lizards. Chro-

Steps in the evolution of heteromorphic sex mosome Res 17: 91–98 (2009a).

chromosomes. Heredity 95: 118–128 (2005).

258 Cytogenet Genome Res 2009;127:249–260 Ezaz /Sarre /O’Mealy /Marshall Graves /

Georges Ezaz T, Moritz B, Waters PD, Graves JAM, Janzen FJ, Phillips PC: Exploring the evolution Moritz C: The evolution of a highly variable sex Georges A, Sarre SD: The ZW sex micro- of environmental sex determination, espe- chromosome in Gehyra purpurascens (Gek-

chromosomes of an Australian dragon lizard cially in reptiles. J Evol Biol 19: 1775–1784 konidae). Chromosoma 90: 111–119 (1984). share no homology with those of other rep- (2006). Muller HJ: A gene for the fourth chromosome of

tiles or birds. Chromosome Res 17: 965–973 Kawagoshi T, Uno Y, Matsubara K, Matsuda Y, Drosophila . J Exp Zool 17: 325–336 (1914). (2009b). Nishida C: The ZW micro-sex chromosomes Nanda I, Zend-Ajusch E, Shan Z, Grutzner F, Fridolfsson AK, Cheng H, Copeland NG, Jen- of the chinese soft-shelled turtle (Pelodiscus Schartl M, et al: Conserved synteny between kins NA, Liu HC, Raudsepp T, et al: Evolu- sinensis, Trionychidae, Testudines) have the the chicken Z sex chromosome and human tion of the avian sex chromosomes from an same origin as chicken chromosome 15. Cy- chromosome 9 includes the male regulatory

ancestral pair of autosomes. Proc Natl Acad togenet Genome Res 125: 125–131 (2009). gene DMRT1: A comparative (re)view on avi-

Sci USA 95: 8147–8152 (1998). Kawai A, Nishida-Umehara C, Ishijima J, Tsuda an sex determination. Cytogenet Cell Genet

Gamble T: A review of sex determining mecha- Y, Ota H, Matsuda Y: Different origins of 89: 67–78 (2000). nisms in geckos (Gekkota: Squamata). Sex bird and reptile sex chromosomes inferred Nanda I, Haaf T, Schartl M, Schmid M, Burt DW: Dev 4 (2010). from comparative mapping of chicken Z- Comparative mapping of Z-orthologous

Georges A, Ezaz T, Quinn AE, Sarre SD: Are rep- linked genes. Cytogenet Genome Res 117: genes in vertebrates: implications for the tiles predisposed to temperature-dependent 92–102 (2007). evolution of avian sex chromosomes. Cyto-

sex determination? Sex Dev 4 (2010). Kawai A, Ishijima J, Nishida C, Kosaka A, Ota H, genet Genome Res 99: 178–184 (2002). Gorman GC: The Chromosomes of the Reptilia, Kohno S et al: The ZW sex chromosomes of Odierna G, Kupriyanova LA, Caprigilone T, a Cytotaxonomic Interpretation, in Chiarel- Gekko hokouensis (Gekkonidae, Squamata) Olmo E: Further data on sex chromosomes li AB, Capanna, E (eds): Cytotaxonomy and represent highly conserved homology with of Lacertidae and a hypothesis on their evo-

Vertebrate Evolution, pp 349–424 (Academ- those of avian species. Chromosoma 118: 43– lutionary trend. Amphibia Reptilia 14: 1–11 ic Press, London, New York 1973). 51 (2009). (1993). Gorman GC, Aitkins L: Confirmation of an X-Y Kejnovsky E, Hobza R, Cermak T, Kubat Z, Vys- Odierna G, Heulin B, Guillaume C, Vogrin N, sex determining mechanism in lizards (Ano- kot B: The role of repetitive DNA in structure Aprea G, et al: Evolutionary and biogeo-

lis ). Copeia 1968: 159–160 (1968). and evolution of sex chromosomes in plants. graphical implications of the karyological

Gorman GC, Atkins L: Chromosomal hetero- Heredity 102: 533–541 (2008). variations in the oviparous and viviparous morphism in some male lizards of the genus King M: The Evolution of Sex Chromosomes in forms of the lizard Lacerta (Zootoca) vivipa-

Anolis . Am Nat 100: 579–583 (1966). Lizards, in Calaly J, Tyndale-Biscoe H (eds): ra . Ecography 24: 332–340 (2001). Gorman GC, Gress F: Sex chromosomes of a py- Evolution and Reproduction, pp 55–60 (Aus- Ogata M, Ohtani H, Igarashi T, Hasegawa Y, gopodid lizard, Lialis burtonis . Experientia tralian Academy of Science, Canberra 1977). Ichikawa Y, Miura I: Change of the heteroga-

26: 206–207 (1970). King M: Chromosome Change and Speciation in metic sex from male to female in the frog.

Gorman GC, Stamm B: The Anolis lizards of Lizards, in Atchley W, Woodruff D (eds): Es- Genetics 164: 613–620 (2003). Mona, Redonda, and La Blanquilla: chromo- says on Evolution and Speciation, pp 262– Ohno S: Sex Chromosomes and Sex-Linked somes, relationships and natural history 285 (Cambridge University Press, Cam- Genes (Springer-Verlag, Heidelberg 1967).

notes. J Herpetol 9: 197–205 (1975). bridge 1981). Ohno S: Evolution of sex chromosomes in mam-

Graves JA: Interactions between SRY and SOX King M: Chromosomal and Immunogenetic mals. Annu Rev Genet 3: 495–524 (1969). genes in mammalian sex determination. Data: A New Perspective on the Origin of Ohno S, Stenius C, Christian LC, Becak W, Be-

Bioessays 20: 264–269 (1998). Australia’s Reptiles, in Olmo E (ed): Cytoge- cak ML: Chromosomal uniformity in the

Graves JAM: Weird animal genomes and the n e t i c s o f A mp h i bi a n s a n d R e p t i l e s ( B i r k h äu s - avian subclass Carinatae. Chromosoma 15: evolution of vertebrate sex and sex chromo- er, Basel 1990). 280–288 (1964).

somes. Ann Rev Genet 42: 565–586 (2008). King M, King D: Chromosomal evolution in the Olmo E: Animal Cytogenetics 4, Chordata 3, a. Graves JAM, Shetty S: The Evolution of Sex lizard genus Varanus (Reptilia). Aust J Biol Reptilia (Gebrüder Borntraeger, Stuttgart

Chromosomes in Higher Vertebrates, in Sci 28: 89–108 (1975). 1986). Clarke M (ed): Comparative Genomics, pp King M, Mengden GA, King D: A pericentric- Olmo E: Trends in the evolution of reptilian 153–205 (Kluwer Academic Publishers, inversion polymorphism and a ZZ/ZW sex- chromosomes. Integr Comp Biol 1–8 (2008). Massachusetts 2000). chromosome system in Varanus acanthurus Olmo E, Signorino G: Chromorep: A reptile Harlow P: Temperature Dependent Sex Deter- Boulenger analyzed by G- and C-banding chromosomes database (http://193.206.118.

mination in Lizards, in Valenzuela N, Lance, and Ag staining. Genetica 58: 39–45 (1982). 100/professori/chromorep.pdf, 2005). VA (eds): Temperature Dependent Sex De- Matsubara K, Tarui H, Toriba M, Yamada K, Olmo E, Odierna G, Capriglione T: Evolution of termination in Vertebrates, pp 11–20 (Smith- Nishida-Umehara C, Agata K, et al: Evidence sex-chromosomes in lacertid lizards. Chro-

sonian Institution, Washington DC, 2004). for different origin of sex chromosomes in mosoma 96: 33–38 (1987). Harlow PS, Taylor JE: Reproductive ecology of snakes, birds, and mammals and step-wise Organ C, Janes DE: Sex chromosome evolution

the jacky dragon ( Amphibolurus muricatus ): differentiation of snake sex chromosomes. in sauropsids. Integr Comp Biol 48: 512–519

An agamid lizard with temperature depen- Proc Natl Acad Sci USA 103:18190–18195 (2008).

dent sex determination. Aust Ecol 25: 640– (2006). Organ CL, Moreno RG, Edwards SV: Three tiers 652 (2000). Matsuda Y, Nishida-Umehara C, Tarui H, Ku- of genome evolution in reptiles. Integr Comp

Hillier LW, Miller W, Birney E, Warren W, Hard- roiwa A, Yamada K, Isobe T, et al: Highly Biol 48: 494–504 (2008). ison RC, Ponting CP, et al: Sequence and conserved linkage homology between birds Peccinini-Seale D, Rocha CFD, Almeida TMB, comparative analysis of the chicken genome and turtles: Bird and turtle chromosomes are Araújo AFB, De Sena MA: Cytogenetics of provide unique perspectives on vertebrate precise counterparts of each other. Chromo- the Brazilian whiptail lizard Cnemidophorus

evolution. Nature 432: 695–716 (2004). some Res 13: 601–615 (2005). littoralis (Teiidae) from a Restinga area (Bar- Hutchinson MN, Donnellan SC: and Matsunaga S: Junk DNA promotes sex chromo- ra de Maricá) in southeastern Brazil. Braz J

genetic variation in the Australian lizards of some evolution. Heredity 102: 525–526 Biol 64: 661–667 (2004). the genus Pseudemoia (Scincidae: Lygosomi- (2009). Perrin N: Sex reversal: A fountain of youth for

nae). J Nat Hist 26: 215–264 (1992). McBee K, Bickham JW, Dixon JR: Male hetero- sex chromosomes? Evolution 63: 3043–3049 gamety and chromosomal variation in Ca- (2009).

ribbean geckos. J Herpetol 21: 68–71 (1987).

Lizard Sex Chromosomes Cytogenet Genome Res 2009;127:249–260 259 Pokorná M, Kratochvíl L: Phylogeny of sex-de- Shetty S, Griffin DK, Graves JA: Comparative Uetz P: Embl (EMBL: http:// termining mechanisms in squamate reptiles: painting reveals strong chromosome homol- www.reptile-database.org, 2009). are sex chromosomes an evolutionary trap? ogy over 80 million years of bird evolution. Veyrunes F, Waters PD, Miethke P, Rens W, Mc-

Zool J Linn Soc 156: 168–183 (2009). Chromosome Res 7: 289–295 (1999). Millan D, Alsop AE, et al.: Bird-like sex chro- Quinn AE: Evolution of Sex-determining Mech- Shine R, Elphick MJ, Donnellan SC: Co-occur- mosomes of platypus imply recent origin of

anisms in Reptiles: Institute for Applied rence of multiple, supposedly incompatible mammal sex chromosomes. Genome Res 18: Ecology, p. 196 (University of Canberra, modes of sex determination in a lizard popu- 965–973 (2008).

Canberra 2008). lation. Ecol Lett 5: 486–489 (2002). Vidal N, Hedges SB: The molecular evolutionary Quinn AE, Georges A, Sarre SD, Guarino F, Ezaz Sinclair AH, Berta P, Palmer MS, Hawkins JR, tree of lizards, snakes, and amphisbaenians.

T, Graves JA: Temperature sex reversal im- Griffiths BL, et al: A gene from the human C R Biol 332: 129–139 (2009).

plies sex gene dosage in a reptile. Science 316: sex-determining region encodes a protein Viets BE, Ewert MA, Talent LG, Nelson CE: Sex 411 (2007). with homology to a conserved DNA-binding determining mechanisms in squamate rep-

Quinn AE, Ezaz T, Sarre SD, Graves JM, Georges motif. Nature 346: 240–244 (1990). tiles. J Exp Zool 270: 45–56 (1994). A: Extension, single-locus conversion and Skaletsky H, Kuroda-Kawaguchi T, Minx PJ, Volff JN, Nanda I, Schmid M, Schartl M: Gov- physical mapping of sex chromosome se- Cordum HS, Hillier L, et al: The male-spe- erning sex determination in fish: regulatory

quences identify the Z microchromosome cific region of the human Y chromosome is a putsches and ephemeral dictators. Sex Dev 1: and pseudo-autosomal region in a dragon mosaic of discrete sequence classes. Nature 85–99 (2007).

lizard, Pogona vitticeps . Heredity (2009a). 423: 825–837 (2003). Woram RA, Gharbi K, Sakamoto T, Hoyheim B, Quinn AE, Radder RS, Sarre SD, Georges A, Smith CA, Clifford V, Western PS, Wilcox SA, Holm LE, et al: Comparative genome analy- Ezaz T, Shine R: Isolation and development Bell KS, Sinclair AH: Cloning and expression sis of the primary sex-determining locus in

of a molecular sex marker for Bassiana of a DAX1 homologue in the chicken em- salmonid fishes. Genome Res 13: 272–280

duperreyi , a lizard with XX/XY sex chromo- bryo. J Mol Endocrinol 24: 23–32 (2000). (2003). somes and temperature-induced sex rever- Smith CA, Roeszler KN, Ohnesorg T, Cummins Wright JW: Evolution of the X1 X 2 Y sex chromo-

sal. Mol Genet Genomics 281: 665–672 DM, Farlie PG, et al: The avian Z-linked gene some mechanism in the scincid lizard Scin-

(2009b). DMRT1 is required for male sex determina- cella laterale (Say). Chromosoma 43: 101–108

Radder RS, Quinn AE, Georges A, Sarre SD, tion in the chicken. Nature 461: 267–271 (1973). Shine R: Genetic evidence for co-occurrence (2009). Yonenaga-Yassuda Y, Rodrigues MT: Supernu- of chromosomal and thermal sex-determin- Solari AJ: Sex Chromosomes and Sex Determi- merary chromosome variation, heteromor-

ing systems in a lizard. Biol Lett 4: 176–178 nation in Vertebrates. (CRC Press, Boca Ra- phic sex chromosomes and banding patterns (2008). ton, FL, 1993). in microteiid lizards of the genus Micra- Rest JS, Ast JC, Austin CC, Waddell PJ, Tibbetts Solleder E, Schmid M: XX/XY sex chromosomes blepharus (Squamata, Gymnophthalmidae).

EA, et al: Molecular systematics of primary in Gekko gecko (Sauria, Reptilia). Amphibia Chromosome Res 7: 21–29 (1999).

reptilian lineages and the tuatara mitochon- Reptilia 5: 339–345 (1984). Yonenaga-Yassuda Y, Rodrigues MT, Pellegrino

drial genome. Mol Phylogenet Evol 29: 289– Srikulnath K, Matsubara K, Uno Y, Thongpan A, KCM: Chromosomal banding patterns in 297 (2003). Suputtitada S, et al: Karyological character- the eyelid-less microteiid lizard radiation: Rodionov A, Chelysheva LA, Solovei IV, Miako- ization of the butterfly lizard (Leiolepis The X 1 X1 X 2X 2 :X1 X 2 Y sex chromosome sys- shina IuA: Chiasma distribution in the reevesii rubritaeniata, Agamidae, Squamata) tem in Calyptommatus and the karyotypes of lampbrush chromosomes of the chicken Gal- by molecular cytogenetic approach. Cyto- Psilophthalmus and Tretioscincus (Squama-

lus gallus domesticus : Hot spots of recombi- genet Genome Res 125: 213–223 (2009). ta, Gymnophthalmidae). Genet Mol Biol 28: nation and their possible role in proper dis- Stiglec R, Ezaz T, Graves JA: A new look at the 700–709 (2005). junction of homologous chromosomes at the evolution of avian sex chromosomes. Cyto- Yoshimoto S, Okada E, Umemoto H, Tamura K,

first meiotic division. Genetika 28: 151–160 genet Genome Res 117: 103–109 (2007). Uno Y, et al: A W-linked DM-domain gene, (1992). Suzuki K: On the chromosomes of the domestic DM-W, participates in primary ovary devel-

Sarre SD, Georges A, Quinn A: The ends of a fowl. Zool Mag (Japan) 42: 358–359 (1930). opment in Xenopus laevis. Proc Natl Acad Sci

continuum: Genetic and temperature-de- Tokunaga S: Temperature-dependent sex deter- USA 105: 2469–2474 (2008). pendent sex determination in reptiles. Bioes- mination in Gekko japonicus (Gekkonidae, Zeng XM, Wang YZ, Liu ZJ, Fang ZL, Wu GF,

says 26: 639–645 (2004). Reptilia). Dev Growth Differ 27: 117–120 Papenfuss TJ, Macey JR: Karyotypes on nine Schartl M: A comparative view on sex determi- (1985). species in the genus Phrynocephalus , with

nation in medaka. Mech Dev 121:639–645 Traut W, Sahara K, Otto TD, Marec F: Molecu- discussion of karyotypic evolution of Chi-

(2004). lar differentiation of sex chromosomes pro- nese Phrynocephalus . Acta Zool Sinica 43: bed by comparative genomic hybridization. 399–410 (1997).

Chromosoma 108: 173–180 (1999).

260 Cytogenet Genome Res 2009;127:249–260 Ezaz /Sarre /O’Mealy /Marshall Graves /

Georges Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.