2002 The American Genetic Association 93:91–99 Mutation in a Sex-Determining Gene in Rainbow Trout: Detection and Genetic Analysis

In rainbow trout (Oncorhynchus mykiss), the acknowledged sex-determining sys-tem is genetic sex determination (GSD) with female homogamety (XX-XY). Sub-sequently, mitotic gynogens are all expected to be females. Unexpected maleness was fortuitously observed in a mitotic gynogenetic family of rainbow trout (13 males out of 27). An equal ratio of males and females suggested the possible seg-regation of some Mendelian sex-influencing factor. In order to perform a compre-hensive analysis of the inheritance and expression of the factor involved, the trans-mission of maleness was studied across the next three generations, using both conventional and/or meiotic and mitotic gynogenetic offspring. On the whole, males as well as intersexes were observed in crosses between two expected carrier par-ents, and in gynogenetic offspring of expected carrier females, but not in crosses between one expected carrier parent and one normal XX control. Sex ratios in the different crosses often fitted Mendelian proportions, but not always. Both excess and lack of maleness were observed. The simplest hypothesis consistent with most results is a one-locus model, assuming the existence of a mutation (termed mal