The widespread aquatic plant genus Callitriche is taxonomically very challenging, but noteworthy in many evolutionary aspects including a high overall diversity, extensive phenotypic plasticity, remarkable reproductive systems and a large variation in ploidy levels and chromosome numbers. We conducted a multi‐level systematic study on 346 individuals of 25 taxa from 21 mostly European countries. Flow cytometric estimation of genome size, chromosome counting and direct sequencing of ITS and trnT‐trnL DNA markers combined with RFLPs of the ITS region were applied in order to unravel the phylogenetic relationships among Callitriche taxa and to clarify the origin of polyploid species and hybrids. Additionally, ITS sequences from a recent worldwide phylogenetic study of the genus were included for comparison. We demonstrate that most of the traditionally recognized European Callitriche taxa are well defined by a combination of genome size and molecular markers. Several species showed remarkable intraspecific genetic variation; previously unknown cryptic taxa were revealed within C. stagnalis, C. truncata and North American C. heterophylla. The origin of selected polyploid taxa was investigated in detail. Diploid C. cophocarpa was confirmed to be the parental species of tetraploid C. platycarpa, but we did not find direct evidence for the putative allopolyploid origin of this species. The complex of C. brutia included three taxa; of these, C. hamulata is probably an allooctoploid derivative of C. brutia var. brutia and C. cophocarpa/C. platycarpa. The third member, C. brutia var. naftolskyi, was newly reclassified at the subspecies level; for the first time, chromosome numbers are provided for this poorly known taxon. For a single triploid sample, our results suggested an autopolyploid origin from C. stagnalis. Four Callitriche hybrids were revealed, two of which are newly described and validated here as C. ×nyrensis and C. brutia nothosubsp. neglecta. A tentative intrageneric concept of two sections (Callitriche, Pseudocallitriche) is adopted, with the need for a more detailed evaluation in the future.

中文翻译：

---

DNA测序和基因组大小的结合阐明了Callitriche（Plantaginaceae）类群的复杂进化史

广泛的水生植物属Callitriche在分类学上非常具有挑战性，但在许多进化方面值得关注，包括较高的总体多样性，广泛的表型可塑性，出色的生殖系统以及倍性水平和染色体数目的巨大差异。我们对来自21个主要欧洲国家的346个25类群的个体进行了多层次的系统研究。应用流式细胞术估计基因组大小，ITS的染色体计数以及ITS和trnT-trnL DNA标记与ITS区的RFLP结合直接测序，以阐明Callitriche之间的系统发育关系并明确多倍体物种和杂种的起源。此外，还包括了该属最近的全球系统发育研究中的ITS序列，用于比较。我们证明，大多数传统上公认的欧洲Callitriche类群通过基因组大小和分子标记的组合得到了很好的定义。几个物种显示出明显的种内遗传变异。以前未知的隐隐类目被发现于葡萄角螺，特兰卡角和北美异头藻中。详细研究了选定的多倍体类群的起源。二倍体C. cophocarpa被证实是四倍体C. platycarpa的亲本种，但我们没有找到该物种推测的同源多倍体起源的直接证据。布鲁氏梭菌的复合体包括三个分类单元。其中，Hamulata C.可能是C. brutia var的同种异种倍体衍生物。brutia和C. cophocarpa / C. platycarpa。第三名成员C. brutia var。naftolskyi，在亚种水平上进行了新的重新分类；首次为这个鲜为人知的分类单元提供了染色体编号。对于单个样品三倍体，我们的结果表明从多倍体的起源C. stagnalis。四Callitriche揭示了杂种，其中两个新描述并在此处验证为C.×nyrensis和C. brutia nothosubsp。忽视。暂时采用两部分（Callitriche，Pseudocallitriche）的类内概念，将来需要进行更详细的评估。