Strain differences in cultured fish — Isozymes and performance traits as indicators

Abstract

A comparison of genetic changes in broodstocks or strains of five cultured fish species, namely, Japanese char, Nile tilapia, Mossambica tilapia, Japanese common carp, and the guppy, showed them to be influenced by the intensity of selection pressure and/or by the history of culture. Allele distributions at polymorphic loci were mainly governed by genetic drift and also by introgressive hybridization or gene exchange. Such genetic changes are expected to occur also at loci controlling performance traits. Using the guppy, Poecilia reticulata, as a model fish, 10 different strains consciously selected for body colour patterns, body size, and finnage, and which were genetically differentiated into six race level groups, were investigated for strain differences in eight growth-related and reproductive traits. Strain differences were significant for all traits and phenotypic differences, expressed as [d]m='P1 - P2||(P1 + P2), were significant in at least one out of eight traits for every strain pair. Of the eight quantitative traits, survival at Day 60, fertility ratio, and brood size showed higher variation, as seen from their coefficients of variation and [d]m range. The correlation of overall [d]m between strains and their corresponding Nei's genetic distance was low (r = −0.104) but individual trait correlations with Nei's D were significant for survival at Day 0 and male body length at Day 60. Results suggest that strain differences are characterized by a random segregation of alleles both at biochemical and trait loci but that occasionally, linkage may occur. The utility of isozyme markers and [d]m the index are discussed.