Phylogenetics and population structure of the western North American endemic Pacific Laurasian clade of Isoëtes

Premise

Isoëtes is a genus of small, semi-woody, hydrophilic, heterosporous lycophytes with a cosmopolitan, global distribution. However, local populations tend to be found in narrow, patchy, and highly fragmented mesic to aquatic habitats, many of which are currently under threat. In this study, we sought to uncover how this patchy distribution has affected the evolutionary history of one of the two lineages of Isoëtes found on the West Coast of North America—the Pacific Laurasian clade (PLC).

Methods

We used a combination of population genetic and multilocus molecular phylogenetic approaches to infer the relationships among the three described species in this clade and to determine the degree of isolation among the sampled populations.

Results

We discovered that the populations studied are highly structured and that two of the species, as currently circumscribed, are not monophyletic. Instead, our phylogenetic results suggest that there are at least eight distinct “species-level” clades within the PLC. Of these eight, five appear to have been the result of a rapid radiation.

Conclusions

Our results suggest that the existing taxonomy does not reflect the actual diversity in the PLC and warrants further investigation.