Adaptive gamete-recognition divergence in a hybridizing Mytilus population

Area of Science:

• Evolutionary biology
• Genetics
• Marine biology

Background:

• Gamete-recognition proteins evolve rapidly, but the timing of their divergence (within or between species) is debated.
• Understanding this divergence is key to explaining speciation and reproductive isolation.
• Mytilus blue mussels hybridize in nature, providing a model system to study these processes.

Purpose of the Study:

• To investigate the evolutionary divergence of a candidate gamete-recognition protein, Lysin-M7, within Mytilus galloprovincialis.
• To determine if Lysin-M7 divergence occurs within species and correlates with reproductive isolation.
• To assess the role of selection in Lysin-M7 evolution, particularly in populations experiencing secondary contact and hybridization.

Main Methods:

• Examined Lysin-M7 evolution in sympatric and allopatric populations of Mytilus blue mussels.
• Analyzed genetic variation and population differentiation of Lysin-M7 alleles.
• Used maximum-likelihood estimates (dN/dS) to infer selective pressures on Lysin-M7.

Main Results:

• Found adaptive divergence of Lysin-M7 alleles into two clades (G and G(D)) within Mytilus galloprovincialis, with significant amino acid differences.
• Selective pressure (dN/dS > 1) indicated adaptive evolution driving divergence between and within Lysin-M7 clades.
• Lysin-M7 allele frequencies differed significantly among populations, with G(D) alleles elevated in secondary contact zones.

Conclusions:

• Lysin-M7 exhibits adaptive divergence within Mytilus galloprovincialis, driven by selection.
• Elevated frequencies of specific Lysin-M7 alleles in hybridizing zones suggest a role in reproductive isolation.
• Pleistocene secondary contact likely fueled Lysin-M7 divergence in M. galloprovincialis.