Triploidy in Parthenogenetic Species of the Teiid Lizard, Genus Cnemidophorus

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.

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.

MITOCHONDRIAL-DNA ANALYSES AND THE ORIGIN AND RELATIVE AGE OF PARTHENOGENETIC LIZARDS (GENUS CNEMIDOPHORUS). IV. NINE SEXLINEATUS-GROUP UNISEXUALS

Mitochondrial DNAs (mtDNAs) from nine morphologically distinct unisexual species and five bisexual species of lizards, all from the sexlineatus species-group of Cnemidophorus, were compared using restriction endonucleases. The unisexual lizards have mtDNAs that are identical at all or nearly all of the 128 sites cleaved. Although differing little in sequence, some mtDNAs differed in size due to the presence of tandem sequence duplications. Phylogenetic analysis of cleavage maps indicates that the mtDNAs of the unisexuals are most similar to that of the bisexual species C. inornatus. Considerable mtDNA diversity exists among C. inornatus populations, and one geographically restricted subspecies, C. i. arizonae, was identified as the most probable maternal ancestor of all nine unisexuals. All but one of these are triploid, and all have at least one C. inornatus gene complement. This, together with the homogeneity of their mtDNAs, suggests that all stem from one or a small number of allodiploid females (presumably parthenogenetic) that originated in a restricted geographic area in the recent past. These data, when combined with those from allozyme studies, preclude the possibility that most of the triploid unisexuals could have arisen via fertilization of an unreduced diploid ovum from one species by a haploid sperm from a different species.

Widespread failure to complete meiosis does not impair fecundity in parthenogenetic whiptail lizards

Parthenogenetic species of whiptail lizards in the genus Aspidoscelis constitute a striking example of speciation by hybridization, in which first-generation hybrids instantly attain reproductive isolation and procreate as clonal all-female lineages. Production of eggs containing a full complement of chromosomes in the absence of fertilization involves genome duplication prior to the meiotic divisions. In these pseudo-tetraploid oocytes, pairing and recombination occur exclusively between identical chromosomes instead of homologs; a deviation from the normal meiotic program that maintains heterozygosity. Whether pseudo-tetraploid cells arise early in germ cell development or just prior to meiosis has remained unclear. We now show that in the obligate parthenogenetic species A. neomexicana the vast majority of oocytes enter meiosis as diploid cells. Telomere bouquet formation is normal, but synapsis fails and oocytes accumulate in large numbers at the pairing stage. Pseudo-tetraploid cells are exceedingly rare in early meiotic prophase, but they are the only cells that progress into diplotene. Despite the widespread failure to increase ploidy prior to entering meiosis, the fecundity of parthenogenetic A. neomexicana is similar to that of A. inornata, one of its bisexual ancestors.

High levels of chromosome instability in polyploids of Saccharomyces cerevisiae

The yeast Saccharomyces cerevisiae was used to study the genetic consequences of polyploidy in a unicellular organism. Isogenic diploid (2N), triploid (3N) and tetraploid (4N) strains with a genetically marked chromosome VII (cyh2-leu1-CEN7-ade6) were constructed and were used to follow the loss of one, two or three chromosome VII's during mitosis. We found that as ploidy increased, the frequency of loss of a single chromosome VII increased: Loss of one copy of chromosome VII occurred at a rate nearly 30-fold higher in triploids and approximately 1000-fold higher in tetraploids than in the diploid. Loss of two or three copies occurred at an even greater frequency. These findings suggest either that aneuploidy (3N-1, 3N-2, 4N-1, 4N-2, 4N-3) increases genome instability or that multiple chromosome loss events occur at high frequency. Polyploidy appears to dramatically increase chromosome loss, presumably due to the inability of the cell to undergo proper chromosome segregation. The biological significance and possible causes for the instability of polyploidy in unicellular organisms such as yeast are discussed.

Parthenogenesis

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Gynogenesis and triploidy in the viviparous fish Poeciliopsis

Three all-female strains of the viviparous fish Poeciliopsis occur in the Río Fuerte of Sinaloa, Mexico. Poeciliopsis lucida, a bisexual species, provides sperm for these monosexual forms which I designate as Cx, Cy, and Cz. Form Cy is a triploid that when test-mated to males of various species produces all-female, triploid offspring devoid of paternal characters. Both Cx and Cz are diploid and express characteristics of both parents.