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Systematic Study of Tribe Indigofereae (Leguminosae: Papilionoideae)

Indigofera hebepetala. Photo: B. Schrire.

The tribe Indigofereae (6 genera, c. 800 species) includes the third largest genus of legumes, Indigofera, with c. 720 species. The most recent family-wide phylogenies (published in 2004 and 2005) indicate a sister group relationship between tribe Indigofereae and the Millettioid-Phaseoloid alliance of tribes. While each of these groups has 100% bootstrap support, the sister relationship is poorly supported.

Relationships within tribe Indigofereae are now relatively well established. However, support remains poor for the sister group relationship between the putatively basally branching endemic Madagascan genus Phylloxylon and the rest of the tribe. It is unclear if Phylloxylon is sister to all the remaining genera of the tribe, or to the clade including only Cyamopsis, Indigastrum, Microcharis and Rhynchotropis, with these five together sister to Indigofera.

Taxon sampling of Indigofereae now covers the full range of morphological, ecological, and biogeographical diversity in the tribe, with over 320 critically selected representative species analysed. In addition, more appropriate sampling of outgroups from the sister tribes Millettieae, Abreae and Phaseoleae has been undertaken, and a morphological data set of 80 characters and an ITS indel data set have been included in a combined analysis. The results suggest that all species in the very species rich genus Indigofera are accommodated in one of four well supported clades, each of which is distinct biogeographically and diagnosable with morphological apomorphies. Mapping ecological and geographical characters onto a phylogenetic tree and finding the resulting patterns of geography and ecology congruent with the phylogeny suggests that habitat and geographical distribution are good predictors of phylogenetic relationships. A rates analysis of nucleotide substitutions confirms a post-Eocene diversification of the extant lineages, and implicates dispersal to explain the worldwide distribution of the tribe.

The primary purpose of the current project is to produce a benchmark paper combining all the data gathered so far for Indigofereae, and the large species-rich genus Indigofera, in a rigorous phylogenetic analysis.

Included in the project are the following objectives: 1) To analyse a comprehensive range of basally branching millettioid-phaseoloid genera recently sampled as putative outgroups to Indigofereae. Previous analyses were guided by less informative legume phylogenies and other more phylogenetically distant genera were used earlier as outgroups; 2) Preliminary results of critical sampling to fill gaps has indicated increased support for the one previously poorly supported clade within Indigofera. Detailed analysis should further emphasise the unique pattern noted so far, that all species in this super-large genus fall within one of only four distinct clades; 3) The Madagascan endemic genus Vaughania (11 species) will be formally reinstated as a clade within Indigofera; 4) The ages of various constituent clades will be estimated in order to fix the root of the Indigofereae stem clade; 5) To review morphological character evolution in the tribe based on the combined analysis; 6) To analyse the biogeographical and ecological affinities within Indigofereae based on ideas developed in Hubbell (2001) and Schrire et al. (2005).

Since starting in 2001, a paper has been published in a conference proceedings volume, and a benchmark paper is to be published in 2007.

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