On the causes of geographically heterogeneous parallel evolution in sticklebacks

Area of Science:

• Evolutionary biology
• Population genetics
• Ecology

Background:

• The three-spined stickleback (Gasterosteus aculeatus) is a key model organism for studying wild parallel evolution.
• Previous research highlighted consistent genetic differentiation between freshwater and marine ecotypes, primarily in the Eastern Pacific.
• Limited geographic sampling in prior studies may have overestimated the prevalence of parallel evolution.

Purpose of the Study:

• To investigate the geographic scale of parallel evolution in marine and freshwater three-spined stickleback ecotypes.
• To identify genomic regions involved in adaptation to different environments across the species' global range.
• To understand the factors influencing the prevalence of parallel adaptation.

Main Methods:

• Analysis of population genomic data from global samples of marine and freshwater stickleback ecotypes.
• Detection of genomic loci showing consistent genetic differentiation.
• Simulations and empirical data analysis to explain observed patterns.

Main Results:

• Most parallel evolution signatures were unique to the Eastern Pacific region.
• Trans-oceanic marine-freshwater differentiation was limited to a few shared genomic regions, including three chromosomal inversions.
• Stochastic loss of alleles and selection against freshwater variants likely reduced standing genetic variation in other regions.
• Elevated linkage disequilibrium in the Eastern Pacific suggests secondary contact after isolation.

Conclusions:

• Parallel marine-freshwater differentiation in three-spined sticklebacks is less prevalent globally than previously assumed based on Eastern Pacific studies.
• Geographic factors and historical processes significantly shape the patterns of adaptation and genetic differentiation.
• Understanding these patterns is crucial for comprehending the mechanisms of rapid evolution in natural populations.