Mitochondrial DNA and allozyme variation in Ontario cultured rainbow trout spawning in different seasons

Abstract

Rainbow trout ( Oncorhynchus mykiss) from 7 Ontario trout farms as well as fish collected from the Ganaraska River and a descendant stock maintained at Normandale Fish Culture Station, Ontario Ministry of Natural Resources, were analyzed for restriction fragment length polymorphisms in mitochondrial DNA (mtDNA) and variation at protein coding loci. Our objective was to determine (1) the genetic similarity of fish from different farms and (2) if the progeny of fish spawning in different seasons are genetically different. The 340 rainbow trout sampled for mtDNA variation could be categorized into 27 haplotypes. The aquaculture stocks had approximately one-half the mtDNA haplotype variation of that of wild and descendant Ganaraska rainbow trout. Polymorphism was detected at 15 protein coding loci in the sample of 670 trout analyzed. The aquaculture stocks did not have reduced enzyme heterozygosity relative to the naturalized population. Fish from different farms showed varying degrees of genetic similarity based on both mtDNA and allozyme variation. Highly significant genetic differences were detected among fish from the same farm but spawning at different times of the year. The mtDNA analysis grouped fish more according to spawning season than to farm of origin whereas the allozyme analysis did not. This suggests that females spawning in a particular season are derived from a limited number of matriarchal lineages which differ from those in other seasons. The lack of a similar pattern in allozyme variation can be attributed to gene flow, primarily male-mediated, among fish spawning in different seasons.