Meiotic Mechanisms of Naturally Occurring Unisexual Vertebrates

Genetic consequences of several proposed or observed oogenic pathways for unisexual vertebrates are tabulated, and compared with known modes of oogenesis or with inheritance data. Poeciliopsis Cx has a selective maturation; inheritance is haploid and matroclinous. Triploid Carassius auratus gibelio reportedly suppress Meiosis I. The triploids Ambystoma tremblayi, A. platineum, and Cnemidophorus uniparens suppress cytokinesis at a premeiotic mitosis. Ova of diploid Lacerta armeniaca undergo two meiotic divisions; the somatic ploidy level is reportedly restored by re-entry of the second polar body. On the other hand, a premeiotic mitosis without cytokinesis is more probably the oogenic pathway used by unisexual Lacerta, since it is consistent with the observations (two meiotic divisions), but avoids the difficulties associated with homozygosity, which would rapidly result from suppressing the second meiotic division. In fact, nothing that is known of inheritance patterns of naturally-occurring unisexual vertebrates is inconsistent with the hypothesis that most, like the two species of Ambystoma and C. uniparens, form ova by suppressing cytokinesis at the last premeiotic mitosis, followed by two meiotic divisions based on pseudobivalents rather than synapsed homologues. Although most unisexual vertebrates probably are fixed heterozygotes, the absence of evidence for antiquity of any of them suggests that they have limited evolutionary potential. Known unisexual anamniotes are gynogenetic, while unisexual amniotes are parthenogenetic. This dichotomy possibly has parallels in experimental studies and natural occurrences of parthenogenesis in bisexual vertebrates.