Date of Award
Dissertation - Restricted
Doctor of Philosophy (PhD)
The Amazon molly fish, Poecilia formosa, described nearly 50 years ago as the first unisexual vertebrate, is a diploid, gynogenetic species. Eggs develop after activation by sperm, that do not fuse with egg nuclei and make no genetic contribution to developing embryos. Along with related triploid biotypes and sexual congeners, P. formosa exists as part of a species breeding complex in southern Texas and northeastern Mexico. Although several mechanisms have been proposed to account for the isogenic, matriclinous inheritance shown by all-female Poecilia, the precise mechanism of reproduction in these fish is unknown.
The present study provides cytological, ultrastructural and cytophotemetric evidence that diploid and triploid unisexuals of Poecilia reproduce by apomixis. Synapses of homologous chromosomes and the first meiotic division are suppressed and supplanted by a maturation division that is mitotic in character, i.e., it does not effect genetic segregation or a reduction in chromosome number. These studies also detail changes associated with nucleolar ribonucleoproteins during early stages of oogenesis in unisexual and bisexual Poecilia. No evidence was found for amplification of ribosomal DNA prior to the appearance of multiple nucleoli in diplotene oocytes of these fishes.
Comparative electrophoretic studies on muscle extracts from these fishes show species-specific patterns of low molecular weight proteins that can be used as genetic markers to monitor paternal inheritance in triploid unisexuals and in bisexual, diploid interspecific hybrids.
Quantitative studies of male reproductive schedules were used to estimate the availability of sperm in naturally occuring populations of Poecilia. Qualitative studies that document the occurrence of superfetation in both sexual and asexual forms of Poecilia suggest that the occurrence of multiple broods of embryos within the ovary of a single female and seasonal differences in the reproductive activity of females may be significant factors in moderating competitive interactions among the three types of females in nature. The maintenance and evolutionary success of unisexual-bisexual breeding complexes thus depends in part on an available supply of sperm, on the reproductive schedules of both bisexual and unisexual females, on their capacity for superfetation and their capability to store sperm for prolonged periods. Apomictic reproduction by unisexual Poecilia may function to maintain adaptively favorable heterozygous genomes without the effect of segregational load imposed by recombination in the sexual species.