Genetic Basis of Heat Induced Sex Determination in Nile Tilapia (Oreochromis Niloticus L.), and Genetic Variability of the Species Local Populations

Abstract

Oreochromis niloticus commonly known as Nile tilapia is the most cultured tilapine cichlid in the sub-Saharan Africa. Most of the culture practices are of mixed sexes with the inherent problem of overpopulation leading to stunted growth. Mono-sex culture of all-male individuals is now encouraged by sex reversal through heat treatment technology. However the method is still at development stage. The aim of the present study was to determine the genetic basis of heat induced sex-determination in 0. niloticus as an approach towards improving heat treatment technology and to assess the genetic variability of the invasive 0. niloticus for conservation and management within Lake Victoria Nyanza gulf and the Yala swamp. Juvenile 0. niloticus were subjected to heat treatment at temperatures ranging from 28°C to 36°C using 26°C as control. There was a positive correlation between treatment temperatures and resultant male sex ratios (p<0.01). However, the survival rates of the fry showed a negative correlation with the temperature (p<0.01). The optimal temperature that provided for optimal sex reversal towards males and survival rates of the fry was found to be 36± 1°C, which gave a male proportion of 86.31 % and fry survival rate of 65.25%. To study genetic basis of heat induced sex determination three sex linked markers UNH846, Abur36 and AburlOO were selected, genotyped and sequenced. The marker Abur36 identified the sex of 95% of the heat induced individuals. The putative function of this gene indicated that the gene could be a male biased hormone producer or transcriptional factor whose activity is influenced by elevated temperature within a narrow range before the onset of the temperature sensitive gonadal sexual differentiation. Determination of the genetic diversity and purity of 0. niloticus was carried out based on eight microsatellite markers and mitochondria DNA control region and compared with that of 0. esculentus. No admixture between the two species was detected. However there were low levels of nuclear admixture primarily from 0. niloticus to 0. esculentus. Genetic diversity of 0. niloticus (h =0.829, 1t: = 0.008) was higher than that of 0. esculentus (h = 0.816; 1t: = 0.004). All populations of the two species retained unique haplotypes and alleles in their respective lakes. Pairwise FST values for population differentiation of both species were all highly significant (P < 0.001). Therefore the four populations of 0. niloticus are potential sources of broodstock choice from which all-male fingerlings could be produced using heat treatment technology.