The Origin and Evolution of Parthenogenesis in Heteronotia

Home , Heteronotia, Heteronotia binoei

The Origin and Evolutionof Parthenogenesis in Heteronotia binoei (Gekkonidae): Evidence for Recent and Localized Origins of Widespread Clones

Craig Moritz

Department of Zoology, University of Queensland, Brisbane, Queensland, Australia 4072 Manuscript received August22, 1990 Accepted for publicationMay 11, 1991

ABSTRACT The parthenogenetic form of the gecko lizard species Heteranatia binoei has an unusually broad geographic range and high genetic diversity.Restriction enzyme analysis revealed two basic types of mitochondrial DNA (mtDNA)among the parthenogens. One typeis restricted to western populations. The other type, analyzed in detail here, was widespread, being found in populations from central to western Australia. The diversity within this widespread type was low. The variation among parthenogens from central to western Australia was similar to that found within local populations of the sexual species that provided the mtDNA, and was an order of magnitude less than the differentiation shown between sexual populations acrossthe same geographic distance. Phylogenetic analysis revealed that the widespread type of mtDNA in the parthenogens is most closely related to mtDNAs from western populationsof the “CA6” sexualparent. These data suggest that these parthenogeneticclones arose recently withina small geographic area, most probably in Western Australia.The parthenogens must have spread rapidly to occupy much of the central and western Australian deserts. This rapid and extensive range expansion provides strong evidence that parthenogenesis can be a successful strategy for lizards in an environment withlow and unpredictable rainfall.

ESPITE intense interest, a general and convinc- lated because they are less able to speciate, rather than D ing explanation for the maintenance of sexual being more prone toextinction. Evidence on the age reproduction has eluded evolutionarybiologists (BELL of a variety of existing parthenogenetic lineages would 1982; MICHOD and LEVINS1988). Much has been resolve this question and would provide abetter foun- made of correlationsbetween the distributions of dation for interpreting the propertiesof these lineages parthenogenetic us. sexual organisms with various (MAYNARDSMITH 1986). physical and biotic attributes of the environment(e.g., Genetic studies have produced valuable insights into VANDEL1928; CUELLAR1977; GLESENERand TILMAN the origin and evolution of parthenogenetic and other 1978). However, these comparisons usually either ig- unisexual vertebrates. Studies of allozymes and chro- nore historical effects or make weak inferences about mosomes have revealed that most arose via hybridi- the history of the organisms from the biogeographic zation (reviewed by VRIJENHOEKet al. 1989). In some patterns themselves. cases, it appears that repetitive hybrid origins have Two aspects of the history of parthenogenetic line- generated considerable genotypic diversity (e.g., VRI- ages areparticularly relevant. First, are theorigins of JENHOEK, ANGUSand SCHULTZ1977; PARKER and parthenogenetic lineages typically restricted in space SELANDER1976; MORITZ et al. 1989b). or time? WILLIAMS(1975) suggested that, in low fe- Analysis of mitochondrial DNA has proved espe- cundity organisms, parthenogenesis may be rare sim- cially useful for identifying the maternal parent of ply because it is difficult to evolve. The evolution of parthenogenetic lineages and in making inferences parthenogenesisfrom sexual reproduction may re- about their origins (BROWN and WRIGHT 1979; re- quire a special constellation of ecological and genetic viewed in MORITZ et al. 1989a).A comprehensive conditions that arerarely acheived(TEMPLETON 1982; analysis of 15 distinct parthenogenetic lineages of MORITZet at. 1989a; VRIJENHOEK1989). whiptail lizards (Cnemidophorus) revealedthat all had Second, are extant lineages of parthenogens recent extremely low mtDNA diversity in comparison to or ancient? It is generally assumed that parthenoge- their sexual relatives and that several otherwise dis- netictaxa are ephemeral because of therarity of tinct lineages shared the same mtDNA type (DENS- radiations (e.g.. genera) that are entirely parthenoge- MORE,WRIGHT and BROWN 1989; DENSMOREet al. netic (BELL 1982).However, STANLEY(1979) sug- 1989; MORITZ, WRIGHTand BROWN1989; VYASet gested that parthenogens may be taxonomically iso- al. 1990). This low diversity was interpreted to mean

Genetics 129: 2 1 1-2 19 (September, (199 1) 212 C. Moritz

that all of these lineages arose recently and that all ‘J had geographically restricted origins. .N 8N The present study extends the mtDNA approach to NmNml the analysis of the parthenogenetic form of the Het- 2 eronotia binoei complex. This complex includes several (as yet, unnamed) sexual species and triploid parthenogenetic lineages (MORITZ 1983). The parthenogens arose via repetitive hybridizations between two of the sexual species, specifically, the“CA6” and “SM6” forms (MORITZ et al. 1989b, 1990). These hybridizations produced two main, chromosomally distinguish- able, types of triploid: the “A” group with one SM6 and two CA6 genomes, and the “B + C” group with the opposite, one CA6 and two SM6 genomes (see Figure 1 in MORITZ 1991). FIGURE 1 .-Map of south central and south Western Australia showing the geographic distribution of mtDNA variants in parthen- The parthenogenetic form has an unusually broad ogenetic H. binoei. The open boxes representthe mtDNA type geographic range for a continental unisexual verte- found in western populations, the filled boxes, the widespread type brate, ranging from the western coast to central Aus- of mtDNA analyzed here. Letters identify the haplotypes as defined tralia (over 1800 km). It also has extraordinarily high in Table 2. The numbers represent sexual populations included in the phylogenetic analysis and are identified in Table 1. genetic diversity (MORITZ 1984; MORITZet al. 1989b). These attributes raised the possibilities that the par- RESULTS thenogens were substantially older than other unisexual lineages, or that they arose via hybridization across mtDNA diversity among parthenogens: In a pre- a broad geographic front. liminary study, mtDNAs from 155 parthenogens and Thispaper presents adetailed comparison of 50 localities were screened with the enzymesXbaI and mtDNA in parthenogenetic H. binoei and their sexual Bgl I. This andsubsequent comparisons of a subset of relatives. The specific aims were to test the above samples with another 12 enzymes revealed that the hypotheses by estimating the time(s) and place(s) of parthenogens have two basic types of mtDNA that origin of the parthenogenetic from the relativediver- differ by ca. 12.0% of sequence (data notshown). One type is restricted to the western populations of par- sity and phylogeny of mtDNA. More generally, the thenogens andis derived fromwestern populations of analysis aimed to test whether the observations for the SM6 sexual parent (Figure 1). Detailed analysis of whiptail lizards are general. Did parthenogenetic H. these western genomes is complicated by length vari- binoei, anindependently evolved, widespread, and ation and will be reported elsewhere (C. MORITZ and genetically diverse set of lineages, also have recent A. HEIDEMAN, manuscriptin preparation). This paper and geographically restricted origins? focuses onthe second,more widespread type of mtDNA that was found in 78 parthenogens from 28 MATERIALS AND METHODS localities in western and central Australia (Figure 1, Mitochondrial DNA was preparedfrom field-collected Table 1). lizards as described in DOWLING,MORITZ and PALMER These mtDNAs varied more for length than for the (1990). Each individual was also karyotyped (MORITZ 1984) distribution of cleavage sites. Many had large tandem and preserved specimens were lodged with the University duplications (1.O- 10.4 kb) which had to be character- of Michigan Museum of Zoology and the Queensland Mu- ized in detail before site gains and losses could be seum. assessed (DOWLING,MORITZ and PALMER 1990).The Variation was assessed by digestion of mtDNAs with a size and location of each duplication was established battery of restriction endonucleases, end-labeling with “P- labeled dNTPs, electrophoresis through both1.2% agarose by comparison with nonduplicated mtDNA using re- and 3.5% polyacrylamidegels, and autoradiography striction enzymes for which the cleavage sites had (BROWN1980; DOWLING,MORITZ and PALMER1990). beenmapped (MORITZ 1991).These enzymes in- Sequence divergence was estimated from the proportion cluded BgZII, XbaI and EcoO109 for all samples, and of shared sites, based on inferences from fragment patterns between two and ten others depending on thesample. (for four base recognizing enzymes), or from comparisons The mtDNAs were then grouped according to their of mapped cleavage sites (for 5- and 6-base pair (bp) recog- length variants for analysis with six 4-bp recognizing nizing enzymes), using the maximum likelihood estimators enzymes: HinPI, MboI, MspI, HinfI,RsaI and DdeI. In of NEI and TAJIMA(1983). Nucleotide diversity was calcu- lated as described by NEI and LI (1 979). Phylogenetic analy- this way, fragment changes due to length mutations sis of mtDNAs was done using Wagner parsimonyas imple- could be distinguished from those due to the gain or mented in PAUP (SWOFFORD1984). loss of cleavage sites (e.g., Figure 2). Evolution of Parthenogenesis 213 TABLE 1 samples ofthis mtDNA type(Table 2). The haplotypes Localities from which sexual individuals and parthenogens with differed by between oneand nine restriction sites the widespread typeof mtDNA were sampled,the sample size (Figure 3). The corresponding estimates of sequence and the mtDNA haplotypes present divergence are between 0.0007 and 0.0072 changes per nucleotide, with a mean of 0.002 f 0.0015. The Sample Locality size Haplotypes mean nucleotide diversity among all 78 samples was 0.00 1. Parthenogenetic populations The 15 haplotypes canbe arranged into aminimum Aileron, N.T. 1N Alice Springs, N.T. 6N length unrooted network that has only one parallel 70 km W Alice Springs, N.T. IN site change (Figure 3). This was for the gain of an Bullabulling Stn., W.A. 3 B,A, C RsaI site (1.4 -+ 1.1 + 0.38 kb). The three haplotypes Bullardoo Stn., W.A. 4K at internal nodes (A, D and N are the most common Coondambo Stn.,S.A. 2A Cunyu Stn., W.A. 1 G (Table 2). Each of these occurs in both of the major De Rose Hill Stn., S.A. 2N lineages of triploid parthenogens derived from inde- Earaheedy Stn., W.A. 2 1,J pendent hybridizations between the predicted hybrid Glenayle Stn., W.A. 11 diploid parthenogens and the sexualspecies (chro- Granite Downs Stn., S.A. 5N mosomeclones “A” and “B + C”;Table 2). The Granite Peak Stn., W.A. 1J Kathleen Valley, W.A. 2 D,M haplotypes at the terminal nodes were rarer and were Kirkalocka Stn., W.A. 1D each restricted to a single chromosome type. Lake Violet Stn., W.A. 5D Each of the three common variants was geographi- Laverton Downs Stn.,W.A. 9 D, L callywidespread; the D haplotype inwestern Aus- Leinster Downs Stn., W.A. 2 A, D tralia, the Nhaplotype in central Australia, and the A 17 km S Leonora, W.A. 1E Mt Willoughby Stn., S.A. 6 N, 0 haplotype in both areas (Figure 1). Closely related Munarra Stn., W.A. 1A haplotypes (Figure 3) tended to be close geographi- Neds Creek Stn.,W.A. 4 D,A, F cally (Figure 1). For example, the Nand 0 types were New Springs Stn., W.A. 3D adjacent as were the L and M types. Oakden Hills Stn., W.A. 2A Warburton, W.A. 1A mtDNA diversity among CA6 sexual lizards: Sam- Wirraminna Stn., S.A. 4 A, N ples of mtDNAfrom lizards of the CA6 sexual species Yellowdine, W.A. 1H were compared with the same suite of four base rec- Yundamindra Stn., W.A. 2D ognizing enzymes. This survey included 26 samples Yunndaga Stn., W.A. 5 A,B, D from six localities (Table 1). Comparisons of lizards Sexual Populations: CA6 1. Alice Springs, N.T. 2 from the same locality revealed multiple haplotypes, 2. 70 km S. Alice Springs, N.T. 1 but the divergence between them was low (Figure 3). 3. Granite Downs Stn., SA. 7” I-IV Six haplotypes were found among seven lizards from 4. Priscilla Mound Springs, S.A. 1 a western locality, but the mean sequence divergence 5. Yoothapinna Stn., W.A. 1 among haplotypes was only 0.002 change per nucleo- 6. Ninghan Stn., W.A. 7” v-x 7. Kalbarri, W.A. 1 tide. A sampleof seven individuals from a central 8. Dalgetty Downs Stn., W.A. 4“ XI-XW Australian population included four haplotypes with 9. Brickhouse Stn., W.A. 1 a mean sequence divergence of 0.001 change per 10. Wooramel Stn., W.A. 1 nucleotide. 1 1. Billabong Roadhouse,W.A. 3” xv-XVI 12. Carrarang Stn., W.A. 5” xvll-xvIll When comparing lizards from different popula- Sexual populations: SM6 tions, there wereusually too many differences be- 13. YarraloolaW.A.Stn., 1 tween the complex fragment patterns produced by 14. GlenayleStn., W.A. 1 digestion withfour base recognizing enzymesto allow Thenumbered localitiescorrespond tothe placesshown in the recognition of individual site gains or losses. To Figure 1. Abbreviation: N.T., Northern Territory; W.A., Western Australia; S.A., South Australia. estimate the amount of differentiation among 12 sex- a These individuals were assayed with 4-bp recognizing enzymes; ual populations sampled across the same geographic the othersexual specimens were used for phylogenetic comparison. range as the parthenogens (Table 1, Figure l), cleavage sites for 5- and 6-bp recognizing enzymes were Digestionwith the six 4-bp recognizing enzymes mapped by double digestion (DOWLING,MORITZ and assayed between 172 and 176 cleavage sites per sam- PALMER1990). Four parthenogens representing the ple, equivalent to sequencing ca. 700 bp per individ- extremes of the diversity revealed by the 4-bp recog- ual. In general, fragment changes due to site gains or nizing enzymes (haplotypes D,K, M and N were also losses were rare. Based on site gains and losses alone included in the analysis. [see MORITZ (1991) for discussion of the length vari- Between 34 and 54 sites were mapped per genome ation], 15 haplotypes were identified among the 78 (Table 3, Figure 4). The estimates of sequence diver- 214 C. Moritz

FIGURE2.-Examples of differences among parthenogenetic populations (P) and within a sexual (S) population of H. binoei revealed by digestion with the 4-bp recognizing enzymes, HinPl (left) and MspI (right). The size marker (L) is lambda digested with Aual and BglII. In the parthenogens. mtDNAs in lanes 1-2 and 10- 1 2 have 10.4-kb duplications and those in lanes 3-9 have 8.8-kb duplications. Also samples 3, 5 and 7 have an 0.38-kb deletion in one copy of the duplication [see MORITZ (1991) for details]. All of the fragment differences among the parthenogens for HinPI can be attributed to the length mutations as can several of those for MspI (indicated by open arrowheads). For MspI, samples 1-2 and 10-12 also lack a site present in the otherparthenogens (0.32 + 0.4 1 + 0.74; filled arrowheads). For the sexuals, both enzymes reveal some variation (filled arrowheads).

gence ranged from 0.003 change per nucleotide for two populations of the othersexual parent (SM6), and localities 70 km apart to 0.053 change between sam- fourrepresentative parthenogens with this type of ples fromcentral and western Australia (Table 3). mtDNA, was inferred from the variation in mapped The mean sequencedifference among the western cleavage sites (Figure 4). Two of the CA6 samples CA6 samples was 0.013 f 0.004 change per nucleo- were exhausted before thesites for EcoO 109 andNcoI tide. The value for comparisons between central and could be mapped-all sites for these enzymes were western Australian CA6 samples was 0.037 f 0.005. treated as missing data for these two samples in the In comparison, for theseenzymes, the most divergent phylogenetic analysis. The mtDNAfrom an unde- of the mtDNAs from the parthenogens differed by scribedcentral Australian species of Heteronotia 0.002 f 0.0010 change per nucleotide, a value con- (MORITZ et al. 1990) was used asan outgroupto sistent with theestimate derived from the analysis position the root of the phylogeny. with four base recognizing enzymes (see above). Of the 122 sites mapped, 20 were conserved across Phylogenetic comparisons of mtDNAs: The phy- all mtDNAs and 46 had unique (cladistically uninfor- logeny of the mtDNAs from the 12CA6 populations, mative) variants. This left 56 informative sites for Evolution of Parthenogenesis 215

TABLE 2

Definition of haplotypes A to 0 observed in widespread type of mtDNA from the parthenogens, and their frequencyof Occurrence in each of the major chromosomeforms of parthenogeneticH. binoei

Restriction enzymes Frequency enzymes Restriction Haplo- type Eco0109 BglIl XbaI HinPI MboI Mspl Hinfl Rsal DdeI “A” “B + C” A a a a a a a a a a 4 9 B a a a a a b a a a 2 0 C a a a a a a b a a 1 0 D a a a a a C a a a 17 6 E a a a b a C a a a 1 0 F a a a a b C a a a 1 0 G a b a a a C a a a 1 0 H a a a a a a a C a 1 0 I a a a a a C a a a 0 1 a a a a a a a a a a a a a 0 2 J C a a a a a a a a a K a a d a a 4 0 a b a a a f a a a a a f a a a b L a 4 0 a b a a a b M a a b a d e 1 0 d a a a a a a a N d g a C b 8 9 d a b a a a b a 0 d g a C b 2 0 The lower case letters within the table refer to different fragment patterns observed for each of the nine enzymes. Each haplotype is defined by a unique combination of fragment pattern variants.

A. points of interest. First, the parthenogens are mono- M *aJ‘ phyletic, forming a clade defined by variants at 10 restriction sites. The samples included the extremes 8. of the diversity revealed in the assays with 4-bp rec- +d 11- 11- I +’pill ognizing enzymes and include localities fromboth E \.I/.’ central and western Australia. Second, the parthenogens and western CA6 populations form a clade de- ”. fined by variation at four restriction sites. This clade iIV excludes thecentral CA6 samples. Third, the par- C. thenogens and all CA6 samples form a clade distin- VI1 guished from the other sexual parent (the SM6 samples) by variants at 34 restriction sites. This unambig- uously identifies the CA6 form as the maternalsexual parent of the original hybrids that gave rise to these widespread clones. These same three phylogenetic groups were present in the UPGMA phenogram de- -m +-d \ tI rived from the estimates of sequence divergence.

+ -* +f A+p J -\ DISCUSSION .f IX Recent and geographically localized origins:The I+= diversity of the widespread type of mtDNA in the 0 parthenogens is similar to that seen within a popula- FIGURE3.-Minimum length networks for mtDNA variants re- tion of sexual lizards and is an order of magnitude vealed among parthenogens (A) and within sexual populations (B less thanthe variation between sexual populations and C). Lower case letters indicate the cleavage sites lost (-) or sampled over the same geographic area(Table 4). gained (+) for EcoO109 (e), DdeI (d), Hinff (f), HinPI (p), MboI (m), MspI (s), RsaI (r) and XbaI (x). The arrows indicate the direction The absolute value of nucleotide diversity among assumed to identify losses or gains and are notnecessarily the true mtDNAs from the parthenogens, 0.001 changes per direction of change. The only site showing homoplasy is circled in nucleotide, is among the lowest yet reported for ter- A. Abbreviations for haplotypes in A follow Table 2. restrial vertebrates (4. AVISEet al. 1987; BALLet al. 1989). phylogenetic analysis. Use of the exhaustive branch These data provide strong evidence for a narrow and bound optionof PAUP revealed six shortest trees bottleneck in the evolution of these parthenogens that at 102steps with a consistency index of 0.55. greatly reduced the variation in mtDNA, but not the The strict consensus tree (Figure 5) revealed three variation in proteins or chromosomes (cf. MORITZ 216 C. Moritz

TABLE 3 Variation in mtDNA among populations of the sexual CA6 species

Central Western Sample 1 2 3 4 5 6 7 8 9 10 11 12

3 0 39 361 3946 30 34 37 39 27 35 28 38 38 2 1.6 34" 28 28 28 27 2926 27 29 29 29 3 3.8 4.4 3752 35 41 4329 37 31 40 40 4 3.9 3.5 5.4 45 33 37 3526 33 30 36 37 53.7 5.0 4.7 405.4 4630 42 42 32 42 42 6 4.0 3.8 3.3 3.8 1.9 48 2943 4132 45 44 7 2.6 3.7 3.0 4.0 1.8 1.9 31" 29 31 31 31 31 8 3.8 3.14.6 4.0 1.6 1.5 1.0 42 42 31 42 42 9 3.6 9 3.4 3.0 5.3 2.4 1.6 1.5 1.7 51 33 46 46 10 3.0 2.9 2.9 2.6 1.7 1.3 1.o 1.o 1.5 35" 34 35 11 3.4 2.7 3.6 4.1 1.7 1.7 0.8 1.o 1.o 0.3 47 47 12 3.7 3.1 3.9 4.0 2.0 1.6 1.3 1.3 1.4 0.3 0.4 49

~~~~ The estimated sequence divergence per nucleotide (X100) is shown below the diagonal, the number of sites mapped is in bold on the diagonal, and the number of shared sites is above the diagonal. The locality numbers correspond to Table 1 and Figure 1. a The sites recognized by Ec00109 andNcoI were not mapped for these samples.

LOCALITY TYM -4 1 1

r SP.

L20.-E13 14 ] WA SM6 FIGURE5.-Minimum-length strict-consensus cladogram for mtDNAs from parthenogenetic (P) and sexual (SM6, CA6) popu- FIGURE4.-Variation in mapped cleavage sites in mtDNAs from lations from central (CA) western (WA) Australia. The mtDNAs parthenogenetic and sexual H. binoei. The full cleavage map for the from sexual lizards are identified by locality number (Table 1) and widespread type of mtDNA in the parthenogens is shown in A. those from the parthenogens by haplotype (Table 2). The numbers Upper case letters indicate conserved sites. Those in parentheses on the interior branches are theminimum number of restriction site vary within the parthenogens. The linearized maps in B show only changes defining the node tothe right (BLRANGE option of the informative sites used for phylogenetic analysis of sexual (CA6 PAUP). Notethat the mtDNA from the parthenogens is most and SM6) populations from central (CA) and western (WA) Aus- closely related to that from western CA6 populations. tralia. The numbers refer to localities mapped in Figure 1. Only the sites present in the ingroup are shown for the outgroup "sp." zations in a small area would capture little variation EcoO109 and NcoI were not mapped for samples 2 and 7. Abbre- in mtDNA, but would freeze a large number of com- viations: AvaI (a), EamHI (b), Ecll (c), EglII (g), EcoO109 (e), EcoRV binations of variants that are segregating at nuclear (v), EcoRI (i), Hind111 (h), MluI (m), Nhel (n), NcoI (o), PvuI (p), Sac11 (s), SpeI (d) and XbaI (x). loci (MORITZet al. 1989a). Subsequent hybridization between these hypothetical diploid parthenogens and 1984; MORITZet al. 1989b). The simplest explanation males to produce the existing triploids wouldfurther for these observations is that the initial hybridization increase diversity for nuclear genes, accounting for events (i.e., the formation of diploid parthenogens) the high diversity of proteins in parthenogenetic H. were restricted to a small geographic area. Given the binoei, but would have no effect on the mtDNA. low diversity of mtDNAand substantial chromosomal The limited mtDNA diversityamong these parthen- and protein polymorphism observed withinsexual ogens also suggests that they arose recently, perhaps populations (MORITZ et al. 1990), repeated hybridi- within the pastfew thousand years. Otherwise the Evolution of Parthenogenesis 217 TABLE 4 Estimates of mtDNA diversity in sexual and parthenogeneticpopulations of Heteronotia and Cnemidophorus lizards

Mean divergence Sample Mean divergence among Comparison size Haplotypes among haplotypes individuals Heteronotia: Parthenogens 78 15 0.002 f 0.00150.001 zk 0.0007 Sexual CA6 Within locality 14 8 0.002 zk 0.0018 0.002 f 0.0015 Among localities (WA)" 8 8 0.014f 0.0042 0.014f 0.0042 Among localities (CA-WA)" 12 0.037 12 2 0.0051 0.037 f 0.0051 Cnemidophorus: Parthenogens ~elox-exsanguis~ 20 0.002 13 0.004 Uniparens' 18 0.002 8 0.003 Tesselatusd 72 0.001 4 0.003 Lemniscatus" 45 4 0.001 0.000 Sexuals Within locality Marmoratusd 20 1 0.000 0.000 Inornatusc 14 6 0.009 0.001 Among localities Marmoratusd 3 0.004 3 0.004 Inornatus' 8 0.043 8 0.043 WA = comparisons among western populations, CA-WA = comparisons between central and western populations. MORITZ,WRIGHT and BROWN(1., 989). ' DENSMOREet al. (1989). DENSMORE,WRIGHT and BROWN(1989). e Vyas et al. (1 990). mtDNA diversity would be increased because of the widespreadclones of parthenogenetic Heteronotia. accumulation of mutations in this rapidlyevolving However, it remains to be determined whether this molecule. Analysis ofthe geographic and phylogenetic result will hold for the western clones of Heteronotia distribution of the mtDNA variants suggests that some that have the other major type of mtDNA. of the 15 haplotypes were inherited from the sexual These results suggest that recent and localized ancestors, whereas others could be derived from post- origins are a common feature of parthenogenetic liz- origin mutations. The common A, D and N haplotypes ards, rather than being peculiar to Cnemidophorus. were widespread and were each found in both chro- Similar, but more limited, observationshave been mosomal lineages, suggestingthat they could predate made for gynogenetic fish (ECHELLEet al. 1989) and the formation of the triploids and may have been amphibians (SPOLSKY and UZZELL 1986; KRAUS inherited from the sexual ancestors. The best candi- 1989), although there are some exceptions, appar- dates for post-origin mutations are those that (1) occur ently due to more widespread (but recent) origins, at the tips of the phylogenetic network, (2) are re- among unisexual fish (AVISE and VRIJENHOEK1988; stricted to one chromosome form and (3) have limited GODDARD,DAWLEY and DOWLING1989). geographic ranges adjacent to closely related haplo- This molecular evidence indicating that all unisex- types. Twelve of the 15 haplotypes are of this type ual vertebrate lineages so far studied arose recently and most differ from widespreadhaplotypes by a greatly strengthens the suggestion that alternatives singlecleavage site change (Figure 3). However, it to sexual reproduction are at a long-term disadvant- cannot be rigorouslyexcluded that some ofthese were age. The general homogeneityof mtDNA in the also inherited from the sexual ancestors. parthenogens compared to the highdiversity The extremely low mtDNA diversity among par- among their sexualrelatives is at odds with thenogens relative to that seen among sexual popula- STANLEY'S(1979) suggestion that they have similar tions is exactly the same as the situation in Cnemido- extinction rates. phorus lizards(reviewed by MORITZ et al. 1989a). That most unisexual taxa have geographically lo- Indeed, the absolute levels of mtDNA diversity are calized origins suggests strong constraints on the ori- very similar in the two independently evolved groups gin of parthenogenesis (MORITZ et al. 1989a; VRIJEN- of parthenogenetic lizards (Table 4). This is a striking HOEK 1989). Thisis consistent with WILLIAM'S(1975) and unexpected result given the much larger geo- suggestion that alternatives to sex could berare simpy graphic rangeand higher genetic diversityof the because they are difficult to evolve. 218 C. Moritz

A recent and rapid rangeexpansion from western of state conservation authorities is gratefully acknowledged. Sup- Australia: The siteof the localized origins of the ported by grants from the National Science Foundation (United States; with W. M. BROWN),the Australian Research Council, the widespreadclones of H. binoei was investigated by National Geographic Society (United States), and the University of comparing mtDNAs from the parthenogens with geo- Queensland. graphic variants in the sexual relatives. The aim was to match the mtDNA in the parthenogens to a partic- LITERATURECITED ular geographic variant of the sexual relatives. This was only partly successful. The phylogenetic analysis AVISE,J. C., and R.C. VRIJENHOEK,1987 Mode of inheritance and variation of mitochondrial DNA in hybridogenetic fishes strongly identifies the CA6 species as the maternal of the genus Poeciliopsis. Mol. Biol. Evol. 4 514-525. sexual parent of these widespread clones. More spe- AVISE,J. C., J. ARNOLD,R. M. BALL,E. BERMINCHAM,T. LAMB,J. cifically, the parthenogens cluster with the western E. NEIGEL, C. A. REEB and N. C. SAUNDERS, populations to the exclusion ofcentral Australian CA6 1987 Intraspecific phylogeography: the mitochondrial DNA samples. However, exactlywhere in western Australia bridge between population genetics and systematics. Annu. Rev. Ecol. Syst. 18: 489-522. these parthenogens arose cannot be resolvedwith BALL,R. M., S. FREEMAN,F. C. JAMES,E. BERMINCHAMand J. C. these data. A western origin is corroborated by the AVISE,1988 Phylogeographic population structure of red- presence in some parthenogens of allozymeand chro- winged blackbirds assessed by mitochondrial DNA. Proc. Natl. mosome markers found only in western CA6and SM6 Acad. Sci. 85 1558-1 562. populations (MORITZ 1984; MORITZ et al. 1989b). BELL,G., 1982 The Masterpiece of Nature. Croom-Helm, London. BROWN,W. M., 1980 Polymorphism in mitochondrial DNAof The recent origin in westernAustralia of wide- humans as revealed by restriction endonuclease analysis. Proc. spread clones of parthenogenetic Heteronotia has im- Natl. Acad. Sci USA 77: 3605-3609. portant implications for our understanding of their BROWN,W. M., and J. W. WRIGHT,1979 Mitochondrial DNA biogeography and ecology. The parthenogenetic lin- analyses and the origin and relative age of parthenogenetic eages appear to have recently and rapidly expanded lizards. Science 203: 1247-1 249. CUELLAR,O., 1977 Animal parthenogenesis. Science 197: 837- their range from western to central Australia. Indeed, 843. it is likely that the parthenogens are still expanding DENSMORE,L. D., J. W. WRIGHT and W. M. BROWN, their range eastward. Thus, interactions with sexual 1989 Mitochondrial DNA analysis and theorigin and relative populations are probably more recent in central than age of parthenogenetic lizards (genus Cnemidophorus). 11. C. in western Australia. In this context, it is notable that neomexicanus and theC. tesselatus complex. Evolution 43: 943- 957. sexual individualsare absent from large areas of west- DENSMORE,L. D., C. MORITZ, J. W. WRIGHTand W. M. BROWN, ern Australia and smaller parts of central Australia. 1989 Mitochondrial DNA analysisand the origin and relative Parthenogenetic females are abundant in both areas age of parthenogenetic lizards (genus Cnemidophorus). IV. Nine (C. MORITZ, unpublished data). sexlineatus group unisexuals. Evolution 43: 469-983. That these genetically diverse parthenogenetic lin- DOWLING,T. E., C. MORITZ and J. PALMER,1990 Nucleic acids 11: Restriction site analysis, pp 250-3 17 inMolecular Systematics, eages havethrived in an area of low and unpredictable edited by D. M. HILLISand C. MORITZ. Sinauer Press, Sunder- rainfall is evidence against the notion that environ- land, Mass. mental unpredictability per se favors sex (reviewedby ECHELLE,A. A., T. E. DOWLING,C. 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DOWLING,1989 Origin this model, the displacement of sexual by partheno- and genetic relationships of diploid, triploid, and diploid-tri- genetic populations could be the result of different ploid mosaic biotypes in the Phoxinus eos-neogaeus unisexual rates of recovery from population crashes rather than complex, pp. 268-280 in Evolutionand Ecology of Unisexual being due to direct competitive interactions (cf. CUEL- Vertebrates, edited by R. M. DAWLEYand J. P. BOGART.New York State Museum, Albany, N.Y. LAR 1977) or to the presence of heterotic or “general KRAUS,F., 1989 Constraints on the evolutionary history of the purpose” genotypes in the parthenogens (g.LYNCH unisexual salamanders of the Ambystoma laterale-texanum com- 1984). This scenario suggested by the historical bio- plex as revealed by mitochondrial DNA analysis, pp. 218-227 geography of H. binoei, is being currently being tested in Evolution and Ecology of Unisexual Vertebrates, edited by R. by experimental manipulation of field populations. M. DAWLEYand J. P. BOGART.New York State Museum, Albany, N.Y. I thank W. M. BROWNfor his guidance in the initial stages of LYNCH,M., 1984 Destabilizing hybridization, general purpose this project, D. VYAS,S. LAVERY andA. HEIDEMANfor laboratory genotypes, and geographic parthenogenesis. Q. Rev. Biol. 59: ;mistance, F. HAMER,S. DONNELLANand D. KING for field assist- 257-290. ance, and J. BULL, L. JOSEPH, P. BAVERSTOCKand R. SLADEfor MAYNARDSMITH, J., 1986 Contemplating life without sex. Nature comments on an earlierversion of the manuscript. The cooperation 324: 300-301. Evolution of Parthenogenesis 219

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