Abstract
BackgroundMarine threespine sticklebacks colonized and adapted to brackish and freshwater environments since the last Pleistocene glacial. Throughout the Holarctic, three lateral plate morphs are observed; the low, partial and completely plated morph. We test if the three plate morphs in the brackish water Lake Engervann, Norway, differ in body size, trophic morphology (gill raker number and length), niche (stable isotopes; δ15N, δ13C, and parasites (Theristina gasterostei, Trematoda spp.)), genetic structure (microsatellites) and the lateral-plate encoding Stn382 (Ectodysplasin) gene. We examine differences temporally (autumn 2006/spring 2007) and spatially (upper/lower sections of the lake – reflecting low versus high salinity).ResultsAll morphs belonged to one gene pool. The complete morph was larger than the low plated, with the partial morph intermediate. The number of lateral plates ranged 8–71, with means of 64.2 for complete, 40.3 for partial, and 14.9 for low plated morph. Stickleback δ15N was higher in the lower lake section, while δ13C was higher in the upper section. Stickleback isotopic values were greater in autumn. The low plated morph had larger variances in δ15N and δ13C than the other morphs. Sticklebacks in the upper section had more T. gasterostei than in the lower section which had more Trematoda spp. Sticklebacks had less T. gasterostei, but more Trematoda spp. in autumn than spring. Sticklebacks with few and short rakers had more T. gasterostei, while sticklebacks with longer rakers had more Trematoda. spp. Stickleback with higher δ15N values had more T. gasterostei, while sticklebacks with higher δ15N and δ13C values had more Trematoda spp. The low plated morph had fewer Trematoda spp. than other morphs.ConclusionsTrait-ecology associations may imply that the three lateral plate morphs in the brackish water lagoon of Lake Engervann are experiencing ongoing divergent selection for niche and migratory life history strategies under high gene flow. As such, the brackish water zone may generally act as a generator of genomic diversity to be selected upon in the different environments where threespine sticklebacks can live.
Highlights
Marine threespine sticklebacks colonized and adapted to brackish and freshwater environments since the last Pleistocene glacial
Genetic structure All the four performed STRUCTURE analyses and the additional DAPC analysis suggested that the lateral plate morphs of threespine stickleback in Lake Engervann belonged to one single gene pool (K = 1)
We argue for finding signs of ongoing divergent niche adaptation among the three lateral plate morphs based on ecologically related measures such as stable isotope values and parasites
Summary
Marine threespine sticklebacks colonized and adapted to brackish and freshwater environments since the last Pleistocene glacial. Throughout the Holarctic, three lateral plate morphs are observed; the low, partial and completely plated morph. Within most populations some individuals tend to be better able to disperse and successfully colonize new environments than others. This is a relevant question for the threespine stickleback, Gasterosteus aculeatus, a highly adaptable euryhaline species commonly observed in salt water, brackish water and fresh water throughout the Holarctic [1,2,3]. As such, studying sticklebacks in brackish water where gene flow from both marine ancestors and freshwater populations occur may increase our understanding of how divergent multifarious adaptive processes act under gene flow. There appears to be a directional pattern of decreased number of lateral plates linked with salinity regimes
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