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

迄今为止，物种和生物多样性的起源过程基本上是用脊椎动物来研究的，尽管无脊椎动物占世界上所有物种的大多数。虽然在许多情况下涉及异域物种形成（为阻碍基因流动而形成的地理边界），但越来越多的证据指向生态驱动的同域物种形成（选择与足够不同的环境参数相互作用，即使在没有地理障碍的情况下也能实现生殖隔离）。湖螺已被证明为研究物种形成机制提供了理想的模式生物。然而，虽然这些系统也往往很复杂，但河流辐射既不常见，也很少研究。在泰国Kaek河的独特环境中，我们在初步研究中发现并评估了不到12个Brotia特有分类群的存在，以及物种的同域、准域和异域发生，为研究物种形成的基本机制和因素提供了一个完美的“自然实验”模型系统。在系统发育和系统地理的框架内，我们建议沿着河流的连续单向地形分析地理和生态的作用，同时利用最新可用的“下一代”技术。创新的限制性位点相关DNA （RAD）测序将用于高分辨率地研究基因组广泛差异，从而评估地理因素与生态因素的影响。在一种综合的方法中，我们将使用贝壳的几何形态计量学、齿壳的形态分化、生活史数据（胎生繁殖策略）以及详细的生态参数（即栖息地、基质偏好）来评估河流蜗牛特有组合中种内和种间遗传分化背景下的表型-环境关联。该项目的目的是：(1)测试明确概述的关于物种形成中潜在地理相关的遗传和/或生态分化的假设，重点是营养特化的作用，以及(2)评估真正原位的存在，即河流适应性辐射。这项示范研究首次使用基因组尺度数据对淡水腹足类进行系统发育和系统地理分析，从而将“物种间”系统发育和“物种内”进化遗传学结合起来，能够比较评估作为物种形成驱动力的外在因素（生态因素与地理因素）。此外，它为进一步判断物种形成的内在（即遗传）原因提供了基本数据，这样不仅可以破译物种和所涉及的辐射的进化史，而且可以更好地理解生物多样性出现背后的机制。

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.