Reproductive isolation in a three-way contact zone

Area of Science:

• Evolutionary Biology
• Speciation Genomics
• Population Genetics

Background:

• Contact zones between divergent populations offer insights into gene flow's role in adaptation and speciation.
• Previous research primarily examined two-way contact zones, overlooking the complexity of multi-population interactions.
• Understanding introgression patterns in natural settings with multiple overlapping populations is crucial.

Purpose of the Study:

• To investigate complex patterns of divergence and introgression in a natural three-way contact zone.
• To analyze the interplay of gene flow, hybridization, and reproductive isolation in three-spined stickleback (Gasterosteus aculeatus).

Main Methods:

• Utilized phenotypic and genomic data to identify and characterize a three-way contact zone.
• Employed genomic cline analyses to detect selection and hybrid incompatibilities.
• Examined immune gene expression profiles in resident and migratory forms and their hybrids.

Main Results:

• Confirmed a three-way contact zone involving freshwater resident, saltwater resident, and saltwater migratory stickleback.
• Observed hybridization, primarily between saltwater resident and migratory forms, yet identified persistent genomic islands of divergence.
• Genomic cline analyses indicated selection and/or hybrid incompatibilities in divergent regions, involving adaptive loci and chromosomal inversions.

Conclusions:

• Reproductive isolation is maintained in this complex three-way contact zone despite ongoing hybridization.
• Chromosomal inversions likely contribute significantly to maintaining reproductive isolation by reducing recombination.
• Distinct immune gene expression profiles exist between forms, but hybrids do not show transgressive expression.