Molecular phylogenetics of Astragalus
Astragalus L. (Fabaceae; subfamily Papilionoideae) is a vast assemblage of herbaceous annuals and perennials with more than 2,500 species distributed mainly in cool arid to semiarid continental regions of the northern hemisphere and South America. The genus is especially diverse in southwest Asia, Sino-Himalayan region, western North America and along the Andes in South America. Molecular phylogenetic studies suggest that Astragalus is monophyletic (Wojciechowski et al., 1993, 1999), and nested in a vast radiation of predominantly herbaceous and temperate papilionoid legumes marked uniquely by loss of one copy of the 25 kb inverted repeat in the chloroplast genome; this "IR-lacking" clade or IRLC, comprises some 45 genera and more than 4,000 species in the tribes Carmichaelieae, Cicereae, Galegeae, Hedysareae, Trifolieae, and Vicieae, plus Callerya and Wisteria of tribe Millettieae, and is one of two clades that comprise Hologalegina, see below (Hu et al. 2000; Wojciechowski et al., 2000). We have also shown that all New World species with aneuploid chromosome numbers form a monophyletic group ("Neo-Astragalus") that is derived from Old World taxa. The few (ca. thirteen) North American species with euploid chromosome numbers are not closely related to Neo-Astragalus but are dispersed among divergent Old World groups. The proposed segregate genera Astracantha (Eurasian) and Orophaca (North American) are clearly nested within Astragalus sensu stricto, while the historically allied genus Oxytropis is not nested within Astragalus but forms a separate clade within the larger "Astragalean" clade. Preliminary estimates of absolute rates of species diversification in Astragalus and larger Astragalean clade suggest it may be higher than in some other, often cited, continental or insular adaptive radiations in angiosperms (Sanderson and Wojciechowski, 1996; Wojciechowski et al., 1999). For more information, see publications for the papers cited here, and visit our Astragalus website . Copies of our datasets have been deposited in TreeBASE.
Supertree of "Hologalegina"
The clade we refer to as "Hologalegina" comprises a very large proportion of members of the legume subfamily Papilionoideae with a temperate distribution, including nine traditionally recognized tribes, some 67 genera, and more than 4,600 species - or about 1/4 of the species diversity of the entire Fabaceae. The monophyly of this clade is supported by cladistic analyses of molecular data and morphological evidence. Using these results as a starting point, we have taken another approach to reconstructing phylogenetic relationships in this large clade, "supertree construction methods". Supertrees are estimates of phylogeny,and one approach for building larger, progressively more inclusive or composite phylogenies from a number of smaller, partially overlapping trees (in terms of taxa) to provide a more comprehensive hypothesis of relationships. Our supertree of Hologalegina (click here) is described in detail in Wojciechowski et al., (2000) [see publications].
Legumes PhyloInformatics dBase
For a modest, but growing, list of links to www sites for information on legume phylogenetics, systematics, and genomics [click here].
Phylogeny of legume family
To be completed.
Diversity of legume-rhizobial symbioses
We are interested in understanding the co-evolution of legumes and rhizobia (nitrogen-fixing bacteria), focusing particularly on the relationship between the phylogenetically diverse group of rhizobial species that nodulate members of the Hologalegina clade, the largest group of temperate herbaceous legumes. While most rhizobia produce nod factors, the chemical signals used to induce nodulation in their legume host(s), which have a saturated fatty acid chain attached to the backbone structure, this group of rhizobia produce nod factors with a modified structure, having an unsaturated fatty acid chain attached instead. To begin to understand the evolutionary significance of this change in nod factor chemistry, we are sequencing the bacterial nodulation genes nodA, nodB, and nodC to examine how closely the evolution of these nod factors parallels that of the host plants from which the rhizobia were taken, and to further resolve phylogenetic relationships among the rhizobia. More generally, we are examining the diversity of rhizobia that nodulate native legumes in Arizona and the southwestern United States region. This project has potential implications in the areas of ecology, agronomy, systematics and evolution.