Molecular phylogenetic analysis of Ceratophyllum L. taxa: a new perspective

Ceratophyllum (Ceratophyllaceae) is among the most enigmatic of angiosperm taxa, with its phylogenetic placement changing almost continuously over the last few decades. Ceratophyllum spp. are difficult to identify using vegetative morphology alone, which is commonly all that is available for these highly clonal plants. A molecular phylogenetic approach was used to examine relationships in the genus and provide molecular markers to facilitate the identification of Ceratophyllum spp. This study included all known Ceratophyllum spp. that have been distinguished in the last two taxonomic treatments of the genus. Sequence data for ITS and matK were used to examine phylogenetic relationships among species. The molecular analyses readily distinguish five clades which correspond taxonomically to C. echinatum, C. demersum, C. australe (including C. tanaiticum), C. submersum and C. muricatum (including C. muricatum subsp. muricatum and C. m. subsp. kossinskyi). In these analyses, accessions of the morphologically distinct C. platyacanthum were not clearly differentiated from some accessions of C. demersum, perhaps as a consequence of the probable polyploid origin of this taxon. Overall, the molecular data disagree with some previous studies based on morphology by indicating (1) the presence of more than two species in the genus, (2) that C. echinatum is not the closest relative of C. submersum, (3) that C. muricatum is related more closely to C. submersum than to C. australe and (4) that C. tanaiticum is resolved in a well-supported clade with C. australe, which is distinct from accessions of C. muricatum. Although C. tanaiticum and C. australe fall in the same clade in the current analysis, their high level of genetic divergence, extreme geographical isolation and significantly different morphologies support their recognition as distinct species. We also discuss the chorological and palaeobotanical aspects of the new results.