Molecular systematics of southern African monkey orange Strychnos L. (Loganiaceae)

Abstract

Strychnos is the largest genus of the Loganiaceae with about 200 species distributed across Africa (including Madagascar), the Americas, Australia and Asia. Recent molecular phylogenetic effort at elucidating relationships globally provided a useful overview for the genus, based on the internal transcribed spacer (ITS) data. However, an understanding of evolutionary and ecological patterns at regional levels is better served by fine scale phylogenetic analysis to resolve species complexes for conservation and allied reasons. In this study, we use plastid (trnL-trnF, trnS-trnG) and nuclear ribosomal ITS sequence data to infer phylogenetic patterns among members of southern Africa Strychnos. We also evaluate sectional validity of the current classification for African members with ITS sequence data. Our findings support the monophyly of Strychnos, although several of the sections are not monophyletic, thus raising the need for sectional reappraisal of the genus. S. xantha is sister to our expanded representation of southern African taxa, while S. aculeata is sister to all African taxa. The uncertain relationships among S. innocua, S. madagascariensis and S. gerrardii were partly resolved in the phylogenetic analysis of combined datasets. S. innocua is sister to the other two species in a well-supported clade. S. gerrardii and S. madagascariensis are also sister taxa that are not yet reciprocally monophyletic, but possess other features to distinguish them. The trnS-trnG marker revealed a relatively large indel event of potential taxonomic value in section Densiflorae. There is also a clear ecological signal in both the plastid and nuclear datasets, as both consistently placed forest species at the base of the phylogeny, and savanna species in more derived positions. We addressed the adaptive significance of such signals as they relate to fruit and growth forms, and submit proposals towards a comprehensive sectional revision of African Strychnos.