Phylogenetic Relationships in the Heterosporous Fern Genus Azolla (Azollaceae) Based on DNA Sequence Data from Three Noncoding Regions

The taxonomic history of Azolla (Azollaceae) is long and complex. Previous studies have employed morphological, cytological, and molecular data in an attempt to circumscribe the seven extant species, but none has been completely successful. In this study, we employ DNA sequence data from three noncoding regions, two derived from the plastid genome (the atpB-rbcL and trnL-trnF regions) and a third from the internal transcribed spacers of the nuclear rRNA genes. Cladistic analyses of these data confirm the division of Azolla into two major clades, corresponding to the traditional classification of the genus into sections Azolla and Rhizosperma. Moreover, the monophyly of A. pinnata plus A. nilotica (sect. Rhizosperma) contradicts newer classifications of the family, in which these species were placed in different subgenera. In section Azolla, DNA sequence data support several past reports in suggesting that A. rubra and A. filiculoides are distinct species and that A. caroliniana is distinct from both A. microphylla and A. mexicana. However, distinct lineages representing A. microphylla and A. mexicana were not found, and these plants appear to represent a single evolutionary lineage.