An empirical genetic assessment of the severity of the northern elephant seal population bottleneck

A bottleneck in population size of a species is often correlated with a sha rp reduction in genetic variation, The northern elephant seal (Mirounga ang ustirostris) has undergone at least one extreme bottleneck, having rebounde d from 20-100 individuals a century ago to over 175,000 individuals today. The relative lack of molecular-genetic variation in contemporary population s has been documented, but the extent of variation before the late 19th cen tury remains unknown. We have determined the nucleotide sequence of a 179 b ase-pair segment of the mitochondrial DNA (mtDNA) control region from seals that lived before, during and after a bottleneck low in 1892. A 'primerles s' PCR was used to improve the recovery of information from older samples. Only two mtDNA genotypes were present in all 150+ seals from the 1892 bottl eneck on, but we discovered four genotypes in five pre-bottleneck seals. Th is suggests a much greater amount of mtDNA genotypic variation before this bottleneck, and that the persistence of two genotypes today is a consequenc e of random lineage sampling. We cannot correlate the loss of mtDNA genotyp es with a lowered mean fitness of individuals in the species today. However , we show that the species historically possessed additional genotypes to t hose present now, and that sampling of ancient DNA could elucidate the gene tic consequences of severe reductions in population size.