Oogenesis in Unisexual Whiptail Lizards (genus Aspidoscelis)

Abstract

Parthenogenesis is a reproductive mode that does not require males. Though theoretically advantageous, its rarity among metazoans suggests otherwise. Paradoxically, some unisexual species within the genus Aspidoscelis appear to be thriving by reproducing through obligate parthenogenesis. Formed via hybridization between different bisexual species, these lizards apparently reap the benefits of both hybrid vigor and higher reproductive potential. Previous studies have demonstrated the high degree of heterozygosity between generations, which contributes to the success of these lineages; a loss of heterozygosity would likely be accompanied by a decrease in fitness. How meiosis is modified in these animals is unknown. The predominant hypothesis suggests that endoreplication takes place prior to meiosis, resulting in a two-fold increase in DNA; however this has not been definitively shown. Further, the downstream modifications that result in a heterozygous gamete have merely been speculated upon. In the following thesis, meiosis within three parthenogenetic species is investigated. Quantification of DNA in germinal vesicles of the diploid parthenogen A. tesselata and the bisexual species A. gularis demonstrated that the parthenogen contains two-fold more DNA, despite equivalent somatic cell ploidy. Chiasmata were present on diplotene-stage chromosomes in both species, indicating that recombination is not bypassed. Additionally, synaptonemal complexes were found during pachytene in each species. Maintenance of heterozygosity is highly dependent on chromosome pairing in the parthenogen. Pairing between homologous chromosomes would result in a decrease; whereas pairing between identical (sister) chromosomes resulting from the additional DNA doubling event would preserve heterozygosity. Using homolog specific FISH probes in the parthenogen A. neomexicana, sister chromosome pairing is revealed. Thus, sister chromosome pairing after an additional DNA doubling allows for maintenance of heterozygosity in Aspidoscelis parthenogens. Secondly, four self-sustaining lineages of a new tetraploid species were generated from the mating between the diploid bisexual species Aspidoscelis inornata and triploid parthenogen A. exsanguis. The identity of these hybrids was confirmed by genotyping analysis. Females retain the ability to reproduce parthenogenetically through the doubling mechanism described in diploid species. These tetraploids have demonstrated how ploidy elevation hypothetically occurs in natural parthenogens. The mechanisms described in this thesis may be utilized in other parthenogens. Recent findings and future directions based on this work are presented.