Departamento de Botânica, Instituto de Biociências, Universidade de São Paulo, São Paulo, SP, Brazil.
Mol Ecol. 2010 Sep;19(18):3981-94. doi: 10.1111/j.1365-294X.2010.04780.x. Epub 2010 Aug 27.
The hypothesis of gene flow between species with large differences in chromosome numbers has rarely been tested in the wild, mainly because species of different ploidy are commonly assumed to be reproductively isolated from each other because of instantaneous and strong postzygotic barriers. In this study, a broad-scale survey of molecular variation was carried out between two orchid species with different ploidy levels: Epidendrum fulgens (2n = 2x = 24 chromosomes) and Epidendrum puniceoluteum (2n = 4x = 52 chromosomes). To test the strength of their reproductive barriers, we investigated the distribution of genetic variation in sympatric and allopatric populations of these two species and conducted crossing experiments. Nuclear and plastid microsatellite loci were used to genotype 463 individuals from eight populations across the geographical range of both species along the Brazilian coastal plain. All six sympatric populations analysed presented hybrid zones, indicating that hybridization between E. fulgens and E. puniceoluteum is a common phenomenon. Bayesian assignment analysis detected the presence of F(1) and F(2) individuals and also signs of introgression, demonstrating a high potential for interspecific gene flow. Introgression occurs preferentially from E. fulgens to E. puniceoluteum. Pure parental individuals of both species display strong genotype-habitat associations, indicating that environment-dependent selection could be acting in all hybrid zones. This study suggests that hybridization and introgression are evolutionary processes playing a role in the diversification of Epidendrum and indicates the importance of investigations of hybrid zones in understanding reproductive barriers and speciation processes in Neotropical orchid species.
物种间染色体数目差异较大的基因流动假说在野外很少得到检验，主要是因为不同倍性的物种通常被认为是彼此生殖隔离的，因为它们存在即时且强大的合子后障碍。在这项研究中，对两个具有不同倍性的兰科植物物种之间的分子变异进行了广泛的调查：Epidendrum fulgens（2n = 2x = 24 条染色体）和 Epidendrum puniceoluteum（2n = 4x = 52 条染色体）。为了检验它们生殖障碍的强度，我们调查了这两个物种在同域和异域种群中的遗传变异分布，并进行了杂交实验。使用核和质体微卫星标记对来自巴西沿海平原两个物种地理分布范围内的 8 个种群的 463 个个体进行了基因型分析。所有 6 个分析的同域种群都呈现出杂交带，表明 E. fulgens 和 E. puniceoluteum 之间的杂交是一种常见现象。贝叶斯分配分析检测到了 F1 和 F2 个体的存在以及基因渐渗的迹象，表明种间基因流动的潜力很高。渐渗发生的方向优先从 E. fulgens 到 E. puniceoluteum。两个物种的纯亲代个体表现出强烈的基因型-生境关联，表明环境依赖的选择可能在所有杂交带中起作用。本研究表明，杂交和渐渗是进化过程，在 Epidendrum 的多样化中发挥作用，并表明在理解新热带兰花物种的生殖障碍和物种形成过程中，对杂交带进行调查的重要性。
