Short-term temporal instability in fine-scale genetic structure of masu salmon

Abstract

Identifying spatial extent and temporal patterns of population genetic structure is important for understanding evolutionary relationships among local populations and for designing conservation programmes. However, little attention has been devoted to understand whether genetic structure is temporally stable or unstable, and how genetic characteristics may change over time. The goal of this study was to assess the temporal stability of the genetic structure of masu salmon (Oncorhynchus masou). To examine the temporal component of genetic variation in genetic structuring within a river system, we used 810 masu salmon collected from eight tributaries of the Atsuta River, Hokkaido, Japan, over three consecutive years (i.e. 2007–09). We assessed the temporal stability of the genetic structure in 15 microsatellite loci by comparing genetic diversity, effective population size and the relationships between genetic and geographical distances among samples collected from different tributaries and over time. Significant genetic differences were observed among most of the temporal samples collected from the same tributary, and analysis of molecular variance indicated that the temporal component explained about 10 times more of the genetic variation (1.98%) than did the spatial component (0.18%). Isolation by distance (IBD) was detected only in 2009. In addition, a significant correlation in population genetic structure was observed between samples collected in 2008 and those in 2009. In 2007, decomposed pairwise regression analysis identified one population as an outlier population and, following exclusion of the outlier sample, IBD was observed in 2007. This population is small and physically isolated by waterfall, in which the effective population size or effective number of breeders would be expected to be small. We found that fine-scale genetic structure in masu salmon populations was temporally unstable, due perhaps to genetic drift in small isolated populations, temporal changes in environmental conditions (e.g. drought), and a combination of these factors. This study also highlights that the genetic structure among populations could easily be affected by temporal and/or sampling issues. Studies focused on temporal genetic structuring are likely to be important for conservation of species inhabiting unstable or isolated habitats such as freshwater fish.