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Phylogeny, diversity, and distribution in Exostema (Rubiaceae): implications of morphological and molecular analyses

Tim McDowell and Birgitta Bremer
Plant Systematics and Evolution
Vol. 212, No. 3/4 (September 1998), pp. 215-246
Published by: Springer
Stable URL: http://www.jstor.org/stable/23643442
Page Count: 32
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Phylogeny, diversity, and distribution in Exostema (Rubiaceae): implications of morphological and molecular analyses
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Abstract

The neotropical genus Exostema comprises 25 species of trees and shrubs, ranging in distribution from Bolivia to Mexico and throughout the West Indies, with most species endemic to the Greater Antilles. Infrageneric relationships and species-level patterns of evolution were investigated in phylogenetic analyses using morphological, molecular, and combined data sets. All data sets resolved three main species groups which correspond to the three sections recognized by McDowell (1996). However, the analyses of ITS sequence data placed the two South American species basal to the three main clades. Otherwise, the morphological and molecular data are highly compatible, and produce a more robust yet consistent phylogeny in the combined data analysis. Morphological evolution in Exostema involves many specializations for xeric habitats, reflecting repeated ecological shifts from moist forest to exposed, seasonally dry environments during the diversification of the genus. Both moth and bee pollination syndromes are found in Exostema, and shifts in pollination ecology appear pivotal to the differentiation of the three sections. Biogeographically, Exostema likely originated in South America and migrated via Central America to the Greater Antilles, where the morphological diversification and speciation are most extensive.

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