Evolution of trichome types and its systematic significance in the genus Phlomoides (Lamioideae - Lamiaceae)

11 February 2019

Eyvazzadeh Khosroshahi, Elmira; Salmaki, Yasaman

Despite the increasing number of molecular phylogenetic studies conducted recently on Phlomoides, this genus is still among the poorly studied taxa in the family Lamiaceae in terms of trichome morphology. In order to test the utility of trichome characters in delimitation of sections, subsection and species, trichomes of 64 species of the genus Phlomoides representing all recognized sections and subsections were examined using stereomicroscopy and scaning electron microscopy. Two basic types of trichomes can be identified: non-glandular and glandular. Both trichome types can be simple or branched. The glandular trichomes were sessile, short stalked or long stalked. Different kinds of branched trichomes were observed in most species of sect. Phlomoides, i.e. symmetrically stellate, stellate with central long branch, bi- or trifurcate. The species of sect. Filipendula were mostly covered by simple trichomes. Moreover, variation in trichome characters appears to have particular value, not only in classification at sectional or subsectional rank, but also in delimitation of species within each section. For example, all studied species of subsect. Fulgentes are characterized by various kinds of stellate trichomes, while the trichome variability in subsect. Tetragonae was sufficiently high for species discrimination. An ancestral character state reconstruction was performed in order to investigate the evolution of trichome types revealing the following evolutionary trends in trichome characters of Phlomoides: (1) branched trichomes are primitive in Phlomoides against the simple ones, (2) long simple non-glandular trichomes are derived against short simple ones, and (3) presence of stalked glandular trichomes is advanced against subsessile or sessile ones.