Differential gene exchange between parapatric morphs of Littorina saxatilis detected using AFLP markers

Abstract

Speciation requires the acquisition of reproductive isolation, and the circumstances under which this could evolve are of great interest. Are new species formed after the acquisition of generalized incompatibility arising between physically separated populations, or may they arise as a result of the action of disruptive selection beginning with the divergence of a rather restricted set of gene loci? Here we apply the technique of amplified fragment length polymorphism (AFLP) analysis to an intertidal snail whose populations display a cline in shell shape across vertical gradients on rocky shores. We compare the FST values for 306 AFLP loci with the distribution of FST estimated from a simulation model using values of mutation and migration derived from the data. We find that about 5% of these loci show greater differentiation than expected, providing evidence of the effects of selection across the cline, either direct or indirect through linkage. This is consistent with expectations from nonallopatric speciation models that propose an initial divergence of a small part of the genome driven by strong disruptive selection while divergence at other loci is prevented by gene flow. However, the pattern could also be the result of differential introgression after secondary contact.

Adaptive responses of independent traits to the same environmental gradient in the intertidal snail Bembicium vittatum

“…This is consistent with the results for another littorine, L. saxatilis, in which 15 of 306 AFLP loci examined showed distinctly large F ST , and all 15 were associated with the same habitat gradient as shell morphology (Wilding et al, 2001;Grahame et al, 2006). It seems unlikely that all these loci are directly associated with the shell dimorphism in L. saxatilis, so it may well be that there is a complex of independent adaptations to this significant intertidal gradient.…”

Population genomics of parallel phenotypic evolution in stickleback across stream–lake ecological transitions

“…Many studies investigating the genetics of adaptation have taken a population genomics approach in which genome-wide patterns of genetic variation are documented in many individuals within a species [4][5][6]. Using this approach, regions of the genome that are under divergent selection between local populations (outlier loci) can be identified by their high level of differentiation compared with the background levels [7][8][9].…”

“…In cases where subpopulations have diverged enough to become reproductively isolated, the genetics of speciation can be examined [3,11]. To date, the majority of population genomics studies on wild species have used anonymous genetic markers [4][5][6]. However, with advances in high-throughput genetics and mounting numbers of completed genome projects, markers with known locations within the genome are increasingly being examined, narrowing the search for underlying genes [12].…”

Phylogeographic analysis reveals a deep lineage split within North Atlantic Littorina saxatilis

“…on the scale of metres) habitatspecific morphological differentiation driven primarily by variation in crab predation and wave exposure [15,17,18]. In each location, pairs of divergent ecotypes display distinct morphologies (E & S, Sweden [19]; RB & SU, Spain [20]; H & M, Britain [21]), partial reduction of gene flow [22][23][24][25] and evidence of reproductive isolation [21,26,27], providing support for multiple independent divergence events and suggesting L. saxatilis as a model of incipient sympatric speciation [17,18,26,28]. Yet, despite its importance as one of the few marine examples of incipient sympatric speciation [29], the necessity of lineage-wide phylogenetic context for model systems of sympatric speciation [30,31] and the established role of historical allopatric divergence in North Atlantic marine species [3,4], little is known about the evolutionary history of L. saxatilis across its trans-Atlantic range.…”