Trichome morphology provides phylogenetically informative signal for generic delimitation in tribe Marrubieae (Lamiaceae)

Trichomes of 60 taxa of tribe Marrubieae were investigated using light and scanning electron microscopy (SEM) in order to assess their importance in generic delimitation. In general, two main types of trichomes were observed: glandular (capitate, peltate or branched) and non-glandular (simple or branched) trichomes. The results revealed trichome micromorphology as useful for taxon delimitation within Marrubieae at various ranks, especially at the generic level in accordance with molecular investigations. Among the characters investigated, regular (non-peltate) glandular trichomes with capitate heads (stalked or sessile) and short simple trichomes were common among all studied species. Peltate glandular trichomes were also common except for Acanthoprasium. Long simple non-glandular trichomes were apomorphic and were found in all species of Ballota. In some species of Pseudodictamnus the glandular trichomes were branched. This condition is likely derived, and might be useful in subgeneric classification. Additional character state assessments in Marrubieae in terms of trichome morphology include: long trichomes are apomorphic versus the short ones, branched trichomes are apomorphic versus the simple ones, smooth surfaces of trichomes are apomorphic versus papillate surfaces, and peltate glandular trichomes are apomorphic with respect to the non-peltate ones.