Phylogeny of the tribe Indigofereae (Leguminosae-Papilionoideae): Geographically structured more in succulent-rich and temperate settings than in grass-rich environments.

This analysis goes beyond many phylogenies in exploring how phylogenetic structure imposed by morphology, ecology, and geography reveals useful evolutionary data. A comprehensive range of such diversity is evaluated within tribe Indigofereae and outgroups from sister tribes. A combined data set of 321 taxa (over one-third of the tribe) by 80 morphological characters, 833 aligned nuclear ribosomal ITS/5.8S sites, and an indel data set of 33 characters was subjected to parsimony analysis. Notable results include the Madagascan dry forest Disynstemon resolved as sister to tribe Indigofereae, and all species of the large genus Indigofera comprise just four main clades, each diagnosable by morphological synapomorphies and ecological and geographical predilections. These results suggest niche conservation (ecology) and dispersal limitation (geography) are important processes rendering signature shapes to the Indigofereae phylogeny in different biomes. Clades confined to temperate and succulent-rich biomes are more dispersal limited and have more geographical phylogenetic structure than those inhabiting tropical grass-rich vegetation. The African arid corridor, particularly the Namib center of endemism, harbors many of the oldest Indigofera lineages. A rates analysis of nucleotide substitutions confirms that the ages of the oldest crown clades are mostly younger than 16 Ma, implicating dispersal in explaining the worldwide distribution of the tribe.