Notes on Isoglossinae (Acanthaceae) in Madagascar, with four new species of Isoglossa

Current generic delimitation in subtribe Isoglossinae of Acanthaceae is discussed using available morphological and molecular data. Particular consideration is given to the small genera endemic to Madagascar — Celerina Benoist, Melittacanthus S.Moore, Sphacanthus Benoist and the illegitimate Conocalyx Benoist — and how these relate to the current circumscription of Isoglossa Oerst. The relative merits and challenges of recognising a broadly circumscribed Isoglossa are considered. Isoglossa s.l. is readily recognisable by combining a bilabiate corolla, with an androecium comprising two bithecous stamens and lacking staminodes, and with the presence of gürtelpollen (i.e. bipororate pollen, circular in apertural view and with a pronounced interapertural “girdle” of tectate exine), whilst the segregate genera as currently defined are not separable morphologically. However, the limited molecular phylogenetic data available to date indicate that Isoglossa s.l. is paraphyletic. Pending further molecular studies of this group to clarify generic delimitation, it is proposed that all new taxa in bithecous Isoglossinae should be described in Isoglossa as an interim solution. Four new species of Isoglossa are described: I. eburnea Onjalal. & I.Darbysh., I. pterocalyx I.Darbysh. & Phillipson, I. pseudoanisotes I.Darbysh. and I. conocalyx I.Darbysh. & Callm., the lattermost of which is based on Conocalyx laxus Benoist nom. illegit. In addition, Justicia onilahensis Benoist (including Ritonia poissonii Benoist) and Anisotes perplexus T.F.Daniel, Letsara & Martín-Bravo are transferred to Isoglossa, and three names within Justicia L. are synonymised within existing taxa in Isoglossinae. Lectotypes are selected for Sphacanthus brillantaisia Benoist, S. humbertii Benoist and Justicia subpaniculata Benoist (=Celerina seyrigii Benoist var. egena Benoist).