Origin of the Hawaiian endemic mints within North American Stachys (Lamiaceae)

Abstract

The Hawaiian endemic mints constitute a major island radiation, displaying a remarkable diversity of floral, fruit, and vegetative features. Haplostachys and Phyllostegia have flowers associated with insect pollination, whereas Stenogyne has flowers typical of bird pollination. The three genera had been thought to be closely related to East Asian members of Lamioideae tribe Prasieae because of the fleshy nutlets borne by Phyllostegia and Stenogyne. We evaluated the origins of the Hawaiian mints using phylogenetic analyses of DNA sequence data from the plastid rbcL and trnL intron loci and the nuclear ribosomal 5S nontranscribed spacer. The Hawaiian genera were found to be monophyletic but deeply nested inside another lamioid genus, Stachys. In particular, they were found to be most closely related to a group of temperate North American Stachys from the Pacific coast, suggesting that the Hawaiian mints derived from a single colonization event from western North America to the Hawaiian Islands. Furthermore, Stachys, which contains amphiatlantic and transberingian clades, was found to be polyphyletic, with some species more closely related to Gomphostemma, Phlomidoschema, Prasium, and Sideritis than to other species of Stachys. Based on chromosomal evidence and our phylogenetic analyses, we hypothesize that the Hawaiian mints may be polyploid hybrids whose reticulate genomes predate the Hawaiian dispersal event and are derived from Stachys lineages with flowers exhibiting insect- vs. bird-pollination characteristics. Thus, the Hawaiian endemic mints may provide yet another insular system for the combined study of polyploidy, hybrid cladogenesis, and adaptive radiation.