Evidence for two unlinked "sex reversal" loci in the Nile tilapia, Oreochromis niloticus, and for linkage of one of these to the red body colour gene

Gynogenetic offspring from heterozygous red (Rr) Oreochromis niloticus females were produced by UV irradiation of sperm and suppression of the second meiotic division. The distance between the red gene and the centromere was estimated to be 4.8 cM. Of 547 gynogenetic offspring that survived to be sexed, 54 (9.9%) were males. There was a significant association between colour and sex - 53 of the male fish were red and only one was wild type. These data provide evidence for genetic linkage between the red gene and a gene that can cause female-to-male sex reversal. Of several fully inbred XX clonal lines of O. niloticus previously developed in our laboratory, only one contained males. To test if this is caused by the same gene as the red-linked autosomal sex reversal gene, a series of test crosses was carried out. Males from this line were crossed to homozygous red females, then some of the offspring, which were all females, were backcrossed to the parental males. If the same gene was causing the presence of males in the gynogenetic offspring and in the clonal line, we would expect that in the backcrosses there would be more males in the wild type than in the red fish. However, the frequency of males was not significantly different between the red and wild-type fish (18/162=11.1% and 18/173=10.4% males, respectively), which leads to the conclusion that different unlinked loci are responsible for the presence of males in the clonal line and in the gynogenetics from the heterozygous red females.