Sex determination in tilapia (Oreochromis niloticus) sex ratios in homozygous gynogenetic progeny and their offspring

A total of 323 potential mitogynes were produced by 14 different females with the intention of developing homozygous clones in an Oreochromis niloticus population, derived from Lake Manzala (Egypt) via the Institute of Aquaculture, Stirling. The sex ratio of 119 mitogynes, their mitotic gynogenetic status proven by gene marker analysis, showed a high proportion of males (35.3%). All female meiogynes were derived from two females which produced males among mitotic gynogenetic progenies. Gynogenetic reproduction of six mitogyne females led to all female-homozygous clones (21 - 241 individuals within clones). Progenies of five mitogyne males mated with different females were all male, all female or predominantly female. The occurrence of two or more minor sex determining factors, which are able to override the XX-XY mechanism when they act in combination and occur in homozygous state might account for the sex ratios observed. No obvious trends could be detected with regard to the influence of shock treatment or environment on sex ratios of the investigated O. niloticus.