Natural Selection at the Edge of Life: Allelic Polymorphism and Recruitment in High Latitude Arctic Char (Salvelinus alpinus) Generated and Maintained by Environmental Extremes

Abstract

Information from extreme habitats of polymorphic populations is expected to answer questions related to evolutionary changes occurring at their niche border. Landlocked and resident/anadromous populations of the Arctic char (Salvelinus alpinus (L.) sp. complex), sampled during three successive years in northern Svalbard, were assessed for life-history characteristics, allele-frequency variation at the polymorphic EST-2* locus, and tested for genotype–environment interactions. While year-class strength correlated positively with mean air temperature during the two summers preceding spawning, the EST-2*100 allele frequency correlated positively with the air temperature in June after birth. By affecting two asynchronous and independent population variables, which are both no doubt correlated with fitness, annual temperature variation is suggested to generate and maintain polymorphism in Arctic char in the High Arctic by modifying year-class strength and selecting for variant alleles influencing cold resistance. Intra- and inter-population comparisons imply an additional selection between the two variant EST-2* alleles to operate ontogenetically, with a fixation on the 90 allele in landlocked and resident individuals and the 100 allele in anadromous individuals. The selective mechanism behind the latter processes is unknown. Because of low substrate specificity, however, esterases may form a reserve of adaptive ability towards environmental stress during contrasting conditions.