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Data from: Genomic divergence between Spanish Littorina saxatilis ecotypes unravels limited admixture and extensive parallelism associated with population history
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Kess, Tony; Galindo, Juan; Boulding, Elizabeth G. — 2019-06-11 The rough periwinkle, Littorina saxatilis, is a model system for studying parallel ecological speciation in microparapatry. Phenotypically parallel wave-adapted and crab-adapted ecotypes that hybridize within the middle shore are replicated along the northwestern coast of Spain, and have likely arisen from two separate glacial refugia. We tested whether greater geographic separation corresponding to reduced opportunity for contemporary or historical gene flow between parallel ecotypes resulted in less parallel genomic divergence. We sequenced double-digested restriction-associated DNA (ddRAD) libraries from individual snails from upper, mid, and low intertidal levels of three separate sites colonized from two separate refugia. FDIST analysis of 4256 SNP markers identified 34.4% sharing of divergent loci between two geographically-close sites, however, these sites each shared only 9.9-15.1% of their divergent loci with a third more-distant site. STRUCTURE analysis revealed that genotypes from only three of 166 phenotypically intermediate mid-shore individuals appeared to result from recent hybridization suggesting that hybrids cannot be reliably identified using shell traits. Hierarchical AMOVA indicated that the primary source of genomic differentiation was geographic separation, but also revealed greater similarity of the same ecotype across the two geographically-close sites than previously estimated with dominant markers. These results from a model system for ecological speciation suggest that genomic parallelism is affected by the opportunity for historical or contemporary gene flow between populations.
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Data from: Size selection by a gape-limited predator of a marine snail: insights into magic traits for speciation
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Boulding, Elizabeth G.; Rivas, María José; González-Lavín, Nerea; Rolán-Alvarez, Emilio; Galindo, Juan — 2016-12-27 The intertidal snail Littorina saxatilis has repeatedly evolved two parallel ecotypes assumed to be wave adapted and predatory shore crab adapted, but the magnitude and targets of predator-driven selection are unknown. In Spain, a small, wave ecotype with a large aperture from the lower shore and a large, thick-shelled crab ecotype from the upper shore meet in the mid-shore and show partial size-assortative mating. We performed complementary field tethering and laboratory predation experiments; the first set compared the survival of two different size-classes of the crab ecotype while the second compared the same size-class of the two ecotypes. In the first set, the large size-class of the crab ecotype survived significantly better than the small size-class both on the upper shore and in the laboratory. In the second set, the small size-class of the crab ecotype survived substantially better than that of the wave ecotype both on the upper shore and in the laboratory. Shell-breaking predation on tethered snails was almost absent within the lower shore. In the laboratory shore crabs (Pachygrapsus marmoratus) with larger claw heights selected most strongly against the small size-class of the crab ecotype, whereas those with medium claw heights selected most strongly against the thin-shelled wave ecotype. Sexual maturity occurred at a much larger size in the crab ecotype than in the wave ecotype. Our results showed that selection on the upper shore for rapid attainment of a size refuge from this gape-limited predator favors large size, thick shells, and late maturity. Model parameterization showed that size-selective predation restricted to the upper shore resulted in the evolution of the crab ecotype despite gene flow from the wave ecotype snails living on the lower shore. These results on gape-limited predation and previous ones showing size-assortative mating between ecotypes suggest that size may represent a magic trait for the thick-shelled ecotype.

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(auteur : Galindo, Juan)

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