A genome size and phylogenetic survey of Mediterranean Tripleurospermum and Matricaria (Anthemideae, Asteraceae)

The study of genome size variation can contribute valuable information on species relationships as well as correlate to several morphological or ecological features, among others. Here we provide an extensive report on genome sizes on genus Tripleurospermum and its closely related genus Matricaria, which are two typically Mediterranean genera particularly widespread and diverse in Turkey, the origin of most of the populations here studied. We analyse and discuss genome size variation in the first relatively complete molecular phylogenetic framework of Tripleurospermum (based on ITS and ETS ribosomal DNA–rDNA–regions). We find cases of intraspecific genome size variation, which could be taxonomically significant. Genome downsizing is also detected as the typical response to polyploidisation in Tripleurospermum taxa, being most conspicuous at the tetraploid level. Several positive correlations with genome size, including those with pollen and stomatal size or cypsela length, among others, are also found. Remarkably, taxa presenting rhizomes tend to present higher genome sizes, confirming a trend to accumulate nuclear DNA in such species, which could be explained by the nutrient reserves availability in their storage organs, allowing genome expansion, or by the lower rates of sexual reproduction in rhizomatous taxa.

基因组大小变异的研究可为物种亲缘关系解析提供重要参考，同时亦可与多种形态或生态特征等建立关联。本研究针对三肋果属（Tripleurospermum）及其近缘属母菊属（Matricaria）的基因组大小展开系统性报道；这两个典型的地中海类群在土耳其分布广泛且类群丰富，本研究多数供试种群均源自该区域。我们基于ITS和ETS核糖体DNA（rDNA）区域，构建了三肋果属首个相对完整的分子系统发育框架，并在此框架内分析、讨论其基因组大小变异情况。研究发现存在具有分类学研究价值的种内基因组大小变异现象。同时在三肋果属类群中，检测到多倍化事件典型伴随的基因组缩减现象，该现象在四倍体类群中最为显著。本研究还发现基因组大小与多项指标存在显著正相关，包括花粉与气孔大小、连萼瘦果长度等。值得注意的是，具根状茎的类群往往拥有更大的基因组尺寸，这印证了此类物种倾向于积累核DNA的趋势；该趋势可通过两类机制解释：一是其储藏器官可提供充足的营养储备，支持基因组扩张，二是根状茎类群的有性生殖速率相对更低。