Alvarez Inés, Wendel Jonathan F
Real Jardín Botánico de Madrid, Consejo Superior de Investigaciones Científicas, Madrid, Spain.
Evolution. 2006 Mar;60(3):505-17.
Interspecific gene flow is increasingly recognized as an important evolutionary phenomenon in plants. A surprising observation is that historical introgression is often inferred between species that presently have geographic and reproductive barriers that would appear to prohibit the inferred sexual exchange. A striking example concerns Gossypium aridum (subsection Erioxylum); previous analyses have shown that populations from Colima (southwestern Mexico) have a chloroplast genome (cpDNA) similar to that of a different taxonomic subsection (Integrifolia) that presently is confined to Baja California and the Galapagos Islands, whereas other G. aridum populations share a cpDNA lineage with each other and with other species in subsection Erioxylum. To evaluate further the possibility that this cpDNA evidence reflects introgression as opposed to some other evolutionary process, as well as to explore patterns of genetic diversity and similarity in both subsections, we conducted amplified fragment-length polymorphism (AFLP) analysis using 50 populations representing all seven species in the two subsections. Genetic diversity is high in G. aridum, and is strongly correlated with geography, as are similarities among the five species in subsection Erioxylum. This subsection is genetically distant from the two species in subsection Integrifolia, whose populations are highly similar inter se. Populations of G. aridum from Colima are genetically distinct from the remainder of the species, and exhibit a comparatively high frequency of AFLP fragments that otherwise are diagnostic of the Integrifolia lineage. These data implicate intersubsectional introgression between presently allopatric and genetically isolated clades, giving rise to a morphologically cryptic, introgressant entity. Biogeographic considerations suggest that this history was initiated following migration of one or more seeds from Baja California to the Colima coast, perhaps during the Pleistocene. We suggest that cryptic and seemingly improbable interspecific introgression and molecular differentiation may be more common than appreciated in angiosperm evolution.
种间基因流动日益被视为植物中一种重要的进化现象。一个令人惊讶的观察结果是，常常能推断出在目前存在地理和生殖障碍的物种之间存在历史渐渗，而这些障碍似乎会阻止所推断的有性交换。一个显著的例子涉及旱地棉（棉属刺叶棉亚组）；先前的分析表明，来自科利马（墨西哥西南部）的种群具有与另一个不同分类亚组（全缘叶棉亚组）相似的叶绿体基因组（cpDNA），而全缘叶棉亚组目前局限于下加利福尼亚和加拉帕戈斯群岛，然而其他旱地棉种群彼此之间以及与刺叶棉亚组中的其他物种共享一个cpDNA谱系。为了进一步评估这种cpDNA证据反映渐渗而非其他某种进化过程的可能性，以及探索两个亚组中的遗传多样性和相似性模式，我们使用代表两个亚组中所有七个物种的50个种群进行了扩增片段长度多态性（AFLP）分析。旱地棉的遗传多样性很高，并且与地理因素密切相关，刺叶棉亚组中的五个物种之间的相似性也是如此。这个亚组在遗传上与全缘叶棉亚组中的两个物种相距甚远，而全缘叶棉亚组的种群彼此之间高度相似。来自科利马的旱地棉种群在遗传上与该物种的其余部分不同，并且表现出相对较高频率的AFLP片段，而这些片段在其他情况下是全缘叶棉谱系的诊断特征。这些数据表明目前异域分布且遗传隔离的分支之间存在亚组间渐渗，从而产生了一个形态上难以区分的渐渗实体。生物地理学方面的考虑表明，这段历史始于一粒或多粒种子从下加利福尼亚迁移到科利马海岸之后，可能发生在更新世。我们认为，隐秘且看似不太可能的种间渐渗和分子分化在被子植物进化中可能比人们所认识到的更为普遍。
