Secondary tetrasomic segregation of MDH-B and preferential pairing of homeologues in rainbow trout.

We examined the inheritance of allelic variation at an isozyme locus, MDH-B, duplicated by ancestral polyploidy in salmonid fishes. We detected only disomic segregation in females. Segregation ratios in males were best explained by a mixture of disomic and tetrasomic inheritance. We propose a two-stage model of pairing in male meiosis in which, first, homologous chromosomes pair and recombine in the proximal region of the chromosome. Next, homeologous chromosomes pair and recombine distally. We suggest that this type of tetrasomic inheritance in which centromeres segregate disomically should be referred to as "secondary tetrasomy" to distinguish it from tetrasomy involving entire chromosomes (i.e., "primary tetrasomy"). Differences in segregation ratios between males indicate differences between individuals in the amount of recombination between homeologous chromosomes. We also consider the implication of these results for estimation of allele frequencies at duplicated loci in salmonid populations.