Genetic discontinuity associated with an environmentally induced barrier to gene exchange in the marine snail Littorina saxatilis

Abstract

The geographical genetic structure of 7 populations of the intertidal marine snail Littorina saxatilis with limited dispersal was studied in NW Spain using 3 classes of genetic markers: 840 amplified fragment length polymorphism (AFLP) loci, 3 microsatellites, and 1 morphological trait (shell size). A sharp genetic division at Cape Finisterre, as evidenced by all 3 markers, delimits 2 regions. This genetic discontinuity coincides with an abrupt change in plankton composition, climate and oceanographic characteristics, suggesting that contemporary influences on gene flow associated with the presence of an ecological barrier maintain the genetic subdivision in NW Spain. However, the genetic structure also accords to some extent with an isolation-by-distance model despite the heterogeneous gene flow due to this ecological barrier. Microsatellites and shell size showed a strong and significant covariation in pairwise differences (F ST and Q ST ) among populations, but no correlation was found between these markers and AFLPs. Similarly, both microsatellites and shell size showed a higher differentiation between the 2 regions than within each region, but this pattern was reversed for AFLPs. Differences in genomic sampling error and homoplasy could account for the discrepancy between different kinds of markers. These results point to the possibility that other marine species may show a similar pattern of intra-specific genetic divergence in NW Spain. 357: 175-184, 2008 local adaptation. This snail lives in the intertidal zone along the Atlantic shores of Western Europe and North America, is ovoviviparous, feeds on the alga microflora and shows a low dispersal capability (1 to 3 m mo -1 ) that typically results in a pattern of isolation-bydistance (Reid 1996). The wide geographic range of this species, coupled with apparently limited dispersal opportunities, has led to a remarkable geographic and microgeographic differentiation (Reid 1996). Indeed, the species is very polymorphic in shell traits both within and between populations and shows strong population structure for allozymes, microsatellites and mtDNA (Ward et al. 1986, Panova et al. 2006, Quesada et al. 2007). Several studies demonstrate a link between ecological adaptation, vertical and/or horizontal distribution, and microhabitat phenotypic differentiation (Rolán-Alvarez et al. 2004, Grahame et al. 2006, Quesada et al. 2007). In many areas, morphologically distinct ecotypes coexist in sympatry or allopatry separated by only a few meters, each showing a significantly higher fitness in its own microhabitat (Rolán-Alvarez et al. 1997). KEY WORDS: Microsatellites · AFLPs · Quantitative traits · Gene flow · Q ST · F ST · Isolation by distance · Ecological barrier Resale or republication not permitted without written consent of the publisherMar Ecol Prog SerThe NW coast of Spain (Fig. 1a) is characterized by fjord-like estuaries that represent discrete habitats isolated from each other by barriers to dispersal and environmental tolerance (Ríos et al. 19...

Genomic divergence between Spanish Littorina saxatilis ecotypes unravels limited admixture and extensive parallelism associated with population history

“…Alternatively, these shared loci may reflect the small complement of shared ancestral standing variation within the southern glacial refuge that enabled rapid adaptive divergence of ecotypes across multiple colonizations (Barrett & Schluter, 2008;Johannesson et al, 2010). Our results also differ from the observed also revealed a potential biogeographic barrier between northern and southern populations corresponding to variation in climatic and oceanographic features that may limit dispersal between sites in the present study (Piñeira, Quesada, Rolán-Alvarez, & Caballero, 2008). The presence of multiple refugia increases the recent evolutionary independence of northern and southern sites, such that genetic backgrounds of divergent traits may have continued to evolve through periods of isolation to be less suitable for the establishment of introduced alleles from distant sites (Conte et al, 2012).…”

“…Due to geographic distance, Burela ecotypes would be expected to experience less frequent introduction of alleles from Corrubedo or Silleiro and vice versa, and this reduction in adaptive admixture will decrease the extent of shared genomic divergence between ecotype pairs. Comparison of genetic structure of L. saxatilis populations across northwestern Spain has also revealed a potential biogeographic barrier between northern and southern populations corresponding to variation in climatic and oceanographic features that may limit dispersal between sites in the present study (Piñeira, Quesada, Rolán‐Alvarez, & Caballero, ). The presence of multiple refugia increases the recent evolutionary independence of northern and southern sites, such that genetic backgrounds of divergent traits may have continued to evolve through periods of isolation to be less suitable for the establishment of introduced alleles from distant sites (Conte et al., ).…”

Historical and contemporary population genetic connectivity of the European short‐snouted seahorse Hippocampus hippocampus and implications for management

“…Instead, it was found that structuring in H. hippocampus is associated with distinct genetic breaks, between the Bay of Biscay and western Iberian coasts and between European and West African (Senegal) Atlantic Ocean coasts. Two other studies (Lundy et al , 1999; Pineira et al , 2008) have proposed a barrier to marine species dispersal between the Bay of Biscay and west Iberian waters, at Cape Finisterre, and a similar genetic break is also suggested for the closely related H. guttulatus (Woodall, 2009). Strong onshore currents from the Gulf Stream that flow east and south of the Cape, together with larval retention by oceanographic features within the Bay of Biscay (Gil, 2008), may be strong barriers to adult or juvenile dispersal by seahorses.…”

Past and present drivers of population structure in a small coastal fish, the European long snouted seahorse Hippocampus guttulatus

“…A major barrier to gene flow between the northern Spanish and southeast Portugese sites was supported in H. hippocampus (Woodall et al 2011b), and a similar pattern is consistent with our analysis of H. guttulatus. Other studies have observed Cape Finisterre in northwest Spain as being associated with genetic differentiation of marine populations (Neiva et al 2012;Piñeira et al 2008;Quesada et al 1998), although a small-scale study of H. guttulatus across this area did not find population differentiation to either side of the cape (Lopez et al 2015). A more southerly barrier to gene flow, between Rio Mondego and Rio Sado in central Portugal, has been suggested for other marine species (Diekmann et al 2005;Pascoal et al 2009).…”