Lineage diversification and evolution of pollinator trapping in Artabotrys (Annonaceae)

Abstract

Evolutionary diversification is influenced by disparate aspects of the biology, ecology and geographical distribution of species. Robust, well-sampled molecular phylogenies have the potential to test which environmental and species attributes likely contributed as evolutionary drivers. Our project investigates lineage diversification and the evolution of pollinator trapping in the species-rich, lianescent genus Artabotrys (c. 100 species; Annonaceae), a characteristic element in Old World tropical and subtropical lowland forests. We are reconstructing the phylogeny of the genus based on an extensive taxon sampling (c. 50 species) and a broad range of chloroplast (matK, ndhF, psbA-trnH, trnL-F) and nuclear (AP3, LFY, ncpGS, PhyA, RPB2) DNA markers. Preliminary results indicate that early diversification in the genus resulted in lineages that are markedly asymmetrical in species numbers: the main species-rich Artabotrys clade (MAC), characterised by flowers with basally concave petals that form a tightly enclosed pollination chamber relative to the species-poor early-divergent grade (comprising A. brachypetalus and A. thomsonii). We hypothesise that (i) the floral chamber in the MAC may function as a pollinator trap, with the pollinators attracted by floral scents but subsequently prevented from leaving the flowers by the closure of the chamber, and (ii) that pollinator trapping is synapomorphic for the MAC, and the putative ‘xylopioid’ traits in A. brachypetalus and A. thomsonii (separation of inner petals at anthesis and extended duration of the apical connivence of petals during floral development) may be plesiomorphic. To investigate these aspects, pollination ecology studies of exemplar species (A. brachypetalus in the early-divergent grade, and A. hongkongensis and A. suaveolens in the MAC) are being conducted, involving assessments of floral phenology, thermogenesis, scent chemistry, pollinator identification, and pollinator trapping; character mapping will be performed to further elucidate pollination chamber evolution and the morphological basis of pollinator trapping. Our preliminary field studies of A. hongkongensis in Hong Kong reveal that the flowers are protogynous, with anthesis over c. 27 hours, and are likely to exhibit pollinator trapping, although the effective pollinator is still to be unequivocally determined. The majority of Artabotrys species possess broad, flat, lanceolate petals (e.g., A. hongkongensis); narrow, terete, filiform petals have evolved independently in several MAC lineages (e.g., A. suaveolens), and may represent an adaptation to a different pollinator guild. Diversification rate analyses will be performed to investigate if pollinator trapping represents a key evolutionary innovation correlated with accelerated diversification in the MAC.