Elucidating the molecular pathway of sex determination in cultured Tilapias and use of genetic markers for creating monosex populations

Abstract

The objectives of this project were to: 1) Identify genetic markers linked to sex-determining genes in various experimental and commercial stocks of O. niloticusand O. aureus, as well as red tilapias; 2) Develop additional markers tightly linked to these sex determiners, and develop practical, non-destructive genetic tests for identifying genotypic sex in young tilapia; A third aim, to map sex modifier loci, was removed during budget negotiations at the start of the project. Background to the topic. A major obstacle to profitable farming of tilapia is the tendency of females to reproduce at a small size during the production cycle, diverting feed and other resources to a large population of small, unmarketable fish. Several approaches for producing all-male fingerlings have been tried, including interspecific hybridization, hormonal masculinization, and the use of YY-supermale broodstock. Each method has disadvantages that could be overcome with a better understanding of the genetic basis of sex determination in tilapia. The lack of sex-linked markers has been a major impediment in research and development of efficient monosex populations for tilapia culture. Major conclusions, solutions, achievements. We identified DNA markers linked to sex determining genes in six closely related species of tilapiine fishes. The mode of sex determination differed among species. In Oreochromis karongaeand Tilapia mariaethe sex-determining locus is on linkage group (LG) 3 and the female is heterogametic (WZ-ZZ system). In O. niloticusand T. zilliithe sex-determining locus is on LG1 and the male is heterogametic (XX-XY system). We have nearly identified the series of BAC clones that completely span the region. A more complex pattern was observed in O. aureus and O. mossambicus, in which markers on both LG1 and LG3 were associated with sex. We found evidence for sex-linked lethal effects on LG1, as well as interactions between loci in the two linkage groups. Comparison of genetic and physical maps demonstrated a broad region of recombination suppression harboring the sex-determining locus on LG3. We also mapped 29 genes that are considered putative regulators of sex determination. Amhand Dmrta2 mapped to separate QTL for sex determination on LG23. The other 27 genes mapped to various linkage groups, but none of them mapped to QTL for sex determination, so they were excluded as candidates for sex determination in these tilapia species. Implications, both scientific and agricultural. Phylogenetic analysis suggests that at least two transitions in the mode of sex determination have occurred in the evolution of tilapia species. This variation makes tilapias an excellent model system for studying the evolution of sex chromosomes in vertebrates. The genetic markers we have identified on LG1 in O. niloticusaccurately diagnose the phenotypic sex and are being used to develop monosex populations of tilapia, and eliminate the tedious steps of progeny testing to verify the genetic sex of broodstock animals.