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To investigate alternative routes to develop YY males and to examine some of the factors responsible for departures from predicted sex ratios in the Nile tilapia (Oreochromis niloticus), YY males were produced (i) by androgenesis from XY male parents; and (ii) by mitotic gynogenesis from XY neofemale parents. Progeny testing of these androgenetic and gynogenetic males, and gynogenetic females, showed that most families (i.e. sibling groups of males and in some cases females) gave monosex progeny only, when individuals were crossed to XX fish (i.e. all-male progeny from tested androgenetic or gynogenetic males; all-female progeny from tested gynogenetic females). There were significant differences among androgenetic families in deviations from expected progeny sex ratios, while among the gynogenetic families, the largest sex ratio deviations occurred in the same family for both male and female gynogenetics. The factor(s) that cause departures from the sex ratios predicted by chromosomal sex determination appear to be autosomal, heritable, polymorphic and able to influence sex ratios in both directions. Furthermore, the results suggest that YY and XX lines could be developed in which such factors had been selected against, for example, by elimination of families showing departures from monosex progeny. |
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