Speciation and adaptation in Antarctic sea spiders: Assessing the relative importance of genetic drift and natural selection using comparative population genomic and morphological approaches

对物种形成和适应过程的分析对于了解生物多样性是如何产生的以及它如何对不断变化的环境作出反应至关重要。研究这种过程的理想生态系统是南大洋。南极洲周围寒冷的沃茨中有着丰富和高度特有的底栖动物。最近的研究发现，这些物种中有许多是由离散的遗传分支组成的，这些遗传分支被解释为以前未被识别的物种，在更新世辐射。这些隐藏的物种可以通过分子方法（例如DNA条形码）和在某些情况下通过细致的形态学分析来区分。通常，物种形成事件被解释为是由最近的冰川驱动的。这种假设是，物种形成是偶然发生的，当以前的大种群被隔离到小的异域避难所在冰川极大期，在那里他们受到随机遗传漂变的强烈影响。由于分子技术的限制，很少探索物种形成的替代模型。其他生态系统的研究表明，生态物种形成，即，由于强差异选择而导致的分歧是一个明显的替代模型。在这里，我们建议使用高分辨率的人口基因组方法，再加上形态分析，以测试竞争的南大洋物种形成模型在自然和非实验室环境。作为一个测试案例，我们将研究遗传漂变和选择的模式在一个特别成功的南大洋底栖物种：海蜘蛛（Pycnogonida）。在我们早期研究的基础上，我们将分析两个物种群中中性和非中性标记的基因组模式：Colossendeis megalonyx和Pallenopsis patagonica。这两个海蜘蛛物种复合体是很好的模式，以解决这一建议的目标，因为几个特点：1）存在许多遗传上不同的隐蔽物种，2）形态支持这些物种中的一些，3）他们广泛的分布范围，包括南极大陆架和亚南极地区受过去的冰川影响较小，4）他们的低流动性。如果异域物种形成主要由冰川避难所种群的遗传漂变驱动，是主要的进化力量，我们预计中性等位基因的随机固定和人口瓶颈的签名在受冰影响严重的地区最高。或者，如果自然选择是促进物种形成的核心进化力量，我们预计在基因和表型水平上，特别是在同域（生态特征置换）的选择强烈的签名。我们将研究整个基因组的变异，以评估随机与非随机变异。拟议的研究将在系统地检验南大洋自然选择在进化中的相对重要性方面迈出重要的第一步。

The analysis of speciation and adaptation processes is fundamental to understand how biodiversity is generated and how it responds to changing environments. An ideal ecosystem to study such processes is the Southern Ocean. The cold waters around Antarctica host a rich and highly endemic benthic fauna. Recent studies have found that many of these species consist of groups of discrete genetic clades interpreted as previously unrecognized species that radiated in the Pleistocene. These cryptic species can be distinguished with molecular methods (e.g. DNA barcoding) and in some cases with meticulous morphological analyses. Typically the speciation events are interpreted as being driven by recent glaciations. The assumption is that speciation occurred by chance when formerly large populations were isolated into small allopatric refugia during glacial maxima, where they were then subjected to the strong effects of random genetic drift. Alternative models of speciation are rarely explored due to previous limitations in molecular techniques. Studies from other ecosystems suggest that ecological speciation, i.e., divergence due to strong differential selection, is one obvious alternative model. Here we propose to use high-resolution population genomic approaches coupled with morphological analyses to test competing Southern Ocean speciation models in a natural and non-laboratory setting. As a test case we will study patterns of genetic drift and selection in a particularly successful group of Southern Ocean benthic species: the sea spiders (Pycnogonida). Building on our earlier studies, we will analyse genomic patterns of neutral and non-neutral markers in two species flocks: Colossendeis megalonyx and Pallenopsis patagonica. These two sea spider species complexes are good models to address the objectives of this proposal because of several characteristics: 1) The presence of many genetically divergent cryptic species, 2) morphological support some of these species, 3) their broad distribution range including both Antarctic continental shelf and sub-Antarctic regions less affected by past glaciations, 4) their low mobility. If allopatric speciation driven largely by genetic drift in glacial refugial populations is the primary evolutionary force, we expect random fixation of neutral alleles and signatures of population bottlenecks to be highest in heavily ice-affected regions. Alternatively, if natural selection has been the central evolutionary force promoting speciation, we expect strong signatures of selection on geno- and phenotype level in particular in sympatry (ecological character displacement). We will investigate variation across whole genomes to assess random vs. non-random variation. The proposed study will make an important first step in systematically testing for the relative importance of natural selection in evolution in the Southern Ocean.