Evolution, Biogeography and Taxonomy of the Southwest Pacific Tribe Coronanthereae (Gesneriaceae)

Abstract

Tribe Coronanthereae (Gesneriaceae) is an enigmatic group of nine genera of plants found in southern South America, Australia, New Zealand, Lord Howe Island, New Caledonia and the Solomon Islands. Some authors have proposed that this group, with unusual tree forms and berry-fruited species unlike most Gesneriaceae, is a relictual lineage of Gondwanan origin. This thesis is comprised of four studies utilizing traditional and modern methods: morphology-based taxonomy, scanning electron microscopy and two molecular techniques (gene sequencing and DNA fingerprinting). The aim of these studies was to explore various aspects of the evolution, biogeography and taxonomy of this angiosperm group. A molecular phylogeny based on both nuclear (intergenic transcribed spacer or ITS) and chloroplast DNA spacer regions (trnL-F, trnE-T, psbA-trnK) helped to resolve the relationship amongst sampled species of Coronanthereae, as well as amongst the tribes within Gesnerioideae. The monophyly of the subfamily Gesnerioideae was strongly supported. Tribe Coronanthereae was resolved as a member of the subfamily Gesnerioideae, found almost exclusively in the New World. The monophyly of tribe Coronanthereae was also strongly supported. Within Coronanthereae, three clades were recovered: 1) a berry-fruited clade of epiphytes and subshrubs with three species from southern South America (Asteranthera ovata, Mitraria coccinea and Sarmienta repens) and one species from Australia (Fieldia australis); 2) a clade of shrubs with coherent anthers from New Zealand (Rhabdothamnus solandri and New Caledonia (Coronanthera spp.), and 3) a tree clade of species located in Australia (Lenbrassia australiana), Lord Howe Island (Negria rhabdothamnoides) and New Caledonia (Depanthus glaber). At least two long-distance dispersals were proposed to explain the current distribution of species. Also, two separate origins of actinomorphic flowers were inferred, as were three shifts from bird pollination syndromes to alternative syndromes. The first comparative pollen morphology survey, based on scanning electron microscopy, was completed for all members of tribe Coronanthereae. The tribe was noted to be a eurypalynous group, with a range of variability that allowed for differentiation of genera. Pollen was examined in the context of the molecular results of three clades recovered. Overall, Coronanthereae pollen morphology was found to be similar to other Gesnerioideae. Key characteristics were small to medium-sized grains, monad, tricolp(or)ate, spheroidal to prolate with microreticulate, reticulate, perforate and rugulate exine patterns. Coronanthera was most distinctive within the tribe in having spheroidal rugulate pollen grains of a very small size (~10µM) more similar to subfamily Cyrtandroideae than Gesnerioideae. Depanthus and Lenbrassia pollen had smooth exines with perforate lumina that were also readily distinguished from other clades. The other genera in the tribe had pollen similar to types seen in other tribes of Gesnerioideae. Amplified fragment length polymorphisms (AFLP), a DNA fingerprinting technique, was applied to the New Zealand endemic, Rhabdothamnus solandri, to investigate its population structure. Few studies at the population-level have been completed in Gesneriaceae. The genetic diversity at species level was found to be high. Populations from offshore islands were as variable as mainland ones. Two clusters appeared within fourteen populations sampled, one northern and one southern. A northern origin was hypothesized for the current distribution of the species. Pleistocene factors were discussed. AFLP was a suitable technique to investigate genetic diversity in a continental island species showing no variation using the ITS marker. The taxonomic revision of Coronanthera and Depanthus, found in New Caledonia and the Solomons, was undertaken to re-examine the species circumscriptions and recognize new taxa collected since the original 1883 treatment. Nine new species from New Caledonia have been described. Trichomes were most informative for discriminating between species, and roughly divided Coronanthera into three loose groups: species with scabrid indumentum, those with glabrous indumentum, and a third with a distinctive felted appearance of inflorescences and new growth. Molecular phylogeny results supported two clades of glabrous and scabrous species, but placed Coronanthera squamata with the scabrous group. Scanning electron micrographs of pollen show a low level of variation amongst species examined. Depanthus was reduced to a single species, D. glaber, as there was less variation between the type specimens of D. pubescens and D. glaber, than was seen within different parts of just one individual tree.