THE GENUS ATHROISMA (ASTERACEAE, HELIANTHEAE)

The palaeotropical genus Athroisma (Asteraceae, Heliantheae) is revised for the first time, and its general morphology is described. Twelve species and two subspecies distributed in East Tropical Africa, Madagascar, and in Asia from north-eastern India to the Sunda Islands are recognized. Four new species are described (A. inevitabile and A. pusillum from East Africa, and A. pinnatifidum and A. psilocarpum hom Madagascar). One species is reduced to subspecies rank (A. gracile ssp. psyllioides). Some species have conflicting character patterns which intergrade. Thus, the delimitation of species is discussed, and a hypothesis of introgressive hybridization in the genus is formulated. Distribution patterns in East Africa are examined in relation to introgression in three species complexes, connected to A. gracile. In two of the cases, the character distribution is explained by a single dispersal event and subsequent introgressive hybridization. A hypothesis of the phylogenetic relationships within the genus is presented, based on a cladistic analysis of 36 morphological characters. The stability of the single most parsimonious cladogram is tested (decay, skewness and random character weighting), and found to be low except for three nodes. An analysis with putative hybrid groups included is also presented. A key to all taxa is supplied as well as nomenclatural consideration, descriptions, drawings and distribution maps. Chromosome counts of three species are reported (2n=20). (C) 1995 The Linnean Society of London