Intricate evolutionary history ofCallitriche(Plantaginaceae) taxa elucidated by a combination ofDNAsequencing and genome size
Abstract
The widespread aquatic plant genusCallitricheis 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 andtrnT-trnLDNA markers combined with RFLPs of the ITS region were applied in order to unravel the phylogenetic relationships amongCallitrichetaxa 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 EuropeanCallitrichetaxa 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 withinC. stagnalis,C. truncataand North AmericanC. heterophylla.
The origin of selected polyploid taxa was investigated in detail. DiploidC. cophocarpawas confirmed to be the parental species of tetraploidC. platycarpa, but we did not find direct evidence for the putative allopolyploid origin of this species.
The complex ofC. brutiaincluded three taxa; of these,C. hamulatais probably an allooctoploid derivative ofC. brutiavar.brutiaandC. cophocarpa/C. platycarpa. The third member,C. brutiavar.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 fromC. stagnalis. FourCallitrichehybrids were revealed, two of which are newly described and validated here asC. xnyrensisandC. brutianothosubsp.neglecta.
A tentative intrageneric concept of two sections (Callitriche,Pseudocallitriche) is adopted, with the need for a more detailed evaluation in the future.