Allohexaploid speciation of the two closely related species Myriophyllum spicatum and M-sibiricum (Haloragaceae)

Myriophyllum L. (Haloragaceae R.By.) is a cosmopolitan aquatic angiosperm with ca. 60 species. Polyploids are not uncommon in this genus, but there has been little research conducted on their polyploidization. In this study, molecular markers were used to examine the speciation of the two closely related species, Myriophyllum spicatum and Myriophyllum sibiricum. Three distinct types of sequences were detected from each individual of the two species using the low copy nuclear gene G3pdh. The results indicate that the two species M. spicatum and M. sibiricwn were both allohexaploids, and that they were formed by the hybridization and polyploidization of the different diploid parental lineages. In contrast, Myriophyllum alterniflorum formed an independent Glade, suggesting that it was not their diploid ancestor. The chloroplast DNA (cpDNA) data showed that the two allopolyploids were both monophyletic and signaled the possible combination in the hybridization. Quaternary climatic oscillations might have had a great effect on the speciation and dispersal of the allohexaploids. The polyploidy highly likely contributed to the current cosmopolitan distribution. Moreover, we found evident concerted evolution of the nuclear ribosomal Internal Transcribed Spacer (nrITS) region towards the maternal lineage in these two allopolyploids. Myriophyllum seems to be a good model to explore this concerted evolution.