Molecular systematics of the genus Astragalus L. (Fabaceae): Phylogenetic analyses of nuclear ribosomal DNA internal transcribed spacers and chloroplast gene ndh F sequences

Abstract

Comparative sequencing of internal transcribed spacers (ITS) and 5.8S gene of nuclear ribosomal DNA was carried out to examine phylogenetic relationships among subgenera and sections of Old World Astragalus as well as the recent segregate genera Barnebyella and Ophiocarpus. For a subset of these taxa (43 accessions), the nrDNA ITS data were supplemented by sequences from the chloroplast ndhF gene. Phylogenetic trees resulting from separate analyses of the nrDNA ITS and ndhF sequences were in conflict mainly on the position and relationships of Ophiocarpus aitchisonii, Astragalus hemsleyi, A. grammocalyx, A. coelicolor, A. capito, A. epiglottis and A. annularis. Excluding these taxa, phylogenetic analysis of a combined nrDNA ITS-ndhF data matrix was also conducted, so that in the resulting tree, most clades were more resolved and better statistically supported than those were in the separate analyses. Our results indicate that the monotypic segregate genera Barnebyella (= A. migpo), Ophiocarpus (= A. ophiocarpus) and morphologically isolated annual species A. dipelta (= Didymopelta turkestanica), A. schmalhausenii (= Sewerzowia turkestanica) and A. vicarius (= S. vicaria) are clearly nested within Astragalus. Our results confirm earlier studies that shows A. vogelii is allied with the genera Colutea and Oxytropis rather than with any Astragalus species. It is therefore excluded from Astragalus and elevated to the new generic rank and named as Podlechiella Maassoumi and Kazempour Osaloo. None of the eight traditionally recognized Astragalus subgenera Epiglottis, Trimeniaeus, Phaca, Hypoglottis, Calycophysa, Tragacantha, Cercidothrix and Calycocystis are monophyletic. Similarly, the monophyly of Podlech’s new subgenera Trimeniaeus, Astragalus and Cercidothrix is not supported. Among the many species-rich sections analyzed here, only Cenanthrum, Chronopus, Laxiflori, Lotidium, Incani and Amodendron are monophyletic.