Nomenclatural and taxonomic changes within the Leucas clade (Lamioideae; Lamiaceae)

Ongoing molecular studies of the subfamily Lamioideae have demonstrated that several genera do not correspond to monophyletic groups as currently circumscribed. Leucas R.Br. and Otostegia Benth. are particularly problematic in this respect. To confront the issue of monophyly taxonomically, the genus Rydingia A.-C.Scheen & V.A.Albert is described here with four species formerly assigned to Otostegia. The number of species of Moluccella L. is increased to three with the recognition of Moluccella aucheri (Boiss.) A.-C.Scheen which is transferred from Otostegia. The number of species of Isoleucas O.Schwartz is increased to two with the recognition of Isoleucas somala (Patzak) A.-C.Scheen, which is transferred from Otostegia. It was also necessary to distinguish two different groups of Leucas: 1) Leucas sensu stricto and 2) the "African Leucas clade", which preliminarily may be referred to as Leonotis (Pers.) R.Br. sensu lato due to nomenclatural priority.