Molecular species delimitation, taxonomy and biogeography of Sri Lankan Gesneriaceae

Abstract

The plant family Gesneriaceae is represented in Sri Lanka by six genera: Aeschynanthus, Epithema, Championia, Henckelia, Rhynchoglossum and Rhynchotechum, with 13 species (plus one subspecies/variety) of which ten are endemic including the monotypic genus Championia, according to the last revision in 1981. They are exclusively distributed in undisturbed habitats, and some have high ornamental value. The species are morphologically diverse, but face a problem of taxonomic delineation, which is further complicated by the presence of putative hybrids. Sri Lanka and Indian Peninsula, represent the Deccan plate of the ancient Gondwanan supercontinent. The presence of a relict flora may indicate the significance of the geological history of the Deccan plate for the evolution of angiosperms. The high degree of endemism here, along with their affinities to the global angiosperm flora paints a complex picture, but its biogeographic history is still unclear. The pantropical family Gesneriaceae distributed in Sri Lanka and South India is therefore an appropriate study group in this context. Besides, the family itself has a complex but largely unresolved biogeographical history especially concerning the origin and diversification of Old World Gesneriaceae. Modern approaches for the taxonomic studies were applied, integrating morphological and molecular data. Multiple samples were collected for each species across their geographical distribution. Nuclear ITS and chloroplast trnL-F sequences for the taxa from Sri Lanka were used to generate regional genus phylogenies of all six genera, using maximum parsimony. The rate of evolution of the nuclear ITS region versus chloroplast trnL-F was varied greatly across the six genera studied. Molecular delimitations were mostly congruent with the classical taxonomy. Over 65 taxonomic characters were studied in detail to recognize synapomorphies for clades and taxa. A complete taxonomic revision of Gesneriaceae in Sri Lanka, including lectotypification, was conducted based on both, the molecular and morphological data. This resulted in the recognition of 14 species in the six genera, including one newly described species H. wijesundarae Ranasinghe and Mich.Möller. Henckelia communis and H. angusta were not supported molecularly as two separate entities but are recognized as two species because of consistent morphological differences between them. Henckelia humboldtiana is proposed to represent a species complex due to its highly variable and inconsistent molecular and morphological diversity and overlap with H. incana and H. floccosa; more research is needed here. National conservation assessments were conducted, and all 14 species were recognized as threatened. Biogeographic affinities of Sri Lankan Gesneriaceae were elucidated, generating a dated phylogeny using an existing matrix of four plastid gene regions; trnL-F, matK, rps16 and ndhF, amended by sequences generated in this study. The final combined matrix included 175 taxa including newly generated sequences for the 13 Sri Lankan taxa. Phylogenetic trees were generated using parsimony, maximum likelihood and Bayesian inference. Molecular dating was carried out using BEAST and ancestral area reconstruction using BioGeoBears. These analyses indicated that the six genera of Gesneriaceae arrived in Sri Lanka separately and sometimes different time periods. One lineage dated back to the early diversification of the subfamily Didymocarpoideae (generally regarded as the Old World Gesneriaceae), which occurred around the KT boundary, before the Deccan plate was connected to Asia.