Gene-centromere mapping of 25 loci in rainbow trout

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Abstract
We have mapped 25 enzyme loci in relation to their centromeres by half-tetrad analysis in gynogenetic progeny from three hatchery strains of rainbow trout (Salmo galrdnerl). The observation of close to 100 percent heterozygous gynogenetic diploids for five unlinked loci suggests that near-complete interference is common in rainbow trout chromosomes. Estimates of gene-centromere recombination rates in triploids are similar to the estimates in gynogenetic diploids. Thus, the near absence of homozygotes at these loci cannot be explained by reduced survival of homozygotes. Furthermore, we found no evidence in 69 comparisons of departure from equality of the two homozygous classes, as would be expected if an allele was linked to a recessive allele that reduced survival. We also found surprisingly little evidence of differences among families within strains or among strains. The average proportion of heterozygous progeny for all loci is 0.56; this corresponds to a fixation index of 0.44 after only one generation. However, the high frequency of loci at which almost all gynogenetic progeny are heterozygous indicates that it is infeasible to produce homozygous lines by gynogenesis involving retention of the second polar body in rainbow trout.