A search for null alleles at the microsatellite locus of chum salmon (Oncorhynchus keta Walbaum)

Population studies with the use of microsatellite markers face a problem of null alleles, i.e., the absence of a PCR product, caused by the mutations in the microsatellite flanking regions, which serve as the sites of primer hybridization. In this case, the microsatellite primer associated with such mutation is not amplified, leading to false homozygosity in heterozygous individuals. This, in turn, results in biased population genetic estimates, including the excess of homozygotes at microsatellite loci. Analysis of the population structure of a Pacific salmon species, chum salmon (Oncorhynchus keta Walbaum), revealed the presence of null alleles at the Oke3 microsatellite locus in the population samples, in which an excess of homozygotes was observed. The analysis was performed using different combinations of modified primers chosen to match the Oke3 locus. The use of these primers enabled identification of true heterozygotes among those individuals, which were previously diagnosed as homozygotes with the use of standard primers. Removal of null alleles eliminated the excess homozygotes in the chum salmon samples described. In addition to the exclusion of false homozygosity, the use of modified primers makes it possible to introduce polymorphic primer variants associated with certain microsatellite alleles into population studies.