Do the same genes underlie parallel phenotypic divergence in different Littorina saxatilis populations?
Bottom Line: Contrary to our expectations, we found that most outliers were country specific, suggesting that much of the genetic basis of divergence is not shared among locations.First, a polygenic basis of divergent traits might allow for multiple distinct molecular mechanisms generating the same phenotypic patterns.Second, additional, location-specific axes of selection that we did not focus on in this study may produce distinct patterns of genetic divergence within each site.
Affiliation: Animal and Plant Sciences, University of Sheffield, Sheffield, S102TN, UK.Show MeSH
Related in: MedlinePlus
Mentions: Here, we study parallel divergence in the marine snail Littorina saxatilis, a model system for divergent adaptation and speciation. Littorina saxatilis inhabits shores across Europe and North America. It occurs in a variety of habitats, including rocky areas such as boulder fields and steep cliffs, but also in salt marshes and soft substrates in brackish water (Reid 1996). Most studies so far have focussed on two rocky shore ecotypes, adapted either to crab predation or to wave action (Rolán-Alvarez 2007; Butlin et al. 2008; Johannesson et al. 2010), which occur on distinct parts of the same shores. Such ecotype pairs can be found in geographically distant locations on shores with quite distinct topologies (Fig. 1B). We will refer to these ecotypes as ‘crab ecotype’ and ‘wave ecotype’, even though additional selection pressures (which may vary among locations) may also play a role in their divergence. They differ phenotypically from each other along multiple axes, including shell characteristics (e.g. shell thickness, aperture size), behaviour (wary vs. bold) and size (crab ecotype larger) (Johannesson et al. 2010; Butlin et al. 2014). Hybridization occurs in relatively narrow contact zones (few metres), but gene flow is limited due to assortative mating, immigrant inviability and habitat choice (Johannesson et al. 1995b, 2010; Rolán-Alvarez 2007; Webster et al. 2012).
Affiliation: Animal and Plant Sciences, University of Sheffield, Sheffield, S102TN, UK.