225 APPLICATION OF GENETIC TECHNIQUES FOR THE PRODUCTION OF MONOSEX MALE TILAPIA IN AQUACULTURE: EARLY EXPERIENCES FROM THE PHILIPPINES G.C. Mair*, L.R. Dahilig, E.J. Morales, J.A. Beardmore and D.O.F. Skibinski School of Biological Sciences, University of Wales Swansea, Swansea SA2 8PP, Wales, U.K. Cite as: Mair, G.C., Dahilig, L.R., Morales, E.J., Beardmore, J.A. and Skibinski, D.O.F. (1997) Application of genetic techniques for the production of monosex male tilapia in aquaculture: Early experiences from the Philippines. Proceedings of the Fourth Central America Symposium on Aquaculture, Tegucigalpa, Honduras, April 22-24, 1997. 225-227. Introduction The desirability of monosex populations for tilapia culture is well established. Grow-out of monosex male populations prevents or minimizes recruitment and thereby competition between recruits and stocked fish which, in mixed sex populations, can significantly reduce harvested yields. Energy expenditure on male -male and male -female behavioural interactions and on gamete production is also minimized, thereby maximizing growth potential. Several approaches have been developed to achieve monosex male populations, direct hormonal sex reversal being the most commonly applied in the industry today, although monosex hybrids and even manual sexing, are also produced in many hatcheries. All available methods have significant disadvantages as summarised by Mair and Little (1991). Manual sexing is labour intensive and susceptible to errors such that sex ratios greater than 90% male are rarely achieved. Hybridisation can produce consistently high percentages of males, especially if Oreochromis urolepis hornorum is used as the paternal parent. However, for most freshwater aquaculture, O. niloticus is usually the species of choice due to its high growth potential, and any dilution through hybridization usually results in loss of performance which is only partially compensated by the enhanced growth in monosex populations. Sex reversal, if properly applied, can be effective in producing sex ratios up to 98% male. However, inconsistencies in application often result in lower sex ratios and environmental and human health concerns have been raised about the direct use of synthetic hormones in aquaculture. Genetic manipulation of sex and the YY male technology Recent research has concentrated on genetic manipulation of sex determining mechanisms as a means of producing monosex populations. Research studies in tilapia have demonstrated that sex determination is predominantly monofactorial, similar to that in humans. In the two important cultured species O. niloticus and O. mossambicus the female has the homogametic genotype XX, and the male is heterogametic XY. It has been demonstrated that a breeding programme combining hormonal feminization and progeny testing, can result in the production of novel YY male genotypes, which sire all- or nearly all-male progeny (Varadaraj and Pandian, 1989 and Mair et al. in press). The most recent and comprehensive study in O. niloticus demonstrated that YY males can be mass produced in crosses of YY males with feminized YY female s (Mair et al., in press). These YY males can then be used as broodstock for the mass production of male progeny known as genetically male tilapia (GMT). The performance of GMT O. niloticus has been compared, in on-station pond trials in the Philippines, where this technology has been developed, with that of normal mixed sex tilapia (MST), and hormonally sex reversed males (SRT). The results (Mair et al., 1995) showed that GMT produced significantly higher yields than both MST (58.8%) and SRT (31.0%). This paper reports the results from on-farm evaluation of GMT in a range of Philippine culture systems and discusses the progress of initial attempts to disseminate the technology and its products in the Philippines