Molecular Mechanisms of Sex Determination in Cultured Tilapias

Tilapias are among the most important aquaculture commodities worldwide. Commercial production of tilapia is based on monosex culture of males. Current methods for producing all-male fingerlings, including hormone treatments and genetic manipulations, are not entirely reliable, in part because of the genetic complexity of sex determination and sexual differentiation in tilapias. The goals of this project are to map QTL and identify genes regulating sex determination in commonly cultured tilapia species, in order to provide a rational basis for designing reliable genetic approaches for producing all-male fingerlings. The original objectives for this research were: 1) to identify the gene underlying the QTL on LG1 through positional cloning and gene expression analysis; 2) to fine map the QTL on LG 3 and 23; and 3) to characterize the patterns of dominance and epistasis among QTL alleles influencing sex determination. The brain aromatase gene Cyp19b, a possible candidate for the genetic or environmental SD, was mapped to LG7 using our F2 mapping population. This region has not been identified before as affecting SD in tilapias. The QTL affecting SD on LG 1 and 23 have been fine-mapped down to 1 and 4 cM, respectively, but the key regulators for SD have not been found yet. Nevertheless, a very strong association with gender was found on LG23 for marker UNH898. Allele 276 was found almost exclusively in males, and we hypothesized that this allele is a male-associated allele (MAA). Mating of males homozygous for MAA with normal females is underway for production of all-male populations. The first progeny reaching size allowing accurate sexing had 43 males and no females. During the course of the project it became apparent that in order to achieve those objectives there is a need to develop genomic infrastructures that were lacking. Efforts have been devoted to the development of genomic resources: a database consisting of nearly 117k ESTs representing 16 tissues from tilapia were obtained; a web tool based on the RepeatMasker software was designed to assist tilapia genomics; collaboration has been established with a sequencing company to sequence the tilapia genome; steps have been taken toward constructing a microarray to enable comparative analysis of the entire transcriptome that is required in order to detect genes that are differentially expressed between genders in early developmental stages. Genomic resources developed will be invaluable for studies of cichlid physiology, evolution and development, and will hopefully lead to identification of the key regulators of SD. Thus, they will have both scientific and agricultural implications in the coming years.