Everything in flux - With tropical snails on the trail of "Darwin's secret" about the origin of new species
New research project on the phylogenomics of freshwater snails in the Kaek River, Thailand
Current: On the trail of snails - field research in Thailand (in German)
The emergence of new species is one of the fundamental processes of evolution. Understanding the mechanisms of this speciation has been one of the central goals of biology for more than a century. Important natural scientists have dealt with this question of speciation, above all Charles Darwin, Alfred Russel Wallace and above all Ernst Mayr, the "Darwin of the 20th century". In addition to the now "classical" view, coined by Mayr, of the origin of species through geographical barriers, other possibilities as the cause of speciation have also been increasingly discussed in recent years.
This is where the project, which is now financially supported by the German Research Foundation (DFG), comes in. Using the example of freshwater snails of the genus Brotia, which is also widespread in Thailand, among other places - perhaps now known to some aquarists as a pretty pet (especially Brotia armata or Brotia pagodula) - we will attempt to gain more precise insights into the mechanisms and patterns of speciation. Our preliminary studies have shown that in one particular river in the mountains of Thailand alone, the Kaek River system, diverse varieties of these snails live. But why are so many different species found there of all places? And: Are they even "good" (i.e. independent) species in the sense of a biospecies? If so, how can they be distinguished from each other - and above all: how exactly did they evolve?
In collaboration with the evolutionary biologist Walter Salzburger from the University of Basel, this project will also use "next generation" sequencing methods to gain detailed insights into the relationship of these snails living in close proximity for the first time on freshwater snails. This, together with research into external morphology, reproductive strategy and various other characteristics, will be studied and evaluated as part of a PhD project (Nora Lentge-Maaß).
The Kaek River Project:
Allopatric speciation, i.e. the emergence of species through geographical blockage of gene flow and associated reproductive isolation, has long been discussed as a fundamentally geographical mechanism for the emergence of biodiversity. However, sympatric speciation is gaining more and more attention and is seen as an alternative mechanism. In order to gain insight into the underlying processes and also to determine more precisely the relationship between the two possible speciation modes, further study of closely related species using detailed phylogenetic methods is needed.
So far, speciation studies have mainly used vertebrates as models. Some are now textbook examples, such as the Darwin's finches (Geospizidae) on Galapagos or, among fishes, the cichlids of African and Central American lakes and rivers. In addition to geographical isolation, ecological factors and sexual selection have been identified as responsible mechanisms.
In the investigation of such model systems, biosystematic research now benefits considerably from the rapid development of methods in the field of genome research, thanks to which, in addition to a significantly increased phylogenetic resolution, above all insights into the underlying genetic mechanisms can be gained. RAD sequencing ("restriction-site associated DNA sequencing") has recently been used as one of the innovative methods. This will now also be tested in this project and for the first time on freshwater snails.
Originally, the Hamburg malacologist Rolf Brandt (1968, 1974) described ten morphospecies (at that time including subspecies) of Brotia from the 100 km long section of the Kaek River (accessible until then) on the basis of the external shell morphology. However, due to their distinct shell morphology, these species were not assigned to Brotia alone, but also to a second genus Paracrostoma. In fact, we were able to show in preliminary studies that seven different species of Brotia occur sympatrically or small-scale parapatrically (on both sides of cataracts) in the Kaek River, which can be differentiated shell morphologically, with regard to their radula morphology as well as molecularly. Thus, the Kaek River system is a manageable, geologically highly interesting and ecologically structured environment with regard to the unidirectional topography for the investigation of the actually effective speciation mechanisms.
Publications
Summary of the DFG project:
Radiation in the river - mechanisms of speciation in viviparous freshwater snails endemic to the Kaek River, Thailand. DFG GL 297/26-1
The question of how species, and thus biodiversity as a whole, come into being has so far been investigated mainly on the basis of vertebrates, although invertebrate groups account for the actual diversity of species. In addition to a geographically effective causation (allopatric speciation), studies have recently also shown non-geographical, in particular ecological speciation through selection processes related to the environment. Freshwater snails are demonstrably well-suited model organisms for research into speciation mechanisms, partly because of their limited mobility and the correlated high degree of attachment to habitat conditions. Adaptive radiations, especially in tropical lakes, have already provided important insights, but proved to be complex systems. In contrast, species swarms in rivers have hardly been studied so far, but are more accessible for speciation studies due to the geographically linear topography. Endemic freshwater snails of the genus Brotia in the Kaek River in Thailand, with less than a dozen microgeographically scaled species, provide an ideal model system, as it were, as a natural experiment for studying basic speciation mechanisms and influencing factors. In addition to the molecular phylogeny and phylogeography of the species swarm and its sym-, para- and allopatric species and species pairs occurring along the continuous unidirectional topography of the Kaek River, divergence data will be used at the genomic level with the highest resolution using "next generation" methods - specifically the innovative RAD (restriction-site-associated DNA) sequencing - in order to make verifiable statements on the effectiveness of geographical barriers or other, for example ecologically determined, factors. other, e.g. ecologically determined, mechanisms of genetic isolation can be developed and tested. In an integrative approach, i.e. including morphological data (adaptations of the shell and radula tongue) on intra- and interspecific divergence in riverine snails, such as their life history (viviparous reproductive strategies), hypotheses on spatially correlated genetic or ecological differentiation, in particular on the role of trophic specialisation, in the speciation process are to be tested against the background of ecological parameters (especially habitat and substrate preference) in order to assess the presence of an actually original riverine radiation. The model-based study allows, in comparison with others, not only to assess the influence of extrinsic (i.e. ecological versus geographical) factors in the emergence of biodiversity; at the same time, it provides the necessary data basis for estimating the contribution of possible intrinsic (i.e. genetic) causation of the speciation process.
Doctoral project
Testing a riverine radiation – Mechanism of speciation in viviparous freshwater gastropods endemic to the Kaek River, Thailand
Using the example of species from the group of freshwater snails Brotia, the evolution and speciation in a river system will be studied in the Kaek River as a model.
Employed in the project as a research assistant: Nora Lentge-Maaß (2016-2019)