Natural diversity of Frankia strains in actinorhizal root nodules from promiscuous hosts in the family Myricaceae

Actinorhizal plants invade nitrogen-poor soils because of their ability to form root nodule symbioses with N-2-fixing actinomycetes known as Frankia, Frankia strains are difficult to isolate, so the diversity of strains inhab iting nodules in nature is not known. To address this problem, we have used the variability in bacterial 16S rRNA gene sequences amplified from root n odules as a means to estimate molecular diversity. Nodules were collected f rom 96 sites primarily in northeastern North America; each site contained o ne of three species of the family Myricaceae, Plants in this family are con sidered to be promiscuous hosts because several species are effectively nod ulated by most isolated strains of Frankia in the greenhouse. We found that strain evenness varies greatly between the plant species so that estimatin g total strain richness of Frankia within myricaceous nodules with the samp le size used was problematical. Nevertheless, Myrica pensylvanica, the comm on bayberry, was found to have sufficient diversity to serve as a reservoir host for Frankia strains that infect plants from other actinorhizal famili es. Myrica gale, sweet gale, yielded a few dominant sequences, indicating e ither symbiont specialization or niche selection of particular ecotypes. St rains in Comptonia peregrina nodules had an intermediate level of diversity and were all from a single major group of Frankia.