Rare "Jackpot" Individuals Drive Rapid Adaptation in Threespine Stickleback.

Area of Science:

• Evolutionary Biology
• Genomics
• Population Genetics

Background:

• Recombination is traditionally viewed as key for adaptation by assembling beneficial alleles.
• The transporter hypothesis suggests recombination drives parallel adaptation in Threespine Stickleback.
• Understanding rapid adaptation mechanisms is crucial for evolutionary studies.

Purpose of the Study:

• To investigate the genomic dynamics of rapid parallel adaptation in Threespine Stickleback during freshwater transition.
• To assess the role of standing genetic variation and recombination in this adaptive event.

Main Methods:

• Whole-genome sequencing of an evolutionary time series.
• Analysis of anadromous (sea-run) founder populations and their freshwater descendants.
• Biological kinship analyses to track allele frequency changes.

Main Results:

• Rapid freshwater adaptation was facilitated by a few "jackpot carriers" with large blocks of adaptive alleles.
• Mating among jackpot carriers accelerated the increase of freshwater-adaptive alleles within a few generations.
• The population overcame a severe bottleneck, with inbreeding potentially reducing genetic load and purging deleterious alleles.
• Recombination played a minimal role in this rapid adaptation scenario.

Conclusions:

• Rare individuals with pre-existing adaptive allele combinations can drive rapid population adaptation.
• This "jackpot" mechanism can be more effective than recombination for swift adaptation, especially after bottlenecks.
• Inbreeding may serve as a purging mechanism for genetic load during population recovery.