National Herbarium of NSW, Mrs Macquaries Road, Sydney, NSW, 2000, Australia.
BMC Evol Biol. 2012 Aug 20;12:149. doi: 10.1186/1471-2148-12-149.
Four of the five species of Telopea (Proteaceae) are distributed in a latitudinal replacement pattern on the south-eastern Australian mainland. In similar circumstances, a simple allopatric speciation model that identifies the origins of genetic isolation within temporal geographic separation is considered as the default model. However, secondary contact between differentiated lineages can result in similar distributional patterns to those arising from a process of parapatric speciation (where gene flow between lineages remains uninterrupted during differentiation). Our aim was to use the characteristic distributional patterns in Telopea to test whether it reflected the evolutionary models of allopatric or parapatric speciation. Using a combination of genetic evidence and environmental niche modelling, we focused on three main questions: do currently described geographic borders coincide with genetic and environmental boundaries; are there hybrid zones in areas of secondary contact between closely related species; did species distributions contract during the last glacial maximum resulting in distributional gaps even where overlap and hybridisation currently occur?
Total genomic DNA was extracted from 619 individuals sampled from 36 populations representing the four species. Seven nuclear microsatellites (nSSR) and six chloroplast microsatellites (cpSSR) were amplified across all populations. Genetic structure and the signature of admixture in overlap zones was described using the Bayesian clustering methods implemented in STUCTURE and NewHybrids respectively. Relationships between chlorotypes were reconstructed as a median-joining network. Environmental niche models were produced for all species using environmental parameters from both the present day and the last glacial maximum (LGM).The nSSR loci amplified a total of 154 alleles, while data for the cpSSR loci produced a network of six chlorotypes. STRUCTURE revealed an optimum number of five clusters corresponding to the four recognised species with the additional division of T. speciosissima into populations north and south of the Shoalhaven River valley. Unexpectedly, the northern disjunct population of T. oreades grouped with T. mongaensis and was identified as a hybrid swarm by the Bayesian assignment test implemented in NewHybrids. Present day and LGM environmental niche models differed dramatically, suggesting that distributions of all species had repeatedly expanded and contracted in response to Pleistocene climatic oscillations and confirming strongly marked historical distributional gaps among taxes.
Genetic structure and bio-climatic modeling results are more consistent with a history of allopatric speciation followed by repeated episodes of secondary contact and localised hybridisation, rather than with parapatric speciation. This study on Telopea shows that the evidence for temporal exclusion of gene flow can be found even outside obvious geographical contexts, and that it is possible to make significant progress towards excluding parapatric speciation as a contributing evolutionary process.
在东南澳大利亚大陆，五分之四的 Telopea（山龙眼科）物种呈纬度替代模式分布。在类似的情况下，一个简单的异域物种形成模型被认为是默认模型，该模型确定了遗传隔离在时间地理分离中的起源。然而，分化谱系之间的二次接触可能会导致与地理隔离物种形成（谱系之间的基因流在分化过程中保持不间断）相似的分布模式。我们的目的是利用 Telopea 的特征分布模式来检验它是否反映了异域或地理隔离物种形成的进化模型。使用遗传证据和环境生态位模型相结合的方法，我们集中研究了三个主要问题：目前描述的地理边界是否与遗传和环境边界一致；在密切相关的物种之间的二次接触区域是否存在杂交区；在末次冰期最大冰川期，物种分布是否收缩，导致分布间隙，即使目前存在重叠和杂交？
从代表四个物种的 36 个种群中抽取了 619 个个体，提取了总基因组 DNA。在所有种群中都扩增了 7 个核微卫星（nSSR）和 6 个叶绿体微卫星（cpSSR）。使用 STRUCTURE 中实施的贝叶斯聚类方法和 NewHybrids 分别描述了重叠区的遗传结构和混合特征。使用来自当前和末次冰期（LGM）的环境参数，为所有物种生成了环境生态位模型。nSSR 位点共扩增了 154 个等位基因，而 cpSSR 位点的数据产生了一个由六个 chlorotypes 组成的网络。STRUCTURE 揭示了五个最佳聚类数，对应于四个公认的物种，而 Shoalhaven 河谷以北和以南的 T. speciosissima 种群被进一步划分为两个群体。出乎意料的是，T. oreades 的北部不连续种群与 T. mongaensis 聚集在一起，并通过 NewHybrids 中实施的贝叶斯分配测试被鉴定为杂种群。当前和 LGM 环境生态位模型差异很大，这表明所有物种的分布都因更新世气候波动而反复扩张和收缩，强烈证实了种间存在明显的历史分布间隙。
遗传结构和生物气候建模结果更符合异域物种形成后的多次二次接触和局部杂交的历史，而不是地理隔离物种形成。对 Telopea 的这项研究表明，即使在明显的地理背景之外，也可以找到基因流时间排除的证据，并且有可能在排除地理隔离物种形成作为一个贡献的进化过程方面取得重大进展。
