Morphological and genetic divergence of intralacustrine stickleback morphs in Iceland: a case for selective differentiation?

Abstract

The evolutionary processes involved in population divergence and local adaptation are poorly understood. Theory predicts that divergence of adjacent populations is possible but depends on several factors including gene flow, divergent selection, population size and the number of genes involved in divergence and their distribution on the genome. We analyse variation in neutral markers, markers linked to putative quantitative trait loci and morphological traits in a recent (< 10 000 years) zone of primary divergence between stickleback morphs in Lake Thingvallavatn, Iceland. Environmental factors, especially predation, are clearly implicated in reducing gene flow between morphs. There is continuous morphological and genetic variation between habitats with a zone centre similar to secondary contact zones. Individual microsatellite loci are implicated as being linked to adaptive variation by direct tests as well as by differences in cline shape. Patterns of linkage disequilibria indicate that the morphs have diverged at several loci. This divergence shows parallels and differences with the well‐studied limnetic–benthic stickleback morphs, both in phenotypic divergence and at the genomic level.

Along the speciation continuum in sticklebacks

“…Another parapatric system is represented by the mud and lava habitats within some Icelandic lakes. In some lakes, phenotypic divergence between the habitats is evident in diet and trophic traits (Kristjánsson et al , 2002 a ), but in other lakes it is more related to predator defence traits (Ólafsdóttir et al , 2007 a ). This latter difference is thought to arise because the more complex lava habitat is thought to provide better protection from predatory Arctic charr Salvelinus alpinus (L.).…”

Water temperature, not fish morph, determines parasite infections of sympatric Icelandic threespine sticklebacks (Gasterosteus aculeatus)

“…However, shallow areas in the middle and southern parts of the lake can be considerably warmer. Stickleback morphs inhabiting the mud and lava rock habitats (littoral lava morph, littoral mud morph, and deep water Nitella morph) differ to some extent genetically, morphologically and in terms of feeding ecology that correspond to their specific habitat characteristics (Kristjánsson et al 2002; Ólafsdóttir et al 2007a,b; Ólafsdóttir and Snorrason 2009). For example, the lava morph has shorter spines than the Nitella morph which enables it to enter small holes and crevices typical for the lava rock habitat.…”

“…Longer spines of the Nitella morph, on the other hand, are most likely a counter–adaptation to predation pressure from Arctic charr and brown trout (Kristjánsson et al 2002; Ólafsdóttir et al 2007b). The littoral mud morph represents an intermediate of the two former as it is morphologically more similar to the lava morph but genetically closer to the Nitella morph (Ólafsdóttir et al 2007a; Ólafsdóttir and Snorrason 2009). …”

“…Here, we focus on the metazoan parasites of threespine stickleback in two large Icelandic lakes, Thingvallavatn and Myvatn. In both lakes, stickleback morphs inhabit two main habitat types (Kristjánsson et al 2002; Ólafsdóttir et al 2007a,b; Ólafsdóttir and Snorrason 2009): soft muddy bottoms (referred here to as mud morph) and lava rock bottoms (referred here to as lava morph, Fig. 1).…”

“…In Thingvallavatn, the lava morph lives in cold water and the mud morph in warm water, whereas in Myvatn the lava morph is found in warm water and the mud morph in the cold. In Thingvallavatn, a third stickleback morph is also found in mud habitat in depths of 10–20 m in association with the macrophyte Nitella opaca (referred here to as Nitella morph) (Kristjánsson et al 2002; Ólafsdóttir et al 2007a,b; Ólafsdóttir and Snorrason 2009), where water temperature resembles more to that of the cold lava habitat. We were particularly interested in the relative importance of the biotic component (biotic features of the mud and lava habitat) and the abiotic environmental component (habitat water temperature) in determining the parasite community structure in the stickleback morphs, which could shed light on the question of timing in parasite-mediated divergent selection.…”