Reproduction Control

Introduction

Broodstock and seed supply has been identified as a major constraint in the PD/A CRSP Continuation Plan 1996-2001, resulting in reproductive control becoming one of the CRSP research priorities. Much of the CRSP research effort has been focused on tilapia for which management of unwanted reproduction is an essential part of most culture systems. The objectives identified in this work plan include a series of studies which address this issue from various perspectives.

Steroid-induced sex reversal to obtain monosex populations is now a standard practice in aquaculture and has essentially supplanted hybridization, hand sexing, or predator control. Direct induction of monosex populations through steroid treatment has evolved from techniques developed in the 1970's which apply androgen through treated feed. This is a practical and effective method, but the following issues suggest that alternatives are needed. This technique results in the production of steroid wastes and metabolites that are potential environmental contaminants. Further, tissue clearance of fed steroids during post-treatment stocking in ponds might be another source of contamination (Goudie et al. 1986; Rothbard et al. 1990). One of the proposed studies will examine the potential for residuals in the pond environment. Alternative treatment modalities might reduce the total amount of steroid used in direct induction. Immersion treatment has been effective in salmonids and has reduced exposure time and concentration of steroid to much lower levels compared to oral treatment. Immersion treatment of other species such as cyprinids and cichlids is less developed and needs further research. However, pilot studies demonstrated that short-term immersion of tilapia resulted in successful masculinization. The studies proposed in this work plan aim to follow up on the initial results and develop guidelines for application under hatchery conditions.

Another alternative to reproductive control for tilapias is more complex and involves both a direct induction phase to develop a unique broodstock and a breeding phase to produce seed stock for grow-out. This approach tries to utilize androgenesis for broodstock production. Development of a basic protocol utilizing androgenesis is critical for the success of this alternative, but is only one of many issues that needs to be addressed, as the success of this approach depends on the genetic basis of sex determination in tilapias.

The major assumption, namely that sex determination is based on sex chromosome induction, is in fact not well established. Shelton et al. (1983) demonstrated in studies with the Ivory Coast strain of Oreochromis niloticus, that the expected sex ratios from pair spawning were not simple. In 71 progeny groups from single-pair spawned broodstock, 55% of each progeny group were males (when averaged over all groups), which essentially conforms to the expected 1:1 Mendelian ratio. However, the proportion of males varied from 31 to 83%, and 21% of the progeny groups were not within the statistical limits of a 1:1 sex ratio. Wohlfarth and Wedekind (1991) characterized this distribution of progeny sex as typical of a continuously variable trait, although sex ratio was shown to be a stable trait that responded to selection (Lester et al. 1989). Wohlfarth and Wedekind (1991) propose that the mechanism of sex determination in tilapias is based on factors which are concentrated on a single pair of chromosomes (sex chromosomes), but that crossing over occurs, and that additional genetic determinants of sex are situated on other chromosomes (autosomes) as well. Segregation of sex chromosomes results in Mendelian sex segregation in the absence of variation of autosomal sex determinants. The latter issue is related to identification of tilapia strains, which when bred, will yield progeny sex ratios more uniformly similar to the expected 1:1 (Hulata et al. 1981). To obtain information of this nature, studies will be conducted (as part of the regional research) to test various strains for uniformity and predictability of progeny sex ratio.

Assuming sufficient stability of sex determination in tilapia (at least in some select strain), the objective of producing unique broodstock through androgenesis can be addressed. Breeding for all-male progeny of O. aureus. was proposed by Jensen and Shelton (1979) and tested by Hopkins et al. (1979). This technique was modified and further developed for O. niloticus by Scott et al. (1989). Currently, male O. niloticus , which are assumed to be YY, are being tested by researchers in the Philippines. These males were produced by using gynogenetic induction and subsequent steroid-induced sex reversal. In this method steroid treatment is restricted to broodstock development. Thus, the concerns for environmental effects and human consumption of steroid treated fish are reduced or eliminated, respectively. The approach used in the Philippines has two drawbacks, namely, it is time consuming and uses steroids. Both drawbacks can be avoided in a technique which utilizes androgenesis, because chromosome manipulation should directly produce a unique broodstock. Oreochromis niloticus progeny from androgenetic induction are expected to include both males and females in a 1:1 sex ratio, but all males should be YY. Thus, progeny testing for identification of YY males will not be required. In the next step, the YY males will be bred to XX females which will produce only exogenous-steroid free males (XY).

References

Goudie, C. A., W. L. Shelton, and N. C. Parker. 1986. Tissue distribution and elimination of radio-labeled methyltestosterone fed to sexually undifferentiated blue tilapia. Aquaculture 58:215-226.

Hopkins, K. D., W. L. Shelton, and C. R. Engle. 1979. Estrogen sex-reversal of Tilapia aurea. Aquaculture 18:263-268.

Hulata, G. I., S. Rothbard, and G. W. Wohlfarth. 1981. Genetic approach to the production of all-male progeny of tilapia. European Mariculture Society, Special Publication 6:181-190.

Jensen, G. L., and W. L. Shelton. 1979. Effects of estrogens on Tilapia aurea: implications for production of monosex genetic male tilapia. Aquaculture 16:233-242.

Lester, L. J., K. S. Lawson, T. A. Abella, and M. S. Palada. 1989. Estimated heritability of sex ratio and sexual dimorphism in tilapia. Aquaculture and Fisheries Management 20:369-380.

Rothbard, S., Y. Zohar, N. Zmora, B. Levavi-Sivan, B. Moav, and Z. Yaron. 1990. Clearance of 17a-ethynyltestosterone from muscle of sex-inverted tilapia hybrids treated for growth enhancement with two doses of the androgen. Aquaculture 89:365-376.

Scott, A. G., D. J. Penman, J. A. Beardmore, and D. O. F. Skibinski. 1989. The 'YY' supermale in Oreochromis niloticus (L) and its potential in aquaculture. Aquaculture 78:237-251.

Shelton, W. L., F. H. Meriwether, K. J. Semmons, and W. E. Calhoun. 1983. Progeny sex ratios from intraspecific pair spawning of Tilapia aurea and Tilapia nilotica. Pages In: L. Fishelson and Z. Yaron (Editors). International symposium on tilapia in aquaculture. Tel Aviv University, Israel:270-280.

Wohlfarth, G. W., and H. Wedekind. 1991. The heredity of sex determination in tilapias. Aquaculture 92:143-156.

Invesitigation Sets

Monosex Tilapia Production through Androgenesis
Steroid Immersion for Masculinization of Tilapia
Detection of Masculinizing Agents in the Pond Environment