Testing a riverine radiation. Mechanism of speciation in viviparous freshwater gastropods endemic to the Kaek River, Thailand

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The process of how species and biodiversity in general originates is to date essentially studied using vertebrates, although invertebrates represent the majority of all species worldwide. While allopatric speciation (geographical boundaries formed to hamper gene flow), is involved in many cases, an increasing number of evidence points at ecologically driven sympatric speciation (selection interacts with sufficiently distinct environmental parameters to enable reproductive isolation even in the absence of geographical barriers). Lacustrine snails have proven to supply ideal model organisms for studying mechanisms of speciation. However, while these systems also tend to be complex, riverine radiations are both infrequent and rarely studied. In the unique setting of the Kaek River in Thailand we have, in preliminary studies, found and evaluated the existence of less than a dozen endemic taxa of Brotia, with sym-, para- and allopatric occurrences of species, providing a perfectly suitable model system as 'natural experiment' for the study of fundamental mechanisms and factors acting in speciation. Within a phylogenetic and phylogeographic framework we suggest to analyse the role of geography and ecology along the continuous unidirectional topography of a river, at the same time taking advantage of newly available 'next generation' techniques. The innovative RAD (restriction-site associated DNA) sequencing will be used in order to investigate genome wide divergence in high resolution, thus allowing to evaluate the influence of geographical versus ecological factors. In an integrative approach we will use geometric morpho¬metrics of shell, morphological differentiation of radula, supplemented by data on life history (viviparous reproductive strategies) together with detailed ecological parameters (i.e. habitat, substrate preference) to assess phenotype-environment associations in context of intra- and interspecific genetic divergence within the endemic assemblage of riverine snails. It is the aim of the project (i) to test explicitly outlined hypotheses on the potentially geographically correlated genetic and/or ecological differentiation in speciation, with emphasis on the role of trophic specialization, and (ii) to evaluate the existence of a truly in situ, i.e. riverine adaptive radiation. This exemplary study using for the first time genomic-scale data for phylogenetic and phylogeographical analyses in freshwater gastropods, thus combining 'among species' phylogenetics and 'within species' evolutionary genetics, enables the comparative assessment of extrinsic (ecological versus geographical) factors as a driving force of speciation. In addition, it provides the basic data for further judgement on the intrinsic (i.e. genetic) causation of speciation, that way allowing not only to decipher the evolutionary history of the species and radiation involved, but to better understand those mechanisms behind the emergence of biodiversity in general.