High frequency of translocation heterozygotes in odd year populations of pink salmon (Oncorhynchus gorbuscha)

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Abstract
The pink salmon (Oncorhynchus gorbuscha) has a rigid two year life cycle so that populations spawning on the even years do not hybridize with populations spawning on the odd years. Examination of the chromosomes of two populations from an even year (1986) and four populations from an odd year (1987) showed that all individuals from the even year populations had a diploid number’of 52, considered the normal number for the species, while a high frequency of individuals in each of the odd year populations sampled from Washington State to Alaska were translocation heterozygotes with a diploid chromosome number of 53. The chromosome involved in the translocation was the seventh metacentric pair containing the NOR (nucleolus organizer region) adjacent to the centromere. In two populations a simple fission of this chromosome has produced individuals with chromosomes with two acrocentrics replacing the metacentric chromosome, with the larger acrocentric having the NOR adjacent to the centromere on the long arm. In the other two populations individuals with 53 and 54 chromosomes were found in which the acrocentric with the NOR has undergone an inversion so that the NOR is now on the short arm of a small submetacentric chromosome. In one population all of the individuals with 53 and chromosomes were of this type, while in the other case both forms were found. Because these two populations are adjacent to each other in the middle of the range sampled, the rearranged chromosome probably had a single origin.