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Molecular Systematics of Australian Gossypium Section Grandicalyx (Malvaceae)

Tosak Seelanan, Curt L. Brubaker, James McD. Stewart, Lyn A. Craven and Jonathan F. Wendel
Systematic Botany
Vol. 24, No. 2 (Apr. - Jun., 1999), pp. 183-208
DOI: 10.2307/2419548
Stable URL: http://www.jstor.org/stable/2419548
Page Count: 26
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Molecular Systematics of Australian Gossypium Section Grandicalyx (Malvaceae)
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Abstract

Australian Gossypium consists of 17 species classified into three sections. The largest and most poorly understood is the group of 12 species in seet. Grandicalyx that occur in the Kimberley and Cobourg regions of NW Australia. These areas are characterized by annual monsoon rains and dry-season fires. Species in sect. Grandicalyx exhibit a suite of morphological and ecological features that are otherwise unknown in the genus and that apparently evolved in response to seasonal fires and ant mutualism. These features include an herbaceous perennial habit, the ability to regenerate from thickened rootstocks following fires and extended drought, pedicels that recurve following pollination so that the capsules are pendent and open inverted at maturity, and sparsely vestitured, ant-dispersed seeds that bear elaiosomes. To better understand the evolutionary and biogeographic history of the species in sect. Grandicalyx, we sampled widely within and among species and generated three sets of DNA sequences, i.e., for the plastid rpl16 intron, the nuclear 18S-26S ITS of rDNA, and an alcohol dehydrogenase gene. Phylogenetic reconstructions indicate that species in sect. Grandicalyx are monophyletic with little resolution within the clade. 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 earlier (Miocene) divergence from the other extant Australian lineages. The palaeoclimatic record, sequence divergence estimates, and phylogenetic data suggest that diversification of the sect. Grandicalyx species arose via range fragmentation of a more widely distributed ancestor or ancestors. This evolutionary history was accompanied by the development of a prostrate to upright herbaceous, multistemmed habit, adaptation to seasonal fires, and a suite of features associated with myrmecochory.

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