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

Area of Science:

• Evolutionary biology
• Population genetics
• Genomics

Background:

• Most adaptation relies on small-effect mutations, but large-effect beneficial variants can be crucial under specific conditions.
• Estimating the fitness effects of genes driving adaptation is challenging but essential for understanding evolutionary processes.

Purpose of the Study:

• To map fitness quantitatively across a generation in an F2 intercross of marine and freshwater stickleback populations.
• To estimate the fitness effect size of the Ectodysplasin (Eda) gene region during adaptation to a novel freshwater environment.

Main Methods:

• Quantitative trait locus (QTL) mapping was used to identify loci influencing the number of surviving offspring.
• Fitness was measured by tracking allele frequency changes of the freshwater and marine Eda alleles over one generation in an F2 intercross.
• Comparison of Eda allele frequency dynamics in a natural population of stickleback colonizing a new lake.

Main Results:

• A single, large-effect locus near the Ectodysplasin (Eda) gene was identified, significantly impacting offspring survival.
• Females homozygous for the freshwater Eda allele had double the offspring of marine homozygotes, indicating a large selection coefficient (s = 0.50 ± 0.09).
• The freshwater Eda allele frequency increased from 0.50 to 0.58 in one generation, and rose to 95% within 20 years in a recently colonized lake, with a selection coefficient of s = 0.49 ± 0.05.

Conclusions:

• The Ectodysplasin (Eda) gene, or closely linked genes, exerts strong selection during freshwater adaptation in stickleback.
• Ancient genetic variants, carried by colonizing ancestors, can rapidly increase in frequency due to strong selection, contributing significantly to adaptive evolution.