Doctor of Philosophy
Biology, Plant Systematics
Date of Defense
Peter F. Stevens
Peter C. Hoch
Elizabeth A. Kellogg
Brad R. Ruhfel
Psorospermum belongs to the tribe Vismieae (Hypericaceae). Morphologically, Psorospermum is very similar to Harungana, which also belongs to Vismieae along with another genus, Vismia. Interestingly, Harungana occurs in both Madagascar and mainland Africa, as does Psorospermum; Vismia occurs in both Africa and the New World. However, the phylogeny of the tribe and the relationship between the three genera are uncertain. Using freshly collected specimens from my fieldwork as well as extant herbarium specimens, I aimed first, to generate a phylogeny of Psorospermum; second, to investigate its biogeography; and third, to investigate species boundaries within Malagasy Psorospermum.
The results of my molecular phylogenetic work, based on a combined analysis of chloroplast DNA (ndhF; trnL-trnF) and nuclear DNA (ITS), strongly support Harungana and Psorospermum as two genera: Harungana also includes one African species of Vismia, V. rubescens, and Psorospermum includes the other African Vismia and Psorospermum. My results also show that Malagasy Psorospermum are paraphyletic, some African species being nested within the clade of Malagasy Psorospermum suggesting dispersal of the genus westwards back to Africa. I conducted ancestral area reconstruction studies to test this hypothesis. Psorospermum may have reached Madagascar by a single dispersal event from Africa during the Oligocene (ca. 34-22 Ma), followed by diversification on Madagascar after ca. 20-19 Ma. However, two recent dispersal events appear to have occurred out of Madagascar back to Africa in the late Miocene (ca. 5.5 and 5.7 Ma).
Malagasy Psorospermum has not had a taxonomic revision in 60 years and the total number of species is uncertain. I undertook a novel approach to investigate the species boundaries in Psorospermum by integrating species hypotheses delimited by both molecular and morphometric analyses. Herbarium specimens of the putative taxa in each well supported clade in the molecular phylogeny of Malagasy Psorospermum were subjected to morphometric analyses using gaussian mixture models. I identified 27 species of Psorospermum of which 11 are new. The results will be used in a taxonomic revision of Malagasy Psorospermum that is in progress.
Ranarivelo, Heritiana S., "Systematics, Biogeography, and Species Delimitation of the Malagasy Psorospermum (Hypericaceae)" (2017). Dissertations. 690.