Overruled by nature: A plastic response to an ecological regime shift disconnects a gene and its trait

Abstract

In Atlantic salmon, age at maturation is a life history trait ruled by a sex-specific trade-off between reproductive success and survival. Following an ecological regime shift in 2005, many North Atlantic salmon populations currently display smaller size at age and delayed age at maturation. However, whether this change reflects rapid evolution or plastic response is unknown. Some 1500 historical and contemporary salmon from river Etne (Western Norway) genotyped at 50k SNPs revealed three loci significantly associated with age at maturation. These included vgll3 and six6, which collectively explained 36 to 50% of the age at maturation variation in the 1983-1984 period. Strikingly, the combined influence of these genes was nearly absent in all samples from 2013-2016, despite allelic frequencies at vgll3 remaining unchanged. We conclude that the regime shift has led to the sudden bypassing of the influence of vgll3 and six6 on maturation through growth-driven plasticity.