Abstract
BackgroundThe underlying mechanisms and processes that prompt the colonisation of extreme environments, such as caves, constitute major research themes of evolutionary biology and biospeleology. The special adaptations required to survive in subterranean environments (low food availability, hypoxic waters, permanent darkness), and the geographical isolation of caves, nominate cave biodiversity as ideal subjects to answer long-standing questions concerning the interplay amongst adaptation, biogeography, and evolution. The present project aims to examine the phylogeographic patterns exhibited by two sympatric species of surface and cave-dwelling peracarid crustaceans (Asellus aquaticus and Niphargus hrabei), and in doing so elucidate the possible roles of isolation and exaptation in the colonisation and successful adaptation to the cave environment.ResultsSpecimens of both species were sampled from freshwater hypogean (cave) and epigean (surface) habitats in Hungary, and additional data from neighbouring countries were sourced from Genbank. Sequencing of mitochondrial and nuclear loci revealed, through haplotype network reconstruction (TCS) and phylogenetic inference, the genetic structure, phylogeographic patterns, and divergence-time estimates of A. aquaticus and N. hrabei surface and cave populations. Contrasting phylogeographic patterns were found between species, with A. aquaticus showing strong genetic differentiation between cave and surface populations and N. hrabei lacking any evidence of genetic structure mediated by the cave environment. Furthermore, N. hrabei populations show very low levels of genetic differentiation throughout their range, which suggests the possibility of recent expansion events over the last few thousand years.ConclusionsIsolation by cave environment, rather than distance, is likely to drive the genetic structuring observed between immediately adjacent cave and surface populations of A. aquaticus, a predominantly surface species with only moderate exaptations to subterranean life. For N. hrabei, in which populations exhibit a fully ‘cave-adapted’ (troglomorphic) phenotype, the lack of genetic structure suggests that subterranean environments do not pose a dispersal barrier for this surface-cave species.
Highlights
The underlying mechanisms and processes that prompt the colonisation of extreme environments, such as caves, constitute major research themes of evolutionary biology and biospeleology
Isolation by cave environment, rather than distance, is likely to drive the genetic structuring observed between immediately adjacent cave and surface populations of A. aquaticus, a predominantly surface species with only moderate exaptations to subterranean life
For N. hrabei, in which populations exhibit a fully ‘cave-adapted’ phenotype, the lack of genetic structure suggests that subterranean environments do not pose a dispersal barrier for this surface-cave species
Summary
The underlying mechanisms and processes that prompt the colonisation of extreme environments, such as caves, constitute major research themes of evolutionary biology and biospeleology. Caves and other subterranean habitats show marked environmental differences with adjacent surface ecosystems with a sharp boundary, most notably the absence of light and associated ecological and biogeochemical conditions Such habitat differences can constitute significant barriers to gene flow and population connectivity, which in turn lead to high levels of genetic differentiation even at relatively small spatial scales [6, 7]. The estimation of phylogenetic relationships from genetic data of cave-dwellers offers the possibility of elucidating the mechanisms and processes that eventually lead to cave colonisations and the persistence of cave populations This is especially the case when genetic data of both surface and cave-dwelling organisms are coupled with their present-day geographic distributions to infer ancient events To fully understand the mechanisms behind cave colonisation events through present-day phylogeographic patterns it is imperative to incorporate approaches that consider the environment and ecology of the species under study, and the underlying factors that drive their evolution
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