Widely distributed and regionally isolated! Drivers of genetic structure in Gammarus fossarum in a human-impacted landscape.

Martina Weiss,Florian Leese

doi:10.1186/s12862-016-0723-z

Abstract

BackgroundThe actual connectivity between populations of freshwater organisms is largely determined by species biology, but is also influenced by many area- and site-specific factors, such as water pollution and habitat fragmentation. Therefore, the prediction of effective gene flow, even for well-studied organisms, is difficult. The amphipod crustacean Gammarus fossarum is a key invertebrate in freshwater ecosystems and contains many cryptic species. One of these species is the broadly distributed G. fossarum clade 11 (type B). In this study, we tested for factors driving the genetic structure of G. fossarum clade 11 in a human-impacted landscape at local and regional scales. To determine population structure, we analyzed the mitochondrial cytochrome c oxidase 1 (CO1) gene of 2,086 specimens from 54 sampling sites and microsatellite loci of 420 of these specimens from ten sites.ResultsWe detected strong overall genetic differentiation between populations at regional and local scales with both independent marker systems, often even within few kilometers. Interestingly, we observed only a weak correlation of genetic distances with geographic distances or catchment boundaries. Testing for factors explaining the observed population structure revealed, that it was mostly the colonization history, which has influenced the structure rather than any of the chosen environmental factors. Whereas the number of in-stream barriers did not explain population differentiation, the few large water reservoirs in the catchment likely act as dispersal barriers.ConclusionsWe showed that populations of Gammarus fossarum clade 11 are strongly isolated even at local scales in the human-impacted region. The observed genetic structure was best explained by the effects of random genetic drift acting independently on isolated populations after historical colonization events. Genetic drift in isolated populations was probably further enhanced by anthropogenic impacts, as G. fossarum is sensitive to many anthropogenic stressors. These findings highlight the importance of small-scale genetic studies to determine barriers restricting gene flow to prevent further loss of genetic diversity and maintain intact freshwater ecosystems.Electronic supplementary materialThe online version of this article (doi:10.1186/s12862-016-0723-z) contains supplementary material, which is available to authorized users.

Highlights

The actual connectivity between populations of freshwater organisms is largely determined by species biology, but is influenced by many area- and site-specific factors, such as water pollution and habitat fragmentation
We tested for factors driving the genetic structure of G. fossarum clade 11 in a human-impacted landscape at local and regional scales
Using the GENELAND groups in an Analysis of molecular variance (AMOVA), we found that this clustering reflects the population structure better than the sub-catchment structuring, as 71.1 % of the variation was between groups and only 10 % was among populations within groups (Table 2)

Summary

Introduction

The actual connectivity between populations of freshwater organisms is largely determined by species biology, but is influenced by many area- and site-specific factors, such as water pollution and habitat fragmentation. The amphipod crustacean Gammarus fossarum is a key invertebrate in freshwater ecosystems and contains many cryptic species. One of these species is the broadly distributed G. fossarum clade 11 (type B). Biogeographic studies revealed wide ranges for many freshwater invertebrate species [1]. This holds true in particular for species found in temperate and more northern latitudes, which had to recolonize habitats after glacial periods. Over the past 15 years, the paradigm of wide ranges has been increasingly questioned, as molecular studies revealed the presence of morphologically cryptic species in many freshwater invertebrate taxa These cryptic species often show rather small and allopatric ranges (e.g. [7, 8]), instead of the presumed broad distribution of the whole cryptic species complex

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