Alien gene introgression promotes rapid adaptation: an unconscious experiment on Fire-bellied toad (Bombina bombina) at the northern edge of its distribution range

该提案通过评估特定假设，即，(1)位于物种分布边缘的小的孤立的种群可能受到缺乏长期遗传变异的限制;（2）外来等位基因的渐渗可能增强这些种群的适应潜力。在基因渗入的情况下，杂合性和等位基因多样性立即增加，有性生殖产生嵌合基因型，结合了不同套的本地和异地等位基因，为快速适应提供了原料。我们的研究生物是火腹蟾蜍Bombina bombina，它在欧洲有两个不同的进化谱系，即，一个是分布在巴尔干半岛的国家和奥地利的南方血统，另一个是从乌克兰通过波兰到达德国北部和瑞典南部北部分布范围边缘的北方血统。在20个不连锁的遗传位点的初步遗传分析解开了南血统（puerkenaustrian）基因型渗入到三个当地北方血统的人口，大概是由于独立的非官方（和非法）释放业余爬虫学家。有趣的是，这些渐渗群体（1）在几个基因座上的渐渗等位基因是固定的，（2）具有正的群体趋势，使得它们优于当地的原生群体。这种情况提供了一个重复的无意识实验，模仿长距离传播和随后的基因渗入从南方血统（可能适应温暖的夏季条件）到北方人口。我们将进行基因组扫描（RAD-seq）和表达谱分析（RNA-seq）的autochthonous和渗入的铃蟾种群辨别，在基因组规模上，从autochthonous的基因渗入等位基因/表达模式。在全基因组关联研究（GWAS）中，我们将这些模式与健身相关的特征（体型，质量，条件）联系起来。这些数据将被输入我们的合作者（Stephan教授）开发的理论模型，以识别选择性扫描与多基因选择在快速适应中的相对重要性。对候选基因（包括MHC和热休克蛋白基因）的深入分析将揭示功能类别，其中渐渗（外来）等位基因将受到选择的青睐，而其中的自体等位基因被保留。这将为我们提供一个因果关系的理解的原因，为什么渐渗种群的火腹蟾蜍在北方边缘的分布范围优于非渐渗的。最终，该项目的重点是渗入的相关性，作为一种机制，以增加长期遗传变异的数量，从而为快速适应提供适应性遗传物质。

This proposal focusses on genomic sources of adaptive variation for rapid change (thematic group 3), by evaluating particular hypotheses, i.e., (1) small isolated populations at the margin of a species distribution might be constrained by a lack of standing genetic variation and (2) introgression of allochthonous alleles may enhance the adaptive potential of such populations. In case of introgression, heterozygosity and allelic diversity increase instantly and sexual reproduction creates mosaic genotypes combining different sets of authochthonous and allochthonous alleles, providing the raw material for rapid adaptation. Our study organism is the Fire-bellied toad Bombina bombina, which occurs in Europe in two distinct evolutionary lineages, i.e., a southern lineage distributed in the Balkan states and Austria, and a northern lineage reaching from Ukraine through Poland to the edges of the northern distribution range in Northern Germany and Southern Sweden. Initial genetic analyses at 20 unlinked genetic loci unraveled an introgression of southern lineage (putatively Austrian) genotypes into three local northern lineage populations, presumably due to independent unofficial (and illegal) release by amateur herpetologists. Interestingly, these introgressed populations (1) are fixed for introgressed alleles at several loci and (2) have a positive population trend, such that they outperform local authochthonous populations. This scenario provides a replicated unconscious experiment, mimicking long-distance dispersal and subsequent introgression from a southern lineage (potentially adapted to warmer summer conditions) to a northern population. We will perform genomic scans (RAD-seq) and expression profiling (RNA-seq) for both the authochthonous and the introgressed Bombina populations to discern, on a genomic scale, introgressed alleles/expression patterns from authochthonous ones. In genome-wide association studies (GWAS), we will relate these patterns to fitness related traits (body size, mass, condition). These data will be fed into theoretical models developed by our collaborator (Prof. Stephan) to discern the relative importance of selective sweeps vs. polygenic selection in rapid adaptation. In-depth analysis of candidate genes (including MHC and heat shock protein genes) will unravel the functional classes, in which introgressed (allochthonous) alleles will be favored by selection, and in which ones authochthonous alleles are maintained. This will provide us with a causal understanding of the reason, why introgressed populations of the Fire-bellied toad at the northern edge of the distribution range outperform the non-introgressed ones. Ultimately, the project focusses on the relevance of introgression as a mechanism to increase the amount of standing genetic variation and thus to contribute adaptive genetic material for rapid adaptation.