Inversions support both parallel and location-specific adaptations in snail ecotypes

Area of Science:

• Evolutionary biology
• Genomics
• Marine ecology

Background:

• Hybrid zones between ecotypes across environmental gradients offer insights into genomic barriers to gene flow.
• The marine snail Littorina fabalis exhibits dwarf and large ecotypes along wave-exposure gradients in northwestern Europe.
• Previous studies identified strong genetic differentiation and 12 chromosomal inversions in a Swedish hybrid zone.

Purpose of the Study:

• To compare a French hybrid zone with a Swedish one to assess parallel ecotype distribution and genomic architecture.
• To investigate if similar wave-exposure gradients promote parallel ecotype divergence and if genomic architecture contributes to adaptation.
• To test for parallel evolution in ecotype distribution and genomic basis of divergence.

Main Methods:

• Whole-genome sequencing of Littorina fabalis from Swedish and French hybrid zones.
• Comparison of genetic differentiation, chromosomal inversions, and cline patterns between the two locations.
• Genome-wide association studies (GWAS) to link genomic regions to ecotype traits like shell size and color.

Main Results:

• Reversed clines in shell size were observed between France (small snails in exposed areas) and Sweden (small snails in sheltered areas).
• 15 chromosomal inversions accounted for most genetic differentiation, showing similar associations with wave-exposure gradients in both zones.
• GWAS identified inversions associated with shell size in Sweden and shell color in France, with some inversions showing unique patterns in each zone.

Conclusions:

• The same chromosomal inversions contribute to ecotype divergence and local adaptation across similar environmental gradients in geographically distant populations.
• Inversions play a dual role in maintaining ecotype differences and harboring site-specific variation for local adaptation.
• Despite parallel genomic architecture, reversed clines indicate complex interactions between genes, environment, and geography in shaping adaptation.