Monosex Tilapia Production through Androgenesis: Selection of Individualsfor Sex Inheritance Characteristics for Use in Monosex Production

Intraspecific breeding programs have been developed to exploit the sex inheritance mechanism in the tilapia Oreochromis niloticus to produce male populations. These programs are built on the premise that the mechanism of sex inheritance must conform closely to a monofactorial sex determination with a heterogametic male. Sex inheritance in tilapia, however, appears to be more complicated. The sex ratio of an individual spawn often does not conform to the expected 1:1 ratio. A better understanding of sex inheritance in tilapia and the identification of tilapia populations with a minimum variation in progeny sex ratios from individual spawns is needed for a successful intraspecific breeding program to produce male tilapia. Nine families of O. niloticus, each from individual pair spawns, were selected based on the sex ratio in the family. Two families were highly skewed to male (100% male), three were near 50% male, and four were skewed to female. Fish within the same family were mated, and the sex ratios of the progeny were determined. In the two families that were all males, the males were mated to females from a family with a sex ratio near 1:1. Ten sets of progeny per family were sexed, with the exception of one family, from which only eight sets were available. Sex ratios did not appear to be passed on from one generation to another in the fish used in our study. A realized heritability for sex ratio of –0.09 was calculated. No family with skewed sex ratios produced progeny from sibling matings with similarly skewed sex ratios. Family VIII, which had a 1:1 male:female ratio, had a range of 43 to 68% male in its sets of progeny. Family V, which was 22% male, gave 10 sets of progeny, of which five sets were > 70% male. Families II and VII, which were 100% male, when crossed with females from Family III gave sets of progeny ranging from 23 to 79% male. When the percentage of males in the female parent family (III, 40% male) was considered in matings with Families II and VII, 40 and 50% of the spawns, respectively, differed in sex ratios from the female parent family. NINETEENTH ANNUAL TECHNICAL REPORT 40 a solution of MS-222, counted, and 110 randomly selected fish preserved in a 10% formalin solution in a labeled jar. If fewer than 110 fish survived, then all were preserved. Preserved spawns were held a minimum of ten days before being sexed.