A Robertsonian polymorphism in pink salmon (Oncorhynchus gorbuscha) involving the nucleolar organizer region

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Abstract
A chromosome polymorphism involving a Robertsonian rearrangement has been found for two consecutive years in pink salmon from Lake Superior. Although the majority of the fish have a diploid number of 52, as previously reported for Oncorhynchus gorbuscha, 10–15% of the population have 2n = 53, in which one of the metacentric chromosomes has apparently undergone fission. The chromosome pair involved is the pair which contains the nucleolar organizer region (NOR) which is normally found adjacent to the centromere. In all of the individuals with 53 chromosomes, there is a small, acrocentric chromosome representing the short arm of the normal metacentric and a slightly larger, submetacentric chromosome with the NOR on its short arms. This chromosome apparently represents the long arm of the original metacentric in which the NOR has been rearranged to the other side of the centromere. Analysis of artificially induced haploid embryos as well as normal diploid embryos from females with 53 chromosomes suggests that meiotic chromosomes undergo alternate disjunction so that only balanced gametes are produced.