Department of Biological Sciences, University of Rhode Island, Kingston, RI, USA.
Department of Biological Sciences, Louisiana State University, Baton Rouge, LA, USA.
J Evol Biol. 2021 Jan;34(1):128-137. doi: 10.1111/jeb.13731. Epub 2020 Nov 20.
The distributions of many sister species in the sea overlap geographically but are partitioned along depth gradients. The genetic changes leading to depth segregation may evolve in geographic isolation as a prerequisite to coexistence or may emerge during primary divergence leading to new species. These alternatives can now be distinguished via the power endowed by the thousands of scorable loci provided by second-generation sequence data. Here, we revisit the case of two depth-segregated, genetically isolated ecotypes of the nominal Caribbean candelabrum coral Eunicea flexuosa. Previous analyses based on a handful of markers could not distinguish between models of genetic exchange after a period of isolation (consistent with secondary contact) and divergence with gene flow (consistent with primary divergence). Analyses of the history of isolation, genetic exchange and population size based on 15,640 new SNP markers derived from RNAseq data best support models where divergence began 800K BP and include epochs of divergence with gene flow, but with an intermediate period of transient isolation. Results also supported the previous conclusion that recent exchange between the ecotypes occurs asymmetrically from the Shallow lineage to the Deep. Parallel analyses of data from two other corals with depth-segregated populations (Agaricia fragilis and Pocillopora damicornis) suggest divergence leading to depth-segregated populations may begin with a period of symmetric exchange, but that an epoch of population isolation precedes more complete isolation marked by asymmetric introgression. Thus, while divergence-with-gene flow may account for much of the differentiation that separates closely related, depth-segregated species, it remains to be seen whether any critical steps in the speciation process only occur when populations are isolated.
许多姐妹物种在地理上的分布重叠，但在深度梯度上被分隔开。导致深度隔离的遗传变化可能是在地理隔离中进化的，作为共存的先决条件，也可能是在导致新物种的主要分歧过程中出现的。现在，通过第二代测序数据提供的数千个可评分位点赋予的力量，可以区分这些替代方案。在这里，我们重新审视了两个深度隔离、遗传上隔离的加勒比烛台珊瑚 Eunicea flexuosa 生态型的情况。以前基于少数标记的分析无法区分隔离后的遗传交换模型（与二次接触一致）和有基因流的分歧模型（与主要分歧一致）。基于从 RNAseq 数据中衍生的 15640 个新 SNP 标记的隔离、遗传交换和种群大小历史分析，最好支持这样的模型，即分歧始于 800K BP，并包括有基因流的分歧时期，但中间有一个短暂隔离的时期。结果还支持了以前的结论，即最近这两个生态型之间的交流是从浅系线到深系线不对称发生的。对另外两个具有深度隔离种群的珊瑚（Agaricia fragilis 和 Pocillopora damicornis）的数据进行的平行分析表明，导致深度隔离种群的分歧可能始于一个对称交换的时期，但在更完全的隔离之前，有一个种群隔离的时期，其标志是不对称的基因渗入。因此，虽然带有基因流的分歧可能解释了将密切相关的、深度隔离的物种分开的大部分分化，但在种群隔离之前，物种形成过程中的任何关键步骤是否只发生，还有待观察。
