Variants of a major DNA satellite discriminate parental subgenomes in a hybrid parthenogenetic lizard Darevskia unisexualis (Darevsky, 1966).

Hybrid parthenogenetic animals are an exceptionally interesting model for studying the mechanisms and evolution of sexual and asexual reproduction. A diploid parthenogenetic lizard Darevskia unisexualis is a result of an ancestral cross between a maternal species Darevskia raddei nairensis and a paternal species Darevskia valentini and presents a unique opportunity for a cytogenetic and computational analysis of a hybrid karyotype. Our previous results demonstrated a significant divergence between the pericentromeric DNA sequences of the parental Darevskia species; however, an in-depth comparative study of their pericentromeres is still lacking. Here, using target sequencing of microdissected pericentromeric regions, we reveal and compare the repertoires of the pericentromeric tandem repeats of the parental Darevskia lizards. We found species-specific sequences of the major pericentromeric tandem repeat CLsat, which allowed computational prediction and experimental validation of fluorescent DNA probes discriminating parental chromosomes within the hybrid karyotype of D. unisexualis. Moreover, we have implemented a generalizable computational method, based on the optimization of the Levenshtein distance between tandem repeat monomers, for finding species-specific fluorescent probes for pericentromere staining. In total, we anticipate that our comparative analysis of Darevskia pericentromeric repeats, the species-specific fluorescent probes that we found and the pipeline that we developed will form a basis for the future detailed cytogenomic studies of a wide range of natural and laboratory hybrids.

Evolution of parthenogenetic reproduction in Caucasian rock lizards: A review.

Despite numerous works devoted to hybrid origin of parthenogenesis in reptiles, the causes of hybridization between different species, resulting in the origin of parthenogenetic forms, remain uncertain. Recent studies demonstrate that sexual species considered parental to parthenogenetic rock lizards (Darevskia spp.) avoid interspecific mating in the secondary overlap areas. A specific combination of environmental factors during last glaciation period was critical for ectotherms, which led to a change in their distribution and sex ratio. Biased population structure (e.g., male bias) and limited available distributional range favored the deviation of reproductive behavior when species switched to interspecific mates. To date, at least 7 diploid parthenogenetic species of rock lizards (Darevskia, Lacertidae) originated through interspecific hybridization in the past. The cytogenetic specifics of meiosis, in particular the weak checkpoints of prophase I, may have allowed the formation of hybrid karyotypes in rock lizards. Hybridization and polyploidization are 2 important evolutionary forces in the genus Darevskia. At present, throughout backcrossing between parthenogenetic and parental species, the triploid and tetraploid hybrid individuals appear annually, but no triploid species found among Darevskia spp. on current stage of evolution. The speciation by hybridization with the long-term stage of diploid parthenogenetic species, non-distorted meiosis, together with the high ecological plasticity of Caucasian rock lizards provide us with a new model for considering the pathways and persistence of the evolution of parthenogenesis in vertebrates.

Cytogenetic Analysis of the Bimodal Karyotype of the Common European Adder, Vipera berus (Viperidae).

Vipera berus is the species with the largest range of snakes on Earth and one of the largest among reptiles in general. It is also the only snake species found in the Arctic Circle. Vipera berus is the most involved species of the genus Vipera in the process of interspecific hybridization in nature. The taxonomy of the genus Vipera is based on molecular markers and morphology and requires clarification using SC-karyotyping. This work is a detailed comparative study of the somatic and meiotic karyotypes of V. berus, with special attention to DNA and protein markers associated with synaptonemal complexes. The karyotype of V. berus is a remarkable example of a bimodal karyotype containing both 16 large macrochromosomes and 20 microchromosomes. We traced the stages of the asynchronous assembly of both types of bivalents. The number of crossing-over sites per pachytene nucleus, the localization of the nucleolar organizer, and the unique heterochromatin block on the autosomal bivalent 6-an important marker-were determined. Our results show that the average number of crossing-over sites per pachytene nucleus is 49.5, and the number of MLH1 sites per bivalent 1 reached 11, which is comparable to several species of agamas.

XX/XY sex chromosomes in a blind lizard (Dibamidae): Towards understanding the evolution of sex determination in squamates.

The reconstruction of the evolutionary history of sex determination in squamate reptiles (lizards and snakes) is complicated by missing data in many lineages, erroneous reports, and often questionable inferences on state homology. Therefore, despite the large effort, the reconstruction of the ancestral sex determination in squamate reptiles is still controversial. With the hope to shed light on this problem, we aspired to identify the sex chromosome gene content in Dibamus deharvengi, the representative of the family Dibamidae, the putative sister clade to all other squamates. Our analyses revealed XX/XY sex-determination system in D. deharvengi: the X chromosome contains genes with homologues scattered across chicken chromosomes 8, 12, 13, 18, 30, and 33, and the Y chromosome seems to largely degenerate. To the best of our knowledge, this combination has never been reported to form sex chromosomes in any amniote lineage. It suggests that the sex chromosomes can represent an apomorphy of a clade including D. deharvengi. Our findings cover an important gap in the knowledge of sex determination in reptiles and further support multiple independent origins of sex chromosomes in this group.

DNA Environment of Centromeres and Non-Homologous Chromosomes Interactions in Mouse.

Although the pericentromeric regions of chromosomes that are enriched in tandemly repeated satellite DNA represent a significant part of eukaryotic genomes, they remain understudied, which is mainly due to interdisciplinary knowledge gaps. Recent studies suggest their important role in genome regulation, karyotype stability, and evolution. Thus, the idea of satellite DNA as a junk part of the genome has been refuted. The integration of data regarding molecular composition, chromosome behaviour, and the details of the in situ organization of pericentromeric regions is of great interest. The objective of this work was a cytogenetic analysis of the interactions between pericentromeric regions from non-homologous chromosomes in mouse spermatocytes using immuno-FISH. We analysed two events: the associations between centromeric regions of the X chromosome and autosomes and the associations between the centromeric regions of the autosomal bivalents that form chromocenters. We concluded that the X chromosome forms temporary synaptic associations with different autosomes in early meiotic prophase I, which can normally be found until the pachytene-diplotene, without signs of pachytene arrest. These associations are formed between the satellite-DNA-rich centromeric regions of the X chromosome and different autosomes but do not involve the satellite-DNA-poor centromeric region of the Y chromosome. We suggest the hypothetical model of X chromosome competitive replacement from such associations during synaptic correction. We showed that the centromeric region of the X chromosome in association remains free of γH2Ax-dependent chromatin inactivation, while the Y chromosome is completely inactivated. This finding highlights the predominant role of associations between satellite DNA-rich regions of different chromosomes, including the X chromosome. We suppose that X-autosomal transient associations are a manifestation of an additional synaptic disorder checkpoint. These associations are normally corrected before the late diplotene stage. We revealed that the intense spreading conditions that were applied to the spermatocyte I nuclei did not lead to the destruction of stretched chromatin fibers of elongated chromocenters enriched in satellite DNA. The tight associations that we revealed between the pericentromeric regions of different autosomal bivalents and the X chromosome may represent the basis for a mechanism for maintaining the repeats stability in the autosomes and in the X chromosome. The consequences of our findings are discussed.

Meiotic synapsis of homeologous chromosomes and mismatch repair protein detection in the parthenogenetic rock lizard Darevskia unisexualis.

Parthenogenetic species of Caucasian rock lizards of the genus Darevksia are important evidence for reticulate evolution and speciation by hybridization in vertebrates. Female-only lineages formed through interspecific hybridization have been discovered in many groups. Nevertheless, critical mechanisms of oogenesis and specifics of meiosis that provide long-term stability of parthenogenetic species are still unknown. Here we report cytogenetic characteristics of somatic karyotypes and meiotic prophase I nuclei in the diploid parthenogenetic species Darevskia unisexualis from the new population "Keti" in Armenia which contains an odd number of chromosomes 2n = 37, instead of the usual 2n = 38. We revealed 36 acrocentric chromosomes and a single metacentric autosomal chromosome, resulting from Robertsonian translocation. Comparative genomic hybridization revealed that chromosome fusion occurred between two chromosomes inherited from the maternal species, similar to another parthenogenetic species D. rostombekowi. To trace the chromosome behaviour in meiosis, we performed an immunocytochemical study of primary oocytes' spread nuclei and studied chromosome synapsis during meiotic prophase I in D. unisexualis based on analysis of synaptonemal complexes (SCs). We found meiotic SC-trivalent composed of one metacentric and two acrocentric chromosomes. We confirmed that the SC was assembled between homeologous chromosomes inherited from two parental species. Immunostaining of the pachytene and diplotene nuclei revealed a mismatch repair protein MLH1 loaded to all autosomal SC bivalents. Possible mechanisms of meiotic recombination between homeologous chromosomes are discussed.

In vitro derived female hPGCLCs are unable to complete meiosis in embryoid bodies.

The fundamental question about the functionality of in vitro derived human primordial germ cell-like cells remains unanswered, despite ongoing research in this area. Attempts have been made to imitate the differentiation of human primordial germ cells (hPGCs) and meiocytes in vitro from human pluripotent stem cells (hPSCs). A defined system for developing human haploid cells in vitro is the challenge that scientists face to advance the knowledge of human germ cell development. To develop human primordial germ cell-like cells (hPGCLCs) from human pluripotent stem cells (hPSCs) that are capable of giving rise to haploid cells, we applied a sequential induction protocol via the early mesodermal push of female human embryonic and induced pluripotent stem cells. BMP4-induced early mesoderm-like cells showed significant alterations in their expression profiles toward early (PRDM1 and NANOS3) and late (VASA and DAZL) germ cell markers. Furthermore, using retinoic acid (RA), we induced hPGCLCs in embryoid bodies and identified positive staining for the meiotic initiation marker STRA8. Efforts to find the cells exhibiting progression to meiosis were unsuccessful. The validation by the expression of SCP3 did not correspond to the natural pattern. Regarding the 20-day meiotic induction, the derived hPGCLCs containing two X-chromosomes were unable to complete the meiotic division. We observed the expression of the oocyte marker PIWIL1 and PIWIL4. RNAseq analysis and cluster dendrogram showed a similar clustering of hPGCLC groups and meiotic like cell groups as compared to previously published data. This reproducible in vitro model for deriving hPGCLCs provides opportunities for studying the molecular mechanisms involved in the specification of hPGCs. Moreover, our results will support a further elucidation of gametogenesis and meiosis of female hPGCs.

Evolution of the parthenogenetic rock lizard hybrid karyotype: Robertsonian translocation between two maternal chromosomes in Darevskia rostombekowi.

Darevskia rostombekowi, the most outstanding of the seven known parthenogenetic species in the genus Darevskia, is the result of an ancestral cross between two bisexual species Darevskia raddei and Darevskia portschinskii. The chromosomal set of this species includes a unique submetacentric autosomal chromosome; the origin of this chromosome was unresolved as only acrocentric chromosomes are described in the karyotypes of Darevskia genus normally. Here, we applied a suite of molecular cytogenetic techniques, including the mapping of telomeric (TTAGGG) n repeats using fluorescence in situ hybridization (FISH), comparative genomic hybridization (CGH), and whole-chromosome painting (WCP) in both D. rostombekowi and parental (D. portschinskii and D. raddei) species. The obtained results in total suggest that a de novo chromosomal rearrangement via Robertsonian translocation (centric fusion) between two maternal (D. raddei) acrocentric chromosomes of different size was involved in the formation of this unique submetacentric chromosome present in the parthenogenetic species D. rostombekowi. Our findings provide new data in specific and rapid evolutional processes of a unisexual reptile species karyotype.

Cytogenetic mechanisms of unisexuality in rock lizards.

Darevskia rock lizards is a unique complex taxa, including more than thirty species, seven of which are parthenogenetic. In mixed populations of Darevskia lizards, tri- and tetraploid forms can be found. The most important issues in the theory of reticulate evolution of Darevskia lizards are the origin of parthenogenetic species and their taxonomic position. However, there is little data on how meiosis proceeds in these species. The present work reports the complex results of cytogenetics in a diploid parthenogenetic species - D. unisexualis. Here we detail the meiotic prophase I progression and the specific features оf mitotic chromosomes organization. The stages of meiosis prophase I were investigated by immunocytochemical analysis of preparations obtained from isolated primary oocytes of D. unisexualis in comparison with maternal species D. raddei nairensis. It has been shown that in D. unisexualis at the leptotene-zygotene stages the axial elements and the synaptonemal complex (SC) form typical "bouquets". At the pachytene-diplotene stage, 18 autosomal SC-bivalents and thickened asynapted sex Z and w univalents were observed. The presence of SYCP1 protein between the lateral elements of autosomal chromosomes proved the formation of assembled SCs. Comparative genomic hybridization (CGH) on the mitotic metaphase chromosomes of D. unisexualis was carried out using the genomic DNA isolated from the parental species D. raddei nairensis and D. valentini. In the pericentromeric regions of half of the mitotic chromosomes of D. unisexualis, specific regions inherited from maternal species have been found. Following our results, we suggest a model for diploid germ cells formation from diploid oocytes without premeiotic duplication of chromosomes in the oogenesis of diploid parthenogenetic lizards D. unisexualis. Taken as a whole, our findings confirm the hybrid nature of D. unisexualis and shed light on heterozygosity and automixis in diploid parthenogenetic forms.

Extraordinary centromeres: differences in the meiotic chromosomes of two rock lizards species Darevskia portschinskii and Darevskia raddei.

According to the synthesis of 30 years of multidisciplinary studies, parthenogenetic species of rock lizards of genus Darevskia were formed as a result of different combination patterns of interspecific hybridization of the four bisexual parental species: Darevskia raddei, D. mixta, D. valentini, and D. portschinskii. In particular, D. portschinskii and D. raddei are considered as the parental species for the parthenogenetic species D. rostombekowi. Here for the first time, we present the result of comparative immunocytochemical study of primary spermatocyte nuclei spreads from the leptotene to diplotene stages of meiotic prophase I in two species: D. portschinskii and D. raddei. We observed similar chromosome lengths for both synaptonemal complex (SC) karyotypes as well as a similar number of crossing over sites. However, unexpected differences in the number and distribution of anti-centromere antibody (ACA) foci were detected in the SC structure of bivalents of the two species. In all examined D. portschinskii spermatocyte nuclei, one immunostained centromere focus was detected per SC bivalent. In contrast, in almost every studied D. raddei nuclei we identified three to nine SCs with additional immunostained ACA foci per SC bivalent. Thus, the obtained results allow us to identify species-specific karyotype features, previously not been detected using conventional mitotic chromosome analysis. Presumably the additional centromere foci are result of epigenetic chromatin modifications. We assume that this characteristic of the D. raddei karyotype could represent useful marker for the future studies of parthenogenetic species hybrid karyotypes related to D. raddei.

Reorganization of the Y Chromosomes Enhances Divergence in Israeli Mole Rats Nannospalax ehrenbergi (Spalacidae, Rodentia): Comparative Analysis of Meiotic and Mitotic Chromosomes.

The Y chromosome in mammals is variable, even in closely related species. Middle East blind mole rats Nannospalax ehrenbergi demonstrate autosomal variability, which probably leads to speciation. Here, we compare the mitotic and meiotic chromosomes of mole rats. For the first time, we studied the behavior of their sex chromosomes in the meiotic prophase I using electron microscopy and immunocytochemical analysis. Unexpectedly, the sex chromosomes of the 52- and 60-chromosome forms of mole rats showed different synaptic and recombination patterns due to distinct locations of the centromeres on the Y chromosomes. The absence of recombination in the 60-chromosome form, the asymmetric synapsis, and the short-term disturbance in the synaptic co-orientation of the telomeric regions of the X and Y chromosomes were revealed as specific features of mole rat sex bivalents. We suggest several scenarios of Y chromosome alteration in connection with species differentiation in mole rats.

Reticulate Evolution of the Rock Lizards: Meiotic Chromosome Dynamics and Spermatogenesis in Diploid and Triploid Males of the Genus Darevskia.

Knowing whether triploid hybrids resulting from natural hybridization of parthenogenetic and bisexual species are fertile is crucial for understanding the mechanisms of reticulate evolution in rock lizards. Here, using males of the bisexual diploid rock lizard species Darevskia raddei nairensis and Darevskia valentini and a triploid hybrid male Darevskia unisexualis × Darevskia valentini, we performed karyotyping and comparative immunocytochemistry of chromosome synapsis and investigated the distribution of RAD51 and MLH1 foci in spread spermatocyte nuclei in meiotic prophase I. Three chromosome sets were found to occur in cell nuclei in the D. unisexualis × D. valentini hybrid, two originating from a parthenogenetic D. unisexualis female and one from the D. valentini male. Despite this distorted chromosome synapsis and incomplete double-strand breaks repair in meiotic prophase I, the number of mismatch repair foci in the triploid hybrid was enough to pass through both meiotic divisions. The defects in synapsis and repair did not arrest meiosis or spermatogenesis. Numerous abnormal mature spermatids were observed in the testes of the studied hybrid.