Introduction to Tilapia Sex-Determination and Sex-Reversal
Kevin Fitzsimmons
University of Arizona
One of the basic factors of tilapia aquaculture is that male fish grow bigger and faster than the females. Also, in order to avoid unwanted spawning in a production unit, all-male populations are preferred. There are several methods used to skew sex ratios and increase the percentage of males in a population.
The first method developed was to simply cull through a population, discard the females and keep the males. This system is obviously wasteful and inefficient. In the 1960’s and 70’s, Israeli scientists discovered that certain hybrid crosses resulted in skewed sex ratios favoring males. There are several theories regarding the genetic factors involving the number and location of sex genes on particular chromosomes. The use of hybrid crosses is still one of the primary methods of producing mostly male populations. The drawback to this method is that two separate broodlines must be maintained. The crossing must be done very carefully and meticulous records should be kept to insure that the parent species are kept pure. Also, usually only one sex from each species is used for any particular cross because the reciprocal cross (using the other sex from each species) is not as successful. Another problem is that the number of young produced is rarely as high as a single species spawn. Therefor, to maintain a commercial scale hatchery will usually require significant resources and staff.
The more common method of generating mostly male populations is through the use of steroids fed to sexually undifferentiated fry. Newly hatched tilapia are still developing their gonads. Even though they are determined genotypically their phenotype, or morphological characteristics can still be altered. By exposing the fish to forms of testosterone or estrogen, the gonad can switched. Typically the desire is to produce all males, so methyltestosterone is included in the diet for several weeks when the fish start eating. Other hormones have been tested and sex-reversal can also be achieved by immersion in a solution. The hormones cause the gonads to develop as testes instead of ovaries and the fish will also take on male morphological characteristics. The hormone is only needed during the first month and after that the fish are feed normally for the rest of their lives. Using this technique farms can produce populations of greater than 90% male fish. These populations grow faster than equivalent populations of mixed sex fish and have significantly less reproduction in the growout systems.
A novel variation on this scheme is to feed young fish estrogen. This results in a population of all female fish. The morphological female but genetic male fish are then reared to maturity and then mated to normal male fish. The resulting fry have a male father and a male mother and thus will all be male. These young have never been treated with any hormone. Of course this technique requires several years to develop the stocks and extensive progeny testing to determine which fish produce the all male young. Even more complicated breeding plans have been developed to fix breeding lines of male with two male chromosomes that will always produce male offspring.
Additional research is underway to study the basic physiology of sexual differentiation. How the genes are turned on and off, how the genetic complement from each of the parent fish contribute to the genetic makeup of the young and how environmental and chemical stimuli affect the development of the gonads. By better understanding these phenomenon we hope to develop more efficient methods of directing the sexual development of the fish.
