The three Hawaiian endemic mint genera, Haplostachys, Phyllostegia, and Stenogyne (Lamiaceae), had been thought to be closely related to East Asian members of tribe Prasieae because of the fleshy nutlets borne by the latter two genera. Phylogenetic analyses of chloroplast rbcL and 5S nuclear ribosomal non-transcribed spacer (5S-NTS) sequences supported the monophyly of the Hawaiian mints and indicated that a North American Stachys species was a closer relative. We present further refinement of this hypothesis based on a phylogenetic survey of North American Stachys 5S-NTS sequences. As previously found, the Hawaiian clade consists of Haplostachys sister to Phyllostegia plus Stenogyne, which are unresolved with respect to each other. However, the Hawaiian taxa, which are characterized by chromosome number 2n=64 or 66, are deeply nested inside North American Stachys, their closest relative being Stachys rigida, a strictly western North American species with 2n=66. Stachys rigida plus the Hawaiian mints are related to a larger clade of Stachys species, some of which are found mainly in the southeastern part of the United States and have 2n=34 and/or 68, whereas others are endemics of western North America with 2n=66. In turn, this larger group is sister to two clades representing taxa from Texas through Mesoamerica, one of which has chromosome number 2n=32, the other 2n=80-84. The relationships between North American and Eurasian Stachys taxa are still unclear but preliminary rbcL data suggest that they may represent separate lineages. The chromosome number 2n=66 recorded for Stachys rigida, other western Stachys, and some Hawaiian mints has previously been explained by centric fusion from 2n=68, but could instead be indicative of allopolyploidy between n=32 and n=34, which in turn could help explain the radiative evolution of morphological features in the Hawaiian endemic mints.

Key words: Hawaiian mints, molecular systematics, morphological radiation, polyploidy, Stachys