Pollen and trichome morphology of tribe Stachydeae (Lamiaceae) and itsphylogenetic significance

Abstract

: Tribe Stachydeae with 12 genera were divided into two major clades: Melitis and Eurystachys clades consisting of all genera attributed to Stachydeae except Melittis . Recently, a broadly defined Eurystachys clade falling into 12 well-supported clades have been formally named in a phylogenetic nomenclature. In the present study, trichomes of 70 spp. from nine genera and pollens of 53 spp. from six genera were investigated using scanning electron microscopy (SEM) in order to draw the taxonomic conclusion of tribe Stachydeae. Trichome characters that can provide adequate variation in delimitation of taxa were: presence or absence of glandular or nonglandular trichomes, number of cells (uni-/bi-/multicellular), presence of extremely long trichomes (i.e. more than 1000 μm), presence of branched trichomes as well as type of glandular trichomes (peltate or capitate with three subtypes: sessile to subsessile, short-stalked and long-stalked trichomes). Among the investigated characters, extremely long simple trichomes and branched trichomes were apomorphic and were found in Eriostomum and Marrubiastrum clades, respectively. In addition, number of colpi, type of sculpturing as well as shape of lumina as the main features of the investigated pollen grains afforded valuable characters especially in delimitation of several natural groups, i.e. Eriostomum clade, Empedoclea clade, Sideritis clade, Marrubiastrum clade, and Swainsoniana clade. Moreover, a close relationship among two annual clades of Sideritis each of represents a section, i.e. Hesiodia and Burgsdorfia , is suggested by our results. However, in both Stachys and Olisia clades which are morphologically heterogeneous, type of sculpturing shows variation and mostly useful in species discrimination. Overall, trichome and pollen characters can be useful in describing the natural groups within tribe Stachydeae along with other reliable morphological characters.