Ecological speciation in palms on an oceanic island

两个多世纪以来，物种多样性的起源一直困扰着生物学家。查尔斯达尔文认识到，异地繁殖是物种形成的驱动力，但他也认为，在没有地理隔离的情况下，种群可能会分化为不同的物种，这种机制现在被称为共域物种形成。在过去的十年里，数学模型已经表明，同域物种形成在理论上是可能的，但在自然界中记录的例子非常少。今年早些时候，Savolainen和他的同事(这项提议的一些申请者之一)在对两种棕榈树的案例研究中提供了共地物种形成的完整证据，这两种棕榈树是偏远的豪恩岛(LHI)特有的棕榈树(“海洋岛屿上棕榈树的共地物种形成”，自然杂志441：210-213,2006)。在这里，我们建议将我们的研究带到一个更深的水平。我们的目标是在我们的棕榈树案例研究中记录物种分歧的基因组结构和遗传基础，从而带来新类型的数据来测试共生关系中分歧的替代方案，并解开一定在这个独特的LHI系统中运行的生态物种形成的机制。LHI是一个不到12平方公里的亚热带小岛，位于澳大利亚东海岸580公里处。其植物区系由241种维管植物组成，其中105种为特有植物。在LHI上，地理隔离是不现实的，因此它是测试共域物种形成的四个标准的理想地点：1)物种共生，2)姐妹关系，3)生殖隔离，4)较早的异地阶段极不可能。许多植物属，如棕榈科的Howea，由岛上不止一个特有物种代表，很可能是共域物种形成的产物。在我们之前对Howea Palms的实验中，我们发现物种分化的遗传特征表明，在中性条件下，只有四个基因座在两个物种之间的差异比预期的更大，并且这些基因最有可能与共面物种形成过程中受到差异选择的基因相关联。为了在这个独特的岛屿系统中描述Howea生态物种形成的基因组和遗传结构，我们将进行六项活动：a)使用更全面的AFLP基因组扫描，在Howea中识别可能被选择的基因组中最分散的标记基因和so区域；b)使用这些分散的基因座作为BAC文库上的探针；c)使用Kania Palm的BAC序列作为模板，在卷叶棕榈树的基因组同源序列和开放阅读框架(ORF)中鉴定可能包含与物种分化有关的基因的部分(测试Gavrilets对Howea中只有几个基因座参与物种形成的预测)，它可能包含正选择的特征d)确定不同的基因组片段在两个棕榈树物种的基因组中是集中的还是广泛分布的(测试费尔森斯坦关于低重组促进物种形成的经典预测)。E)使用合并模型和多个位点，评估Howea物种分化期间基因流动是否中断，并估计中断的程度。F)与油棕榈树的EST文库相比，为在选择下发现的任何ORF提供可能的功能。

The origin of species diversity has challenged biologists for over two centuries. Charles Darwin recognised that allopatry, species divergence resulting from geographical isolation, is a driving force in speciation, but he also thought that populations could diverge into separate species in the absence of geographical isolation, a mechanism now called sympatric speciation. During the last decade, mathematical models have shown that sympatric speciation is theoretically possible, but extremely few examples have been documented in nature. Early this year, Savolainen and colleagues (some of the applicants of this proposal) provided complete evidence for sympatric speciation in a case study of two species of palm, Howea forsteriana and H. belmoreana, endemic of the remote Lord Howe Island (LHI) ('sympatric speciation in palms on an oceanic island', Nature 441: 210-213, 2006). Here we propose to take our research to a much deeper level. Our goal is to document the genomic architecture and genetic basis for species divergence in our case study of palms, thus bringing new types of data to test alternatives to divergence in sympatry and disentangle the mechanisms of ecological speciation that must have operated in this unique LHI system. LHI is a minute subtropical island of less than 12 km2, situated 580 km off the eastern coast of Australia. Its flora comprises 241 vascular plant species of which 105 are endemic. Geographic isolation is not realistically possible on LHI and thus it is an ideal site on which to test the four criteria for sympatric speciation: 1) species sympatry, 2) sister relationships, 3) reproductive isolation, and 4) that an earlier allopatric phase is highly unlikely. Numerous plant genera, like the palms Howea, are represented by more than one endemic species on the island, which may well be products of sympatric speciation. In our previous experiments of the Howea palms, we found that the genetic signature of species divergence indicated that only four loci differ more strongly between the two species than expected under neutrality and these are those most likely to be linked to genes subject to divergent selection during sympatric speciation. To characterise the genomic and genetic architecture underlying the ecological speciation of Howea in this unique island system, we will conduct six activities: a) Using a more comprehensive AFLP genome scan, identify in Howea the most divergent marker loci and so regions of the genome that may be under selection b) Using these divergent loci as probes on a BAC library, characterise portions of the kentia palm's genome that may contain genes involved in species divergence (testing Gavrilets' prediction of only a few loci being involved in speciation in Howea) c) Using the BACs sequences of the kentia palm as template, identify in the curly palm's genome homologous sequences and open reading frames (ORFs), which may contain the signature of positive selection d) Determine whether the divergent genomic segments are concentrated versus widely distributed in the genomes of the two palm species (testing Felsenstein's classic prediction of speciation being facilitated by low recombination). e) Using coalescence models and multiple loci, evaluate whether gene flow has been interrupted / and estimate the extent of disruption / during species divergence in Howea. f) In comparison with an EST library for the oil palm, provide possible functions for any ORFs found under selection.