Multiple independent evolutionary losses of XY pairing at meiosis in the grey voles

Pavel M. Borodin, Ekaterina A. Basheva, Anna A. Torgasheva, Olesya A. Dashkevich, Fedor N. Golenishchev, Irina V. Kartavtseva, et al.

Chromosome Research The Biology of Chromatin and Chromosomes

ISSN 0967-3849 Volume 20 Number 2

Chromosome Res (2012) 20:259-268 DOI 10.1007/s10577-011-9261-0

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Chromosome Res (2012) 20:259–268 DOI 10.1007/s10577-011-9261-0

Multiple independent evolutionary losses of XY pairing at meiosis in the grey voles

Pavel M. Borodin & Ekaterina A. Basheva & Anna A. Torgasheva & Olesya A. Dashkevich & Fedor N. Golenishchev & Irina V. Kartavtseva & Kazuyuki Mekada & Beth L. Dumont

Received: 5 October 2011 /Revised and Accepted: 23 November 2011 /Published online: 13 December 2011 # Springer Science+Business Media B.V. 2011

Abstract In many eutherian , X–Ychromo- species. Our sample includes ten species of the tribe some pairing and recombination is required for meiotic Microtini and two species of the tribe Lagurini. This progression and correct sex chromosome disjunction. increased breadth of sampling allowed us to identify Arvicoline present a notable exception to this asynaptic species in each major Microtine lineage. meiotic rule, with multiple species possessing asynaptic Evidently, the ability of the sex chromosomes to pair sex chromosomes. Most asynaptic vole species belong to and recombine in male meiosis has been independently the genus sensu lato. However, many of the lost at least three times during the evolution of Microtine species both inside and outside the genus Microtus dis- rodents. These results suggest a lack of evolutionary play normal X–Y synapsis at meiosis. These observa- constraint on X–Y synapsis in Microtini, hinting at the tions suggest that the synaptic condition was present in presence of alternative molecular mechanisms for sex the common ancestor of all voles, but gaps in current chromosome segregation in this large mammalian taxonomic sampling across the arvicoline phylogeny tribe. prevent identification of the lineage(s) along which the asynaptic state arose. In this study, we use electron and Keywords meiosis . recombination . synapsis . immunofluorescent microscopy to assess heterogametic synaptonemal complex . pseudoautosomal region . sex chromosome pairing in 12 additional arvicoline MLH1 . SCP3 . vole .

Responsible Editor: Herbert Macgregor.

P. M. Borodin (*) : E. A. Basheva : A. A. Torgasheva : I. V. Kartavtseva O. A. Dashkevich Institute of Biology and Soil Science, Institute of Cytology and Genetics, Russian Academy of Sciences, Far Eastern Department, Russian Academy of Sciences, Siberian Department, Vladivostok 690022, Russia Novosibirsk 630090, Russia e-mail: [email protected] K. Mekada Experimental Division, P. M. Borodin : O. A. Dashkevich RIKEN BioResource Center, Department of Cytology and Genetics, Tsukuba-shi, Ibaraki 305-0074, Japan Novosibirsk State University, Novosibirsk, Russia B. L. Dumont E. A. Basheva : F. N. Golenishchev Department of Genome Sciences, Zoological Institute, Russian Academy of Sciences, University of Washington, Sankt-Petersburg 199034, Russia Seattle, WA 98195-5065, USA Author's personal copy

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Abbreviations species with asynaptic sex chromosomes also pos- BSA Bovine serum albumin sess unusually large sex chromosomes that contain Cy3 Orange fluorescing cyanine large blocks of constitutive heterochromatin (Marchal et DAPI 4′-6-Diamidino-2-phenylindole al. 2004). Whether this unique karyotypic feature is EM Electron microscopy functionally related to sex chromosome behavior at FITC Fluorescein isothiocyanate meiosis remains to be elucidated. FM Fluorescent microscopy Arvicoline rodents present an especially powerful MLH1 Homolog of prokaryotic mutL 1 mismatch test case for studying X–Y pairing and recombination repair protein at meiosis. Arvicolinae is a speciose mammalian sub- SC Synaptonemal complex family in which many species with asynaptic sex chro- SCP3 Synaptonemal complex protein 3 mosomes have been found (Fig. 1). The availability of closely related species that follow distinct meiotic para- digms provides a unique model system for dissecting the molecular and evolutionary basis for the loss of sex Introduction chromosome pairing and recombination. Most of the voles with asynaptic sex chromosomes The segregation of homologous chromosomes into belong to the genus Microtus sensu lato. All species of daughter cells at meiosis depends on the timely and Palearctic subgenera Microtus, Agricola, Sumeriomys, precise execution of homolog pairing, synapsis, and Iberomys, , and studied so far recombination. Unpaired chromosomes and chromo- are asynaptic (Ashley et al. 1989, 1990; Borodin et al. somes that fail to undergo recombination can activate 1995; Carnero et al. 1991; Jimenez et al. 1991;Rovatsos early meiotic checkpoints, leading to arrest of the et al. 2008;Wolfetal.1988). Normal X–Y synapsis has meiotic cell cycle or programmed cell death. Defects been observed in all studied species of East Asian sub- in meiotic chromosome pairing and recombination are genera Pallasiinus (Ashley and Fredga 1994;Borodinet established mechanisms of infertility in humans al. 1997, 1995;Mekadaetal.2001)andAlexandromys (Hassold and Hunt 2001). (Borodin et al. 2011). Synaptic sex chromosomes have Mammalian heterogametic sex chromosomes present also been found in arvicoline species closely related to a notable deviation from this meiotic routine. The X and the genus Microtus,suchasNeodon sikimensis (Mekada Y chromosomes are largely non-homologous, with only et al. 2002)andChionomys nivalis (Megias-Nogales et short terminal regions of sequence homology. Pairing al. 2003), and in the distantly related species and recombination in these pseudoautosomal regions terrestris and Arvicola sapidus (Borodin et al. 1995; (PAR) is required for meiotic progression and proper Megias-Nogales et al. 2003), talpinus disjunction of the sex chromosomes in many mammalian (Bogdanov et al. 1986), and six species of the genus species (Mangs and Morris 2007). Structural mutations Myodes (Ashley and Fredga 1994; Borodin et al. 1995; in the PAR have been directly linked to spermatogenic Iwasa et al. 1999). failure in humans and mice (Burgoyne et al. 1992; Together, these patterns suggest that the synaptic Mohandas et al. 1992), and divergence in the PAR condition is the ancestral state in arvicoline rodents. between isolated populations could provide an intrinsic Apparently, it has been retained in the East Asian genetic barrier to hybridization, driving the formation of lineage of the genus Microtus, but lost in the other new species. lineage (or lineages) of this genus (Borodin et al. 2011). Despite the clear link between X–Y pairing and However, the synaptic patterns of several lineages in the meiotic progression in many mammals, there are a genus Microtus sensu lato remain unknown. Determin- handful of exceptional species. For example, in ing the XY pairing status in Nearctic voles, which are marsupials and several species, the X and Y considered a sister group to the Asian and Palearctic chromosomes remain unpaired throughout meiosis lineages (Conroy and Cook 2000), and in some early (Deakin et al. 2010). In these species, the sex branches of the tribe Microtini,suchasStenocranius chromosomes are sequestered into a dense sex and (Abramson et al. 2009), could help body, but they never synapse or recombine (Page shed light on the evolutionary origin(s) of XYasynapsis et al. 2006). Curiously, most of the mammalian on the arvicoline phylogeny. Author's personal copy

Evolutionary losses of XY pairing in grey voles 261 Fig. 1 Informal supertree of the arvicoline rodents with a known pattern of X–Ypair- ing.Thetreeisbasedon published molecular phyloge- nies (Abramson et al. 2009; Bannikova et al. 2009, 2010; Conroy and Cook 2000; Jaarola et al. 2004). Nodes receiving less than moderate support are reduced to polyt- omies. Species names are given according to recent publications and the checklist (Wilson and Reeder 2005). The subgenus names are given in brackets; the names used in the original publica- tions are given in square brackets. Numbers in paren- theses indicate the source of information about X–Ysyn- aptic condition: 1 Ashley and Fredga 1994; 2 Borodin et al. 1995; 3 Iwasa et al. 1999; 4 Bogdanov et al. 1986; 5 Megias-Nogales et al. 2003; 6 Mekada et al. 2002; 7 Mekada et al. 2001; 8 Borodin et al. 1997; 9 Borodin et al. 2011; 10 Jimenez et al. 1991; 11 Ashley et al. 1990; 12 Wolf et al. 1988; 13 Ashley et al. 1989; 14 Rovatsos et al. 2008; 15 Carnero et al. 1991. Black lines indicate synaptic spe- cies; red lines, asynaptic

Toward this goal, we applied electron and immuno- Our sample includes five species of the Nearctic grey fluorescent microscopy of surface-spread synaptonemal voles (Microtus (Mynomes) longicaudus Merriam 1888, complexes (SC) to analyze sex chromosome pairing in Microtus (Mynomes) montanus Peale, 1848, Microtus ten species of the tribe Microtini and two species of the (Mynomes) pennsylvanicus Ord, 1815, Microtus (Ped- tribe Lagurini. We identify several independent losses of omys) ochrogaster Wagner, 1842., and Microtus (Aulac- sex chromosome pairing across the vole phylogeny, a omys) mexicanus Saussure, 1861); the widely distributed result that hints at the presence of an as yet uncharac- Microtus (Stenocranius) gregalis Pallas, 1779; two East terized mechanism for segregating unpaired, achiasmate Asian species (Lasiopodomys brandtii Radde, 1861 and sex chromosomes in these species. Lasiopodomys mandarinus Milne-Edwards, 1871); and the Central Asian species Microtus juldaschi (Severtzov, 1879). M. juldaschi had formerly been assigned to the Material and methods genus (Musser and Carleton 2005) in which one synaptic species, N. sikimensis, has already been A total of 23 adult males from 12 species were examined described (Mekada et al. 2002). However, M. in this analysis. Sampling locations are provided in juldaschi was recently assigned to the genus Table 1. Microtus (Blanfordimys) based on an analysis of Author's personal copy

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Table 1 Synapsis between X and Y chromosomes in the species studied

Species Sampling locality No. of No. of cells No. of cells Percent of cells FM EM containing X and Y paired at pachytene

Eolagurus luteus Volgograd Distr., Russia 1 50 94 lagurus East Kazakhstan Distr., Kazakhstan 1 25 86 gud Kabardino-Balkar Rep., Russia 1 50 100 Microtus longicaudus New Mexico, USA 1 41 95 M. montanus New Mexico, USA 1 43 88 M. pennsylvanicus Wisconsin, USA 3 50 96 M. ochrogaster Illinois, USA 4 59 44 M. mexicanus New Mexico, USA 1 46 0 M. juldaschi Sughd Province, Tajikistan 2 100 100 0 M. gregalis Novosibirsk Distr., Russia 4 97 62 0 M. brandtii Chita Distr., Russia 2 100 100 0 M. mandarinus Buryat Rep., Russia 2 100 100 70 F1 M. mujanensis x M. maximowiczii Buryat Rep., Russia 2 100 100 86 the cytochrome b DNA sequence (Bannikova et to human SC lateral element protein SCP3 (1:500, al. 2009).Oursamplealsoincludedaspecimenof Abcam, Cambridge) and mouse monoclonal antibody Chionomys gud Satunin, 1909. One species of this to human mismatch repair protein MLH1 (1:30, genus, Ch. nivalis, has previously been shown to Abcam), rinsed, and fluorescently labeled with goat possess synaptic X and Y chromosomes (Megias- anti-rabbit Cy3-conjugated antibodies (1:500, Jackson, Nogales et al. 2003). Two representatives of the West Grove) and goat anti-mouse FITC-conjugated tribe Lagurini— luteus Eversmann, antibodies (1:30, Jackson). Preparations were pho- 1840, and Lagurus lagurus Pallas, 1773—were tographed using an Axioplan 2 Imaging microscope used in our study as outgroup species. In addition, F1 (Carl Zeiss, Germany) equipped with a CCD camera hybrids were generated from a cross between two (CV M300, JAI Corporation, Japan), CHROMA filter synaptic species, Microtus mujanensis Orlov et sets, and ISIS4 image processing package (MetaSystems Kovalskaya, 1978, and Microtus maximowiczii GmbH, Germany). Schrenk, 1859. Handling and euthanasia of animals Only cells containing complete sets of chromosomes were performed according to protocols approved by were analyzed. Cells were classified as early, middle, or the animal care and use committees at the University late pachytene according to the criteria suggested for of Wisconsin, the Zoological Institute of the Russian surface-spread spermatocyte SC preparations of labora- Academy of Sciences, the Institute of Cytology and tory mice (Moses 1980). Synapsis of the X and Y Genetics of the Russian Academy of Sciences, and the chromosomes was defined by the formation of a clear University of Nagoya. synaptonemal complex between the chromosome axes. Spermatocyte spreads were prepared from fresh testis Accidental associations and crossing of the axes were tissue using the drying-down technique (Peters et al. classified as asynaptic configurations. For cells with 1997). The cell spreads for imaging by electron micros- synaptic sex chromosomes, we scored the presence or copy (EM) were stained with silver nitrate (Howell and absence of recombination in the pairing regions Black 1980) and covered with a plastic film. After light using immunolocalization patterns of MLH1, a microscopic examination, cells were transferred to spec- mismatch repair protein of mature recombination imen grids, examined, and photographed with an electron nodules (Anderson et al. 1999; Baker et al. 1996; microscope (JEM-100, JEOL, Japan) at 80 kV. Froenicke et al. 2002). MLH1 signals were only Cell spreads for imaging by fluorescent microscopy scored if they co-localized with the SC between (FM) were incubated with rabbit polyclonal antibody synapsed sex chromosomes. Author's personal copy

Evolutionary losses of XY pairing in grey voles 263

Results foci on all autosomal bivalents (Fig. 3a). This indicates an absence of recombination between Table 1 shows the frequency of pachytene cells the X and Y chromosomes. containing synapsed XY sex chromosomes in the EM examination of silver-stained spermatocyte species studied. No cells with synapsis of the X spreads from M. longicaudus, M. montanus, M. and Y axes were found by EM analysis for four ochrogaster, and M. mandarinus and FM analysis of species: M. mexicanus, M. juldaschi, M. gregalis, immunostained SCs of E. luteus, L. lagurus, Ch. gud, and M. brandtii. In these species, the axes of the M. pennsylvanicus, and interspecies hybrids between sex chromosomes came together at early pachytene M. mujanensis and M. maximowiczii revealed clear (Fig. 2a) and remained in close proximity through pairing between the X and Y chromosomes in most mid- and late pachytene (Fig. 2b, c), often lingering near but not all cells (Table 1). Synapsis typically occurred the periphery of the nucleus and commonly surrounded at early pachytene. Terminal parts of the axial elements by electron-dense material. Even though the X and Y of the X and Y formed a synaptonemal complex similar axes often overlapped, they never fully aligned with in its appearance to that observed in autosomal bivalents each other to form a true SC. The same chromosome (Fig. 2d). At mid-pachytene, the length of the X–YSC behavior has been described in all other asynaptic spe- decreased, but the axes of the sex chromosomes cies of the genus Microtus (Ashley et al. 1989;Borodin remained attached to each other until the end of pachy- et al. 1995; Carnero et al. 1991; Jimenez et al. 1991). tene (Fig. 2e, f). We detected a single MLH1 focus in the No MLH1 foci were observed on sex chromosomes X–Y pairing region along SCs of synaptic species and examined using FM from M. juldaschi, M. grega- the interspecies hybrids (Fig. 3b–d). This focus lis,orM. brandtii despite the presence of clear appeared at early pachytene and disappeared during

Fig. 2 Electron microphotographs of silver-stained axial species M. montanus (d–f). a, d Early pachytene; b, e elements of X and Y chromosomes in pachytene spermato- middle pachytene; c, f late pachytene. Scale bar,2μm cytes of asynaptic species M. juldaschi (a–c) and synaptic Author's personal copy

264 P.M. Borodin et al. Fig. 3 Immunofluorescent microphotographs of pachytene spermatocytes from M. juldaschi (a), M. pennsylvanicus (b), the F1 hybrid between M. mujanensis and M. maximo- wiczii (c), and M. mandari- nus (d). Red, SCP3; green, MLH1. Arrows indicate MLH1 foci at XY pairing regions. Scale bar,5μm

mid-pachytene. In contrast, MLH1 foci on the auto- XY bivalent with two distinct pairing regions of somes were present until late pachytene. This pattern approximately the same length. One involved the prox- of sex chromosome pairing and recombination has been imal part of the Y and distal part of Xp; another con- observed in all synaptic species of rodents studied so far sisted of the distal parts of Y and Xq (Fig. 4d). Such (Anderson et al. 1999; Ashley and Fredga 1994; circular XY bivalents are observed in human spermato- Borodin et al. 2011, 1997, 1995; Iwasa et al. 1999; cytes (Speed and Chandley 1990), but, as far as we Megias-Nogales et al. 2003; Mekada et al. 2001b, 2002). know, have not been described in the voles. M. mandarinus was the most interesting species in The XY bivalents of M. mandarinus usually terms of the synaptic behavior of its sex chromosomes. contained a single MLH1 focus located very close to The specimens we examined had metacentric X chro- the Y centromere. Surprisingly, the second pairing mosomes and telocentric Y chromosomes. We detected region often contained its own MLH1 focus (Fig. 3d). about 30% cells with unpaired sex chromosomes. In these cells, the X and Y chromosome axes sometimes folded back, mimicking the appearance of normally Discussion paired autosomal bivalents (Fig. 4a). In most cells (about 70%), we observed synapsis between the proxi- To identify the lineages along which the asynaptic state mal part of the Y chromosome and distal part of Xp arose, we plotted all known synaptic and asynaptic (Fig. 4c). However, in some cells, the Y chromosome species on an informal supertree of the arvicolines was paired along its full length (Fig. 4b). This extensive (Fig. 1). and apparently non-homologous synapsis between the All arvicoline species beyond the genus Microtus X and Y chromosomes has been described in M. man- sensu lato studied so far belonged to the synaptic darinus (Gu et al. 1999) and in some other species of group. The three additional species studied here—E. mammals (Basheva et al. 2008; Borodin 1991; Pack et luteus, L. lagurus, and Ch. gud—all exhibited synaptic al. 1993). In about 20% of cells, we observed a circular sex chromosome conformations at pachytene. These Author's personal copy

Evolutionary losses of XY pairing in grey voles 265 Fig. 4 Synaptic configura- tions of X and Y chromo- somes in pachytene spermatocytes of M. mandarinus. a Asynapsis. b Extended synapsis. c Homo- logous synapsis in one pairing region. d Homolo- gous synapsis in two pairing regions. Scale bar,2μm

results corroborate the prior finding of XY synapsis in group to the Palearctic subgenera Microtus, Agri- Ch. nivalis (Megias-Nogales et al. 2003) and strongly cola, Blanfordimys, Sumeriomys,andTerricola (all suggest that XY pairing at meiosis is the ancestral asynaptic) and the Asian subgenera Pallasiinus and condition in Microtine voles. (all synaptic). We found synaptic sex Our analysis uncovers evidence for the loss of chromosomes in four Nearctic species (M. longicaudus, this ancestral condition along several independent M. montanus, M. pennsylvanicus,andM. ochrogaster), Microtine lineages. whereas one species (M. mexicanus) was asynaptic. The M. juldaschi, a species recently assigned to the observed heterogeneity of X–Y pairing patterns within subgenus Blanfordimys (Bannikova et al. 2009), was the Nearctic voles is consistent with another inde- foundtohaveasynapticXYchromosomes.Two pendent loss of XY synapsis. additional species of this subgenus, Microtus Variability in XY pairing patterns was also detected afghanus and Microtus bucharensis,havepreviously in the lineage including M. (Stenocranius) gregalis, M. been shown to lack XY synapsis (Borodin et al. 1995). (Lasiopodomys) brandtii,andM. (Lasiopodomys) This Central Asian subgenus shows some affinity mandarinus. These three species belong to the earliest to the Palearctic phylogenetic lineage of the genus and longest branch sister to all other lineages of the Microtus (Bannikova et al. 2010). To date, all Palearctic genus Microtus sensu lato (Abramson et al. 2009; species examined have asynaptic sex chromosomes, Bannikova et al. 2009; Jaarola et al. 2004). M. gregalis suggesting that XY synapsis was lost along this and M. brandtii are both asynaptic, suggesting that XY lineage. However, support for the monophyletic group asynapsis also evolved along this branch, independently Blanfordimys+Palearctic phylogenetic lineage is weak. from the other asynaptic branches described above. Therefore, we cannot rule out the possibility that the loss In contrast, the sex chromosomes of M. mandarinus of XY synapsis occurred in these groups independently. pair and recombine at pachytene. However, several The monophyly of Nearctic voles is well supported observations favor the argument that XY synapsis in by molecular genetic data (Conroy and Cook 2000; M. mandarinus represents a derived condition. First, Jaarola et al. 2004). They are considered as a sister two other species belonging to the same lineage are Author's personal copy

266 P.M. Borodin et al. asynaptic (M. gregalis and M. brandtii). Second, the homology between the X and Y is not required for sex pattern of XY pairing observed in this species is chromosome pairing in male meiosis (Acosta et al. pronouncedly different from that observed in all 2011). However, data on MLH1 immunolocalization at other arvicolines. The XY pair of this species the XY bivalent of the synaptic species such as in M. contains two pairing regions. Both are relatively maximowiczii, M. mujanensis,andM. fortis described long and both are involved in regular recombination, earlier (Borodin et al. 2011)andinM. mandarinus and similar to the pseudoautosomal pairing regions in human. M. pennsylvanicus described here indicate that homol- Theseregionsmayhaveoriginatedbydenovotrans- ogous recombination does occur in the pairing region. locations of autosomal material, as suggested for the This is clear evidence that the synaptic species of the human pairing regions (Graves et al. 1998). genus Microtus retain sequence homology between the Our results indicate that the ability of the sex chro- XandY. mosomes to pair and recombine in male meiosis has Interestingly, this sequence homology remains been lost independently at least three times during the unaltered even after speciation events. M. mujanensis evolution of Microtine rodents: (1) in the lineage (2n038, FN046) and M. maximowiczii (2n040, including subgenera Stenocranius and Lasiopodomys; FN054), two synaptic species, diverged from each (2) in the Nearctic species M. (Aulacomys) mexicanus; other ∼0.3 Mya. Although their genomes are dis- and (3) in Palearctic phylogenetic lineage including tinguished by several chromosomal rearrangements, subgenera Microtus, Sumeriomys, Terricola, Agricola, the X chromosome of M. mujanensis pairs normally Blanfordimys,andIberomys. This may be an underesti- with the Y chromosome of M. maximowiczii during mate as the latter three subgenera occupy basal positions the meiotic prophase in interspecies hybrids. Future in the Microtine tree and their relation to the Palearctic work comparing the molecular structure of pairing lineage remains controversial (Bannikova et al. 2010; regions in synaptic species versus the orthologous Jaarola et al. 2004). They may have acquired the asyn- regions in asynaptic species could help illuminate aptic condition independently. the genetic mechanisms of XY synaptic gain and We speculate that these losses were facilitated by loss across the vole phylogeny. the emergence of an as yet uncharacterized cellular mechanism for sequestering the sex chromosomes at Acknowledgments This work was supported by research meiosis in the common ancestor of Microtini. It has grants from Russian Foundation for Basic Research (to PMB, been suggested that some components of the axial EAB, AAT, OAD, FNG, IVK), Siberian and Far Eastern Departments of Russian Academy of Sciences (to PMB and elements of SC may participate in the segregation of IVK), National Science Foundation Predoctoral Fellowship sex chromosomes in asynaptic species of mammals (to BLD), and Japan Science Society Sasagawa Scientific (de la Fuente et al. 2007; Page et al. 2003). The Research Grant (to KM). We thank Mrs. Marina Rodionova evolution of a novel mechanism for ensuring proper for her help in SC spreading; Drs. Andrei Dobrotvorsky, Victor Panov, Yuichi Narita, Bruce Cushing, Nancy Cushing, and sex chromosome segregation without imposing a Ms. Roni Yi-Ju Chen for providing of animals; Drs. Sen-ichi Oda, requirement for pairing and recombination would Kazuhiro Koyasu, Masashi Harada, and Vladimir Lebedev for reduce (or eliminate) selective constraint on XY synapsis, valuable advices; and the Microscopic Center of the Siberian setting the stage for subsequent losses of XY pairing in Department of Russian Academy of Sciences for granting access to microscopic equipment. multiple lineages. XY pairing has been completely lost in asynaptic species. Interestingly, in some synaptic species, we observe a high frequency of pachytene cells References with X–Y asynapsis: up to one third in M. mandarinus and two thirds in M. ochrogaster. We do not know whether losses of XY pairing have Abramson NI, Lebedev VS, Bannikova AA, Tesakov AS (2009) Radiation events in the subfamily Arvicolinae (Rodentia): been caused by deletions, rearrangements of the evidence from nuclear genes. Dokl Biol Sci 428:458–461 ancestral pairing region, or by epigenetic modification. Acosta MJ, Romero-Fernandez I, Sanchez A, Marchal JA Acosta et al. (2011) found no correlation between syn- (2011) Comparative analysis by chromosome painting of aptic and asynaptic classes of sex chromosomes and the the sex chromosomes in arvicolid rodents. Cytogenet Genome Res 132(1–2):47–54 presence or absence of sequence homology in their Anderson LK, Reeves A, Webb LM, Ashley T (1999) Distribution euchromatic regions. Thus, the existence of sequence of crossing over on mouse synaptonemal complexes using Author's personal copy

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