CHLOROPLAST GENE SEQUENCE DATA SUGGEST A SINGLE ORIGIN OF THE PREDISPOSITION FOR SYMBIOTIC NITROGEN-FIXATION IN ANGIOSPERMS

Of the approximately 380 families of angiosperms, representatives of o nly 10 are known to form symbiotic associations with nitrogen-fixing b acteria in root nodules. The morphologically based classification sche mes proposed by taxonomists suggest that many of these 10 families of plants are only distantly related, engendering the hypothesis that the capacity to fix nitrogen evolved independently several, if not many, times. This has in turn influenced attitudes toward the likelihood of transferring genes responsible for symbiotic nitrogen fixation to crop species lacking this ability. Phylogenetic analysis of DNA sequences for the chloroplast gene rbcL indicates, however, that representatives of all 10 families with nitrogen-fixing symbioses occur together, wit h several families lacking this association, in a single clade. This s tudy therefore indicates that only one lineage of closely related taxa achieved the underlying genetic architecture necessary for symbiotic nitrogen fixation in root nodules.