Introgressive Hybridization in Cultured Tilapia Stocks in the Philippines

Abstract

Tilapia populations, assumed by culturists to be Nile tilapia Oreochromis niloticus, were sampled from five farms and one research center around Laguna de Bay, Philippines and their identity were investigated by starch gel electrophoresis of polymorphic and monomorphic isozymes and by isoelectric focusing of sarcoplasmic protein markers. A reference stock of O. mossambicus from natural waters was similarly examined.
Data from morphological characters were inconclusive. However, from 20 biochemical loci investigated, five isozyme and two sarcoplasmic protein loci were found to be reliable species specific markers and from this evidence the introgressive hybridization by O. mossambicus was found in all the assumed Nile tilapia populations. The percentage of pure O. niloticus individuals with no O. mossambicus biochemical markers ranged from 0-40 between populations. Individuals heterozygous for all the biochemical marker loci (assumed to be F₁ hybrids) were rare. The mean percentage frequency of O. mossambicus marker alleles in the six populations was 13.6 (range 8.2 to 17.9). Their genic variabilities, indicated by proportion of polymorphic loci (P=0.30 to 0.40) and mean expected heterozygosity (H^e=0.047 to 0.104), were almost all higher than published values for Egyptian O. niloticus (P=0.18, H^e=0.061). For the Philippine O. mossambicus examined and for wild Egyptian fish, the respective values of P and H^e were 0.10 and 0.021 compared to 0.05 and 0.002. For the six Philippine O. niloticus populations, the ratios of observed to expected heterozygosities (H^o:H^e) were all>1.000 (range 1.021 to 1.103).
The importance of conservation of tilapia genetic resources, the need to ascertain the identity of farmed and experimental populations, the provision of single species and hybrid seed for culturists and future approaches to genetic improvement are discussed relative to these results,