Freshwater Colonization, Adaptation, and Genomic Divergence in Threespine Stickleback

Area of Science:

• Evolutionary biology
• Genetics
• Ecology

Background:

• Threespine stickleback (Gasterosteus aculeatus) are ancestrally marine but many populations adapt to freshwater.
• Repeated freshwater adaptation over millions of years makes them a key model for studying rapid evolutionary processes.
• Hybridization between anadromous and freshwater stickleback introduces adaptive alleles into marine populations.

Purpose of the Study:

• To investigate the speed and repeatability of adaptation in Threespine stickleback colonizing freshwater habitats.
• To understand the genomic mechanisms underlying rapid adaptation, including allele frequency changes and haplotype structures.
• To identify the role of selection and recombination in assembling adaptive genetic variation.

Main Methods:

• Analysis of whole-lake experiments in Alaska.
• Tracking allele frequency changes of freshwater-adaptive alleles over generations.
• Examining genomic and phenotypic evolution in response to freshwater colonization.

Main Results:

• Significant and rapid increases in the frequency of freshwater-adaptive alleles (from ~1% to ≥50% in ten generations) were observed.
• Freshwater phenotypes evolved in parallel with genetic changes.
• Intense directional selection on heterozygotes and suppressed recombination in adaptive regions were inferred.

Conclusions:

• Threespine stickleback exhibit astonishingly rapid and repeatable adaptation to freshwater.
• Genomic and phenotypic evolution is driven by intense selection and facilitated by recombination dynamics.
• This system provides broad insights into the mechanisms of rapid adaptation.