Fitness maps to a large-effect locus in introduced stickleback populations

A QTL map of the number of surviving offspring per F2 female detected a single, large-effect locus near Ectodysplasin (Eda), a gene having an ancient freshwater allele causing reduced bony armor and other changes. F2 females homozygous for the freshwater allele had twice the number of surviving offspring as homozygotes for the marine allele, producing a large selection coefficient, s = 0.50 ± 0.09 SE. Correspondingly, the frequency of the freshwater allele increased from 0.50 in F2 mothers to 0.58 in surviving offspring.

We compare these results to observed allele frequency changes at the Eda gene in an Alaskan lake population colonized by marine stickleback in the 1980’s. The frequency of the freshwater Eda allele rose steadily over multiple generations and reached 95% within 20 years, yielding a similar estimate of selection, s = 0.49 ± 0.05. These findings are consistent with other studies suggesting strong selection on this gene (and/or linked genes) in fresh water.

Selection on ancient genetic variants carried by colonizing ancestors is likely to increase the prevalence of large-effect fitness variants in adaptive evolution.