Date of Award

Spring 1977

Degree Type

Thesis - Restricted

Degree Name

Master of Science (MS)



First Advisor

Rasch, Ellen M.

Second Advisor

Balsano, Joseph

Third Advisor

deVlaming, Victor


Comparisons based on chromosome appearance, number and morphology, as well as size and staining intensity of nucleoli, and changes in morphological and chemical composition of the ooplasm revealed no observable cytological differences in the events associated with ovarian maturation of unisexual and bi sexual forms of Poecilia spp . from northeastern Mexico. Analysis of patterns of ovarian maturation in wild caught unisexual and bisexual forms, based on an empirically derived ovarian maturation index, revealed differences in the frequency of post-vitellogenic oocytes from different seasons, different collection sites, and between different microhabitats at the same collection locality. It has been shown that females of Poecilia spp. have asynchronous cycles of ovarian maturation and our data suggest that at any given time there are small but consistent differences in patterns of ovarian maturation for each of these forms. The implication of seasonally correlated and environmentally conditioned differences in the reproductive competencies of these females is discussed in light of a model for oocyte development in Poecilia spp .