Abstract
Cryptic species, i.e. species that are morphologically hard to distinguish, have been detected repeatedly in various taxa and ecosystems. In order to evaluate the importance of this finding, we have to know in how far cryptic species differ in various aspects of their biology. The amphipod Gammarus fossarum is a key invertebrate in freshwater streams and contains several cryptic species. We examined the population genetic structure, genetic diversity and demographic history of two of them (type A and type B) using microsatellite markers and asked whether they show significant differences. We present results of population genetic analyses based on a total of 37 populations from the headwaters of two major European drainages, Rhine and Rhone. We found that, in both species, genetic diversity was geographically structured among and within drainages. For type A in the Rhine and type B in the Rhone, we detected significant patterns of isolation by distance. The increase of genetic differentiation with geographical distance, however, was much higher in type A than in type B. This result indicates substantial interspecific differences in population history and/or the extent of current gene flow between populations. In the Rhine, type B does not show evidence of isolation by distance, and population differentiation is relatively low across hundreds of kilometres. The majority of these populations also show signatures of recent bottlenecks. These patterns are consistent with a recent expansion of type B into the Rhine drainage. In summary, our results suggest considerable and previously unrecognized interspecific differences in the genetic structure of these cryptic keystone species.
Highlights
A comprehensive understanding of biodiversity requires knowledge about the species forming an ecosystem as well as the genetic diversity within these species
Type B was studied on a larger geographical scale than type A and included two major drainages, overall FST was higher in type A (0.38; 95% CI: 0.302–0.475) than type B (0.19; 95% CI: 0.125–0.267)
The two species differ substantially with regard to the extent of genetic differentiation between populations: While type B is characterized by moderate genetic differentiation across hundreds of kilometres, even geographically close populations are markedly distinct in type A, suggesting differences in the time since colonization and/or the extent of intraspecific gene flow
Summary
A comprehensive understanding of biodiversity requires knowledge about the species forming an ecosystem as well as the genetic diversity within these species. More and more molecular studies reveal that cryptic species are common and not restricted to certain taxonomic groups or ecosystems [2]. Several studies found divergence between cryptic species in ecological, physiological or behavioural aspects [1,3,4]. This suggests that the identification of cryptic species and differences between them may be essential for understanding population dynamics and ecosystem functioning. Multiple cryptic species complexes have been identified in freshwater vertebrates Several ecologically important ‘‘species’’ have been shown to consist of several reproductively isolated entities (e.g. [9]), the biological differences between which are often unknown
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