Gender control is a major concern for tilapia producers. Males grow faster than females, which lay eggs monthly as of 4 months of age and stop feeding during the mouthbrooding period.
The use of male monosex populations enables optimization of tilapia growth in aquaculture, as well as food conversion. However, these populations are mainly produced by hormonal inversion techniques, which raise many contentious issues with respect to the environment and consumers.
A CIRAD team has developed an environment-friendly approach based on the masculinizing effect of high temperature, and a genetic approach involving the use of YY male breeders of monosex male progeny.
Producing monosex populations
Indirect approaches such as progeny testing are required to select YY male parents of monosex male progeny since sex chromosomes cannot be distinguished in most fish. Analysis of the sex ratio of offspring can reveal the sexual genotype (XX, XY, YY) of the parents.
The only problem is that it is necessary to wait 4–5 months to be able to identify interesting parents by sexing the progeny. All laboratories working on tilapia have thus been analysing the gonads with the aim of finding molecular markers that would enable early, efficient and simple sexing.
A dimorphic sex-related gene in the brains of fish fry
The CIRAD team decided to study the brains of tilapia fry. This led to the discovery of a gene that was previously considered to be typically present in the gonads of all vertebrates.
This gene was found to have major sexually dimorphic expression, occurring much earlier in the central nervous system than in the gonads. Using mixtures of monosex male and monosex female populations, the researchers assembled batches with different known sex ratios (100%, 75%, 50%, 25% and 0% females) and then conducted blind molecular sexing assays on each batch of 14 day old fry.
The remainder of each batch was preserved and sexed at 4 months using the conventional technique. The sex ratios determined at 14 days corresponded 100% to those determined at 4 months.
This is a further step towards achieving gender control in tilapia using consumer- and environment-friendly approaches.