Taxonomic Revision and Molecular Studies of Garcinia Section Garcinia (Guttiferae)
Saleh, Mohd. Nazre
Garcinia section Garcinia is one of 14 sections of the species-rich pantropical genus Garcinia (Guttiferae/Clusiaceae). In its most recent circumscription the section comprised 43 species, mostly of rain forest understorey trees, distributed from eastern India to Fiji, and in Madagascar (Jones, 1980: unpublished Phd. Thesis, University of Leicester). Its most famous member is the fruit tree, mangosteen (G. mangostana). Taxonomic revision of section Garcinia reduces the number of species from 43 to 16 with five varieties, with distribution from eastern India to Malesia. A total of nine species are excluded, 19 species are reduced to synonyms and five species are insufficiently known to be classified. Within sect. Garcinia, G. acuticosta, G. discoidea, G. exiguous, G. ochraceus and G. sangudsangud are newly described, G. diospyrifolia var. arborea, G. diospyrifolia var. minor and G. malaccensis var. pseudomangostana are new varieties, and G. cataractalis, previously unassigned to any section by Jones (1980) is newly included. Species limits are defined on the basis of combinations of characters such as shape of stamens and presence of pistillode, fruit type, and leaf characters such as shape, size, venation pattern and type of glandular lines. A total of 83 accessions representing 42 Garcinia species were sequenced for the nuclear ribosomal internal transcribed spacer (ITS) region, while 50 accessions (30 spp.) and 24 accessions (16 spp.) were sequenced for the chloroplast non-coding regions trnS-G and trnD-T respectively. Of these, 30 accessions or ten spp. (ITS), 23 accessions or nine spp. (trnS-G) and 17 accessions or nine spp. (trnD-T) belong to sect. Garcinia. Parsimony and Bayesian analyses show that the ITS data are more phylogenetically informative and provide better resolution for sectional and species relationships compared to trnS-G and trnD-T. Most of the sections delimited by Jones (1980) proved to be monophyletic but sect. Garcinia is not. Garcinia maingayi, G. trianii and G. costata, assigned to sect. Garcinia by Jones, are in a separate clade that is strongly supported as sister to sect. Brindonia. These species have significantly different morphological characters from sect. Garcinia, and should be excluded from it. Characters that are important for sectional delimitation are inflorescences that are simple cymes, stamen bundles that are 4-angled or 4-lobed, and fruits with a smooth surface. Four major clades in sect. Garcinia are supported by the shape of the stamen bundles, the shape of the fruit and the stigma, and fruit wall characters. ITS trees are significantly incongruent with plastid trees because of the placement of G. rigida, which could reflect a hybrid origin. Another hybrid species, the cultivated mangosteen, could be the product of hybridisation events between varieties of G. malaccensis if mangosteen is proven to be an obligate agamosperm. However, if mangosteen is a facultative agamosperm, G. malaccensis is likely to be the female parent, but any other Garcinia species could be the putative paternal species. Phylogenetic trees of ITS show that most Garcinia species from east of Wallace’s Line are nested within species from the west. This might reflect dispersal of species across this biogeographic division from west to east when the Sahul and Sunda shelves converged (c. 20 MY).This hypothesis is supported by the estimated divergence of accessions of G. rigida from the east of Wallace’s Line, from their most recent common ancestor to the west of the line, not later than 21.58+/-2.90 MY.