Genetic and morphological diversity and population structure of a polyploid complex of Mimosa (Leguminosae)

Evolutionary processes in plants such as hybridization and polyploidy are relevant to speciation and little studied in subtropical South America. In subtropical South America, the genus Mimosa is highly diverse, comprising 30% of polyploid species. The origin and genetic and morphological structure of polyploids in Mimosa are only very recently investigated, as well as their relevance to speciation, morphological variation, and distribution. Mimosa subser. Dolentes–Brevipedes, a taxonomic complex from that region, exhibits polyploidy, with specific and infraspecific taxa confusedly circumscribed. Studies of its population biology would facilitate the elucidation of evolutionary processes in Mimosa and resolution of taxonomic conflicts. We performed a multidisciplinary study of this complex, analysing seven populations and 87 individuals by means of morphometry, and AFLP fingerprinting throughout its area of distribution. Five populations included several taxa and intermediate individuals. Morphometric analyses revealed three clusters, distinguished mainly by leaf morphology and inflorescence. One population was tetraploid, one was tetraploid/octoploid and the rest octoploid. Genetic differentiation was high (PhiPT = 0.277) and expected heterozygosity was moderate (He = 0.190; H = 0.271). Multivariate molecular analyses revealed genetic divergence between highland and lowland grasslands. Structural genetic analyses revealed three clusters: two mixed, distinctly predominant in lowland/highland grasslands; another, exclusively from Mercedes. Molecular analysis of variance showed significant differences between highland/lowland grasslands and cytotypes. Cytotypes and populations differed partially by morphology. Tetraploids were morphologically similar to the sympatric octoploids, showing similar genetic structure and gene flow; furthermore, tetraploids are involved in origin of these higher polyploids. The systematics of this complex requires revision: subseries Dolentes and some Brevipedes species could be considered as one subseries. Admixture of genetic clusters and intermediates can be explained by hybridization and introgression. A marginal population differed from the others by morphology and genetics thus suggesting incipient speciation at the geographic edges of this complex.植物的进化过程（如杂交（hybridization）与多倍化（polyploidy））与物种形成（speciation）息息相关，但在南美亚热带区域，此类研究仍较为匮乏。南美亚热带地区的含羞草属（Mimosa）物种多样性极高，其中多倍体物种占比达30%。目前针对该属多倍体的起源、遗传与形态结构，及其与物种形成、形态变异和分布格局的关联，仅在近年才逐步开展相关探索。含羞草属Dolentes–Brevipedes亚系（Mimosa subser. Dolentes–Brevipedes）是该区域的一个分类学复合体（taxonomic complex），该类群存在多倍化现象，且其物种及种下分类单元（infraspecific taxa）的界定长期存在混淆。对其种群生物学展开研究，将有助于阐明含羞草属的进化过程，并解决该类群的分类争议。本研究针对该复合体开展多学科研究：在其整个分布区域内，通过形态计量学（morphometry）与AFLP指纹图谱（AFLP fingerprinting）技术，对7个种群共87个个体进行了分析。其中5个种群包含多个分类单元与中间型个体。形态计量分析结果显示可划分为3个聚类群，主要差异体现在叶片形态与花序特征上。种群倍性情况如下：1个种群为四倍体（tetraploid），1个种群兼具四倍体与八倍体（octoploid），其余种群均为八倍体。该类群的遗传分化程度较高（PhiPT = 0.277），期望杂合度（expected heterozygosity）处于中等水平（He = 0.190；H = 0.271）。多变量分子分析显示，高地草原与低地草原的类群存在显著遗传分化。遗传结构分析则揭示了3个聚类群：2个混合聚类群分别主要分布于低地与高地草原；剩余1个聚类群仅分布于Mercedes地区。分子方差分析（Molecular analysis of variance）显示，高地与低地草原的类群以及不同细胞型之间均存在显著差异。细胞型与种群在形态特征上存在部分分化。四倍体与同域分布的八倍体形态特征相似，且具有相似的遗传结构与基因流（gene flow）；此外，四倍体可能是这类更高倍性的多倍体的起源类群。该复合体的分类系统亟需修订：Dolentes亚系与部分Brevipedes物种可合并为一个亚系。遗传聚类群间的基因混合与中间型个体，可通过杂交与渐渗（introgression）得到合理解释。有一个边缘种群在形态与遗传特征上均与其他种群存在显著差异，这暗示在该复合体的地理分布边缘，可能正在发生初期物种形成。