Abstract
Contact zones between divergent forms of the same species are often characterised by high levels of phenotypic diversity over small geographic distances. What processes are involved in generating such high phenotypic diversity? One possibility is that introgression and recombination between divergent forms in contact zones results in greater phenotypic and genetic polymorphism. Alternatively, strong reproductive isolation between forms may maintain distinct phenotypes, preventing homogenisation by gene flow. Contact zones between divergent freshwater-resident and anadromous stickleback (Gasterosteus aculeatus L.) forms are numerous and common throughout the species distribution, offering an opportunity to examine these contrasting hypotheses in greater detail. This study reports on an interesting new contact zone located in a tidally influenced lake catchment in western Ireland, characterised by high polymorphism for lateral plate phenotypes. Using neutral and QTL-linked microsatellite markers, we tested whether the high diversity observed in this contact zone arose as a result of introgression or reproductive isolation between divergent forms: we found strong support for the latter hypothesis. Three phenotypic and genetic clusters were identified, consistent with two divergent resident forms and a distinct anadromous completely plated population that migrates in and out of the system. Given the strong neutral differentiation detected between all three morphotypes (mean FST = 0.12), we hypothesised that divergent selection between forms maintains reproductive isolation. We found a correlation between neutral genetic and adaptive genetic differentiation that support this. While strong associations between QTL linked markers and phenotypes were also observed in this wild population, our results support the suggestion that such associations may be more complex in some Atlantic populations compared to those in the Pacific. These findings provide an important foundation for future work investigating the dynamics of gene flow and adaptive divergence in this newly discovered stickleback contact zone.
Highlights
Divergent natural selection can lead to the evolution of adaptive differences between populations inhabiting contrasting environments [1,2]
Only low plated forms occurred in Lough Feeagh, whereas low, partially and completely plated forms were all observed in Lough Furnace (Fig 3A)
Contact zones between divergent anadromous and freshwater resident stickleback forms provide important opportunities to study the role of introgression in generating phenotypic diversity
Summary
Divergent natural selection can lead to the evolution of adaptive differences between populations inhabiting contrasting environments [1,2]. Strong selection and adaptive divergence as a consequence of differential selection between contrasting environments can lead to the evolution of reproductive barriers, driving progress towards speciation [6,7]. I.e. regions of overlap between divergent populations or species, are often characterised by high levels of gene flow, which give rise to hybrid zones [8,9]. Studying contact zones between populations occupying different habitats that have recently diverged offers the opportunity to examine the role selection plays in maintaining divergent populations despite high potential for gene flow
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