Molecular systematics of Australian Gossypium section Grandicalyx (Malvaceae)

Australian Gossypium consists of 17 species classified into three sections. The largest and most poorly understood is the group of 12 species in sect. Grandicalyx that occur in the Kimberley and Cobourg regions of NW Australi a. These areas are characterized by annual monsoon rains and dry-season fir es. Species in sect. Grandicalyx exhibit a suite of morphological and ecolo gical features that are otherwise unknown in the genus and that apparently evolved in response to seasonal fires and ant mutualism. These features inc lude an herbaceous perennial habit, the ability to regenerate from thickene d rootstocks following fires and extended drought, pedicels that recurve fo llowing pollination so that the capsules are pendent and open inverted at m aturity, and sparsely vestitured, ant-dispersed seeds that bear elaiosomes. To better understand the evolutionary and biogeographic history of the spe cies in sect. Grandicalyx, we sampled widely within and among species and g enerated three sets of DNA sequences, i.e., for the plastid rpl16 intron, t he nuclear 18S-26S ITS of rDNA, and an alcohol dehydrogenase gene. Phylogen etic reconstructions indicate that species in sect. Grandicalyx are monophy letic with little resolution within the dade. Sequence divergence is low in all pairwise comparisons among species, suggesting that the group radiated relatively recently perhaps in the late Pliocene-Pleistocene after an earl ier (Miocene) divergence from the other extant Australian lineages. The pal aeoclimatic record, sequence divergence estimates, and phylogenetic data su ggest that diversification of the sect. Grandicalyx species arose via range fragmentation of a more widely distributed ancestor or ancestors. This evo lutionary history was accompanied by the development of a prostrate to upri ght herbaceous, multistemmed habit, adaptation to seasonal fires, and a sui te of features associated with myrmecochory.