Selection of Individuals for Sex Inheritance Characteristics for Use in Monosex Production 9RCR6

Collaborating Institution

Auburn University

Ron Phelps

Objective
To develop lines of Nile tilapia, Oreochromis niloticus, that will breed true for a given sex ratio.

Significance
There is evidence that in domestic stocks of O. niloticus, sex ratios of individual pairs may vary from 50:50. Shelton et al. (1983) found that 21% of the pair spawns were outside a 95% confidence interval for an expected 50:50 ratio. Karsina (1993) found that 32% of 22 pair spawns did not conform to an expected 50:50 ratio. Lester et al. (1989) found sex ratios from 60 families to range from 2 to 60% male. Abucay (1997) reported sex ratios ranging from approximately 30 to 70% males from presumed XX females x XY males. Baroiller et al. (1993) mated hormone-induced “XX” male O. niloticus to normal XX females. The resultant groups of progeny should have been all female, but for fry held at 27 to 29°C they found that 26% of the spawns contained > 10% males and when at 32 to 36°C, 92% of the spawns had > 10% males.

This variability in sex ratios is a challenge in the development of YY breeding programs. Mair et al. (1995) reported that YY males gave a mean of 96.5% male offspring but with a range of 67 to 100%. In the culture of large tilapia, populations of > 95% males are desirable and any variability less than that can impact the quantity of marketable fish. Lovshin et al. (1990) found that even 5% females in a population reduce adult production by almost half after nine months of culture. For YY production of males to be successful, variability in the percent males produced must be reduced and parent lines that conform to a simple Mendelian inheritance established.

It may be possible to select for individuals which consistently produce progeny with a 50:50 sex ratio. Baroiller et al. (1993) found that some pairs of O. niloticus gave progeny whose sex ratios were influenced by temperature while other pairs were not. Mair et al. (1990) also found some individual pairs which gave progeny that were not influenced by temperature. Wohlfarth and Wedekind (1991) concluded that sex ratio is a heritable trait and responds to selection. They reported that progeny from individuals giving a skewed sex ratio would also produce a skewed ratio while those from fish giving a 50:50 ratio have ratios similar to their parents. Repeated spawns of a single pair have been consistent in the sex ratios of the progeny (Shelton et al., 1983; Wohlfarth and Wedekind, 1991). It is then possible to select for lines of O. niloticus that will give consistent sex ratios near 50:50. The procedure of androgenesis should eliminate any cytoplasmic effects on sex ratios and when the procedure is applied to fish that give consistent sex ratios then the possibility of all-male seed production is greatly enhanced. The combination of androgenically produced YY males from true breeding parent stock crossed with true breeding females should give the consistent results needed for YY male technology to become widely adopted. But before YY male technology can be advanced, true breeding lines of tilapia based on the results of individual pair spawns must be established.

Experimental Design
Site: Auburn University in collaboration with the University of Oklahoma

Study Period: July 1, 1998 to June 30, 1999

Facilities: A minimum of 30 aquaria and 35 hapas would be utilized at Auburn University for the proposed activity.

Plan of Work: Selected males and females from two or more strains, which the sex ratios of the progeny are know, will be mated in an attempt to obtain target sex ratios. These fish will have been identified as part of the Eighth Work Plan. Individuals which have given progeny conforming to 50:50 ratio will be mated and the sex ratio of the progeny determined. Attempts will be made to obtain a minimum of three spawns from each pair. Half of the progeny from each spawn will be held at 27 to 30°C and half at 36°C for 30 days during the period of gonadal differentiation to determine if the sex ratio from a given pair spawn is susceptible to environmental influence.

Progeny of selected spawns will be reared to maturity and mated to other families of similar sex ratio history and the sex ratio of the progeny determined. Families which give repeated spawns not differing from a 50:50 sex ratio and are not susceptible to temperature effects on sex ratio will be held for mating with YY males.

Statistical Design: Null Hypothesis: Sex ratio is not an inheritable characteristic. Sex ratio data will be analyzed by Chi square. The effects of temperature on sex ratio be analyzed by ANOVA.

Regional IntegrationAll regional programs except one are using monosex tilapia as a basic part of their program and would benefit from the knowledge gained from the proposed work. This study is a precursor to a direct collaboration with three of the country programs. It is linked to the Kenyan program in the establishment of sex ratios of selected strains of O. niloticus.

Schedule
The study period is 1 May 1999 to 30 April 2001. Field activities will take place from 7/1/99 to 10/1/99 and from 7/1/00 to 10/1/00. Sex ratios will be determined and data analyzed from November 1999 to March 2000 and from November 2000 to March 2001. A progress report will be submitted by 4/30/00, and a final report by 4/30/01.

References
Abucay, J.S., 1997. The YY male technology and its products. In: G.C. Mair and T.A. Abella (Editors), Tecnoguide on the Genetically Male Tilapia (GMA). Freshwater Aquaculture Center, Central Luzon State University, Nueva Ecija, Philippines, pp. 14-26.
Baroiller, J.F., A. Fostier, C. Cauty, X. Rognon, and B. Jalabert, 1993. Effects of high rearing temperatures on the sex ratio of progeny from sex-reversed males of Oreochromis niloticus. In: R.S.V. Pullin,
J. Lazard, M. Legendre, J.B. Amon Kothins and D. Pauly (Editors), The Third International Symposium on Tilapia in Aquaculture. ICLARM Conf. Proc. 41.
Karsina, E.A. 1993. Production of YY “supermale” Oreochromis niloticus (Linnaeus) using hormonally sex reversed XY females and an attempt to distinguish between XY and XX females using discriminant analysis. M.S. Thesis, Auburn University, AL, 59 pp.
Lester, L.J., K.S. Lawson, T.A. Abella, and M.S. Palada, 1989. Estimated heritability of sex ratio and sexual dimorphism in tilapia. Aquacult. Fish. Manage., 20:369-380.
Lovshin, L.L., A.B. Da Silva, A. Carneiro-Sobrinho, and F.R. Melo, 1990. Effects of Oreochromis niloticus females on the growth and yield of male hybrids (O. niloticus female x O. hornorum male) cultured in earthen ponds. Aquaculture, 88:55-60.
Mair, G.C., J.S. Abucay, J.A. Beardmore, and D.O.F. Skibinski, 1995. Growth performance trials of genetically male tilapia (GMT) derived from YY-males in Oreochromis niloticus L.; On station comparisons with mixed-sex and sex-reversed male populations. Aquaculture, 137:313-322.
Mair, G.C., J.A. Beardmore, and D.O.F. Skibinski, 1990. Experimental evidence for environmental sex determination in Oreochromis species. In: R. Hirano and I. Hanyu (Editors), The Second Asian Fisheries Forum. Asian Fisheries Society, Manila, Philippines, pp. 555-558.
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 (Compilers), International Symposium on Tilapia in Aquaculture. Tel Aviv University, Tel Aviv, pp. 270-280.
Wohlfarth, G.W. and H. Wedekind, 1991. The heredity of sex determination in tilapia. Aquaculture, 92:143-156.