Speciation in Caucasian lizards: climatic dissimilarity of the habitats is more important than isolation time

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 favoured the deviation of reproductive behaviour when species switched to interspecific mates. To date, at least seven 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 two 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.

Parthenogenesis in Darevskia lizards: A rare outcome of common hybridization, not a common outcome of rare hybridization

Hybridization is a common evolutionary process with multiple possible outcomes. In vertebrates, interspecific hybridization has repeatedly generated parthenogenetic hybrid species. However, it is unknown whether the generation of parthenogenetic hybrids is a rare outcome of frequent hybridization between sexual species within a genus or the typical outcome of rare hybridization events. Darevskia is a genus of rock lizards with both hybrid parthenogenetic and sexual species. Using capture sequencing, we estimate phylogenetic relationships and gene flow among the sexual species, to determine how introgressive hybridization relates to the origins of parthenogenetic hybrids. We find evidence for widespread hybridization with gene flow, both between recently diverged species and deep branches. Surprisingly, we find no signal of gene flow between parental species of the parthenogenetic hybrids, suggesting that the parental pairs were either reproductively or geographically isolated early in their divergence. The generation of parthenogenetic hybrids in Darevskia is, then, a rare outcome of the total occurrence of hybridization within the genus, but the typical outcome when specific species pairs hybridize. Our results question the conventional view that parthenogenetic lineages are generated by hybridization in a window of divergence. Instead, they suggest that some lineages possess specific properties that underpin successful parthenogenetic reproduction.

Genotypic similarities among the parthenogenetic Darevskia rock lizards with different hybrid origins

BACKGROUND

The majority of parthenogenetic vertebrates derive from hybridization between sexually reproducing species, but the exact number of hybridization events ancestral to currently extant clonal lineages is difficult to determine. Usually, we do not know whether the parental species are able to contribute their genes to the parthenogenetic vertebrate lineages after the initial hybridization. In this paper, we address the hypothesis, whether some genotypes of seven phenotypically distinct parthenogenetic rock lizards (genus Darevskia) could have resulted from back-crosses of parthenogens with their presumed parental species. We also tried to identify, as precise as possible, the ancestral populations of all seven parthenogens.

RESULTS

We analysed partial mtDNA sequences and microsatellite genotypes of all seven parthenogens and their presumed ansectral species, sampled across the entire geographic range of parthenogenesis in this group. Our results confirm the previous designation of the parental species, but further specify the maternal populations that are likely ancestral to different parthenogenetic lineages. Contrary to the expectation of independent hybrid origins of the unisexual taxa, we found that genotypes at multiple loci were shared frequently between different parthenogenetic species. The highest proportions of shared genotypes were detected between (i) D. sapphirina and D. bendimahiensis and (ii) D. dahli and D. armeniaca, and less often between other parthenogens. In case (ii), genotypes at the remaining loci were notably distinct.

CONCLUSIONS

We suggest that both observations (i-ii) can be explained by two parthenogenetic forms tracing their origin to a single initial hybridization event. In case (ii), however, occasional gene exchange between the unisexual and the parental bisexual species could have taken place after the onset of parthenogenetic reproduction. Indeed, backcrossed polyploid hybrids are relatively frequent in Darevskia, although no direct evidence of recent gene flow has been previously documented. Our results further suggest that parthenogens are losing heterozygosity as a result of allelic conversion, hence their fitness is expected to decline over time as genetic diversity declines. Backcrosses with the parental species could be a rescue mechanism which might prevent this decline, and therefore increase the persistance of unisexual forms.

Phylogeny of caucasian rock lizards (Darevskia) and other true lizards based on mitogenome analysis: Optimisation of the algorithms and gene selection

We generated a phylogeny for Caucasian rock lizards (Darevskia), and included six other families of true lizards (Lacertini), based on complete mitochondrial genome analysis. Next-generation sequencing (NGS) of genomic DNA was used to obtain 16 new mitogenomes of Darevskia. These, along with 35 sequences downloaded from GenBank: genera Darevskia, Zootoca, Podarcis, Phoenicolacerta, Takydromus, Lacerta, and Eremias-were used in the analysis. All four analytical methods (Bayesian Inference, BI; Maximum Likelihood, ML; Maximum Parsimony, MP; and Neighbor-Joining, NJ) showed almost congruent intra-generic topologies for Darevskia and other lizard genera. However, ML and NJ methods on one side, and BI and MP methods on the other harvested conflicting phylogenies. The ML/NJ topology supports earlier published separation of Darevskia into three mitochondrial clades (Murphy, Fu, Macculloch, Darevsky, and Kupinova, 2000), but BI and MP topologies support that the basal branching occurred between D. parvula from the western Lesser Caucasus and the rest of Darevskia. All topologies altered the phylogenetic position of some individual species, including D. daghestanica, D. derjugini, and D. chlorogaster. Reanalysis after excluding four saturated genes from the data set, and excluding genus Eremias gives fully convergent topologies. The most basal branching for true lizards was between Far Eastern Takydromus and the Western Eurasian genera (BI). Comparing phylogenetic performance of individual genes relative to whole mitogenome data, concatenated 16S RNA (the least saturated gene in our analyses) and Cytochrome b genes generate a robust phylogeny that is fully congruent with that based on the complete mitogenome.

Phenotypic divergence, convergence and evolution of Caucasian rock lizards (Darevskia)

Phenotypic evolution can cause either divergent or convergent phenotypic patterns. Even adaptation to the same environment may result in divergence of some elements of phenotype, whereas for other morphological traits it could cause phenotypic convergence. We hypothesize that at least some phenotypic characters diverge monotonically, hence they evolve irreversibly even in very closely related species, and this happens in spite of multiple convergent adaptive patterns. We studied the evolution of phenotype in 13 closely related Caucasian rock lizards (Darevskia), whose phylogenetic relationships are well known. We used head shape and the outlines of three important scales, using geometric morphometrics. We studied the association of the overall head shape, individual principal components of head shape and scale outlines with four predictors: phylogeny, habitat, sex and size. The overall head shape was not correlated with any of these predictors, whereas some principal components were correlated with habitat or phylogeny. Habitat type explained the highest fraction of variation in head shape and anal scale area. The relatedness inferred from the components of phenotype not correlated with habitat was congruent with the phylogenetic tree inferred from molecular data. Although adaptation to local environments may obscure the phylogenetic signal present in phenotype, there are components of phenotype whose evolution is irreversible.

The role of hybridisation in the origin and evolutionary persistence of vertebrate parthenogens: a case study of Darevskia lizards

Obligate parthenogenesis is found in only 0.1% of the vertebrate species, is thought to be relatively short lived and is typically of hybrid origin. However, neither the evolutionary persistence of asexuality in vertebrates, nor the conditions that allow the generation of new parthenogenetic lineages are currently well understood. It has been proposed that vertebrate parthenogenetic lineages arise from hybridisation between two divergent taxa within a specific range of phylogenetic distances (the 'Balance Hypothesis'). Moreover, parthenogenetic species often maintain a certain level of hybridisation with their closest sexual relatives, potentially generating new polyploid hybrid lineages. Here we address the role of hybridisation in the origin and evolutionary lifespan of vertebrate parthenogens. We use a set of microsatellite markers to characterise the origins of parthenogens in the lizard genus Darevskia, to study the distinctiveness of sexual and asexual taxa currently in sympatry, and to analyse the evolutionary consequences of interspecific hybridisation between asexual females and sexual males. We find that parthenogens result from multiple past hybridisation events between species from specific lineages over a range of phylogenetic distances. This suggests that the Balance Hypothesis needs to allow for lineage-specific effects, as envisaged in the Phylogenetic Constraint Hypothesis. Our results show recurrent backcrossing between sexual and parthenogenic Darevskia but neither gene flow nor formation of new asexual lineages. We suggest that, along with their demographic advantage, parthenogens gain additional leverage to outcompete sexuals in nature when the retention of sexual reproductive machinery allows backcrossing with their sexual ancestors.

Comparing developmental stability in unisexual and bisexual rock lizards of the genus Darevskia

Parthenogenetic species are usually considered to be short-lived due to the accumulation of adverse mutations, lack of genetic variability, and inability to adapt to changing environment. If so, one may expect that the phenotype of clonal organisms may reflect such genetic and/or environmental stress. To test this hypothesis, we compared the developmental stability of bisexual and parthenogenetic lizards of the genus Darevskia. We assessed asymmetries in three meristic traits: ventral, preanal, and supratemporal scales. Our results suggest that the amount of ventral and preanal asymmetries is significantly higher in clones compared with their maternal, but not paternal, progenitor species. However, it is questionable, whether this is a consequence of clonality, as it may be considered a mild form of outbreeding depression as well. Moreover, most ventral asymmetries were found in the bisexual species Darevskia valentini. We suggest that greater differences in asymmetry levels among bisexuals may be, for instance, a consequence of the population size: the smaller the population, the higher the inbreeding and the developmental instability. On the basis of the traits examined in this study, the parthenogens do not seem to be of significantly poorer quality.

Phylogenetic relationships of D. rudis (Bedriaga, 1886) and D. bithynica(Mehely, 1909) based on microsatellite and mitochondrial DNA in Turkey

The spiny-tailed lizard of the genus Darevskia have a series of taxonomic revisions, but still their phylogenetic relationships remain uncertain. In the present study, we have assessed taxonomic relationships among Darevskia bithynica and Darevskia rudis populations through estimation of phylogenetic relationships among 96 specimens using microsatellite DNA (Du215, Du281, Du323 and Du418 loci) and 53 specimens using mtDNA (16S rRNA and cytb) from main populations in Turkey. Although D. b. bithynica and D. r. mirabilis were separated based on the PCoA analysis at low level from other D. rudis and D. bithynica populations, the distance values of Nei's genetic distance, Nei's unbiased genetic distance, Fst and Linear Fst were not high among taxa in microsatellite DNA. On the other hand, our phylogenetic analyses (NJ, ML, MP and BI) did not separate D. rudis and D. bithynica populations. Finally, most of the topologically identical trees of phylogenetic analyses and microsatellite results showed that the extant populations of D. rudis and D. bithynica were found to be polytomy. Based on our molecular phylogenetic study, D. rudis complex is still ongoing revisions.