REPRODUCTION CONTROL
Introduction
Reproduction Control remains one of the PD/A CRSP research priorities as part of
the PD/A CRSP Continuation Plan 1996-2001, which identified broodstock and seed supply
as a major constraint to the development of aquaculture.
Reproduction control of tilapia—the species of focus for much of the CRSP research effort—is
critical to the success of most forms of production because tilapia have such a high reproductive
capacity. Tilapia are capable of reproducing at a small size, of breeding multiple times per year,
and of caring for their young—all of these characteristics may lead to the production of
too many young and/or undersize fish for market. To prevent tilapia from diverting energy
required for growth to that required for reproduction, various management practices have
been used, including the production of populations of fish that contain only one sex.
Culture of such monosex populations of tilapia produced by treatment with steroids early in
development has become common practice throughout the world. For the Ninth Work Plan, CRSP
research will focus on measuring the potential limitations of this practice and on developing
alternative methods for producing monosex populations of tilapia.
Feeding 17a-methyltestosterone (MT)
to developing tilapia fry is an effective means of producing monosex
populations (Green et al., 1997); nevertheless, alternative methods require
investigation because of the concerns raised about production of steroid wastes
and metabolites that are potential environmental contaminants. While Ninth Work Plan
Effluents and Pollution studies examine the effects of MT-feeding on pond soils,
alternatives to feeding with MT are also being examined. Other methods for
masculinization of tilapia such as immersion in steroid-containing solutions
will be examined after promising results from previous CRSP research (Gale et al., 1995;
Contreras-Sanchez et al., 1997). Development of techniques for masculinization through
immersion may provide aquaculturists with a safe and cost-effective alternative to
treating fry with food that contains MT.
Another approach to producing monosex populations of tilapia involves
the use of chromosome manipulation techniques (Thorgaard and Allen, 1986).
In the Eighth Work Plan, studies were initiated to develop methods for inducing
androgenesis in tilapia. Such animals have only paternal genetic material because
the maternal genome has been destroyed by ultraviolet radiation. By having only
paternal genetic material, androgenetic tilapia should have two possible sex chromosome
configurations: XX (female) or YY (male). These latter animals should theoretically sire
only male offspring when mated to normal females, and thus would be valuable for use in
producing monosex male populations for culture without treatment with steroid. In the
Ninth Work Plan, studies are proposed to continue development of androgenetic
techniques—namely, to use the irradiation protocol developed in the Eighth Work
Plan in experiments designed to establish subsequent treatment conditions that will return
the fish to the normal diploid complement of chromosomes and thereby optimize survival.
The successful development of androgenetic techniques for production
of monosex populations of tilapia relies on the assumption that sex
determination in this species is based on inheritance of sex chromosomes.
However, Eighth Work Plan studies confirmed and extended earlier work
(Shelton et al., 1983) that the sex ratios of Nile tilapia populations can
deviate significantly from the expected 1:1 Mendelian ratio, suggesting that
sex determination in Nile tilapia may involve genetic determinants on sex chromosomes
and other chromosomes (autosomes). Crosses of tilapia that result in sex ratios
deviating severely from 1:1 males to females may indicate one or both parents with
excessive autosomal influence on sex determination. Such autosomal influence may
explain how some YY males sire populations with sex ratios well below 100% male (Mair et
al., 1995). Perhaps only YY males bred with females that have minimal autosomal influence
on sex determination will result in 100% male offspring. Therefore, in studies proposed for
the Ninth Work Plan, females without such excessive autosomal influence will be identified
by progeny testing of repeated crosses. Those females that always produce 1:1 sex ratios
will then be used in crosses with androgenetically-produced YY males. In addition, the
performance of all-male progeny of such crosses will be compared side-by-side with that
of MT-masculinized males at a variety of CRSP sites.
References
Contreras Sanchez, W.M., M.S. Fitzpatrick, R.H. Milston, and C.B. Schreck, 1997.
Masculinization of Nile tilapia (Oreochromis niloticus) by single immersion
in 17a-methyldihydrotestosterone and trenbolone acetate.
In: K. Fitzsimmons, (Ed.), Proceedings from the Fourth International Symposium on
Tilapia in Aquaculture. Northeast Regional Agricultural Engineering Service, Ithaca, NY,
pp. 783-790.
Gale, W.L., M.S. Fitzpatrick, and C.B. Schreck, 1995. Immersion of Nile tilapia (Oreochromis
niloticus) in 17a-methyltestosterone and mestanolone
for the production of all-male populations. In: F.W. Goetz and P. Thomas, (Eds.), Proceedings of
the Fifth International Symposium on Reproductive Physiology of Fish, Fish Symposium 95,
Austin, TX, p. 117.
Green, B.W., K.L. Veverica, and M.S. Fitzpatrick, 1997. Fry and fingerling production.
In: H. Egna and C. Boyd, (Eds.), Dynamics of Pond Aquaculture. CRC Press,
Boca Raton, FL, pp. 215-243.
Mair, G.C., J.S. Abucay, J.A. Beardmore, and O.F. Skibinski, 1995. Growth
performance trials of genetically male tilapia (GMT) derived from YY-males in
Orechromis niloticus L.: On station comparisons with mixed sex and sex
reversed male populations. Aquaculture, 137: 313-322.
Shelton, W.L., F.H. Meriwether, K.J. Semens, and W.E. Calhoun, 1983. Progeny
sex ratios from interspecific pair spawns of Tilapia aurea and T. nilotica.
In: L. Fishelson and Z. Yaron, (Eds.), International Symposium on Tilapia in
Aquaculture. Tel Aviv University, Tel Aviv, Israel, pp. 270-280.
Thorgaard, G.H. and S.K. Allen, 1986. Chromosome manipulation and markers
in fishery management. In: N. Ryman and F. Utter, (Eds.), Population Genetics
and Fishery Management. Washington Sea Grant Program, University of Washington
Press, Seattle, WA, pp. 319-331.
