Enhanced genetic differentiation of Japanese chum salmon identified from a meta-analysis of allele frequencies
AbstractThe number of individuals returning to Japan, the location of the world’s largest chum salmon hatchery program, has declined substantially over two decades. To find the genetic cause of this severe decline never previously experienced, we analyzed published genetic data sets for adult chum salmon, namely, 10 microsatellites, 53 single nucleotide polymorphisms (SNPs) and a combined mitochondrial DNA locus (mtDNA3), and three isozymes, from 576 locations in the distribution range (n = 76,363). The SNPs were selected for stock identification to achieve high accuracy, were highly differentiated in the distribution range and included important genes related to reproduction, growth and immune responses. By contrasting the genetic differentiation of these genes with the population structure estimated from the neutral microsatellite markers, we identified genes that distort the neutral population structure. We matched the sampling locations of SNPs and isozymes with those of microsatellites based on geographical information, and performed regression analyses of SNP and isozyme allele frequencies of matched locations on the population structure. TreeMix analysis indicated two admixture events, from Japan/Korea to Russia and the Alaskan Peninsula. Meta-analysis of allele frequencies identified three outliers, mtDNA3 (control region and NADH-3), GnRH373 (gonadotropin-releasing hormone) and U502241 (unknown), which showed enhanced differentiation in Japanese/Korean populations compared with the others. GnRH improves stream odor discrimination and has increased expression in adult chum salmon brains during homing migration, suggesting that the current admixture was caused by GnRH373 differentiation. mtDNA plays a key role in endurance exercise training, energy metabolism and oxygen consumption, suggesting that the significant reduction in mtDNA3 allele frequencies reduced aerobic athletic ability, as observed in YouTube videos. Our analyses relied on limited data sets, though they were the best available. Clearly, genome-wide data will be needed to fully address this issue.