FATTY-ACID COMPOSITION OF SYMBIOTIC CYANOBACTERIA FROM DIFFERENT HOST-PLANT (AZOLLA) SPECIES - EVIDENCE FOR COEVOLUTION OF HOST AND SYMBIONT

The total cellular fatty acid contents of 40 recently isolated cyanoba cterial symbionts obtained from seven species of Azolla host plants we re determined by gas-liquid chromatography-mass spectroscopy. A total of 63 fatty acids belonging to seven distinct chemical classes were id entified. Fatty acid compositions varied among the cyanobacteria depen ding on the hosts species. Parameters that differed significantly (at the 99% level of probability) included the concentrations of the 16:0 and 18:3 fatty acids, the total concentrations of the polyunsaturated acids, the total concentrations of the 16-carbon and 18-carbon fatty a cids, the ratios of unsaturated fatty acids to saturated fatty acids, and the total percentages of straight-chain even-carbon-number fatty a cids, unsaturated fatty acids, and branched-chain unsaturated fatty ac ids. The results of an analysis of variance suggested statistical regr ession for the total percentages of these fatty acids and chemical cla sses according to the following linear alignment of cyanobacteria by h ost: Azolla filiculoides, Azolla microphylla, Azolla caroliniana, Azol la mexicana, Azolla rubra, Azolla nilotica, and Azolla pinnata (includ ing Azolla pinnata subsp, pinnata and Azolla pinnata subsp, imbricata) . The seven groups could be divided into two distinct clusters on the basis of the results of a dendrogram analysis of Euclidian distances. The symbionts obtained from A. filiculoides, A. microphylla, A. mexica na, and A. caroliniana constituted one cluster, and the symbionts obta ined from A. rubra, A. nilotica, and A. pinnata constituted a second c luster. A minor dichotomy separated the A. filiculoides symbionts from the other members of the first cluster. The clustering of Azolla cyan obacterial symbionts based on the results of our fatty acid analysis c orrelates remarkably well with the taxonomic grouping of the American Azolla species. This correlation suggests that the cyanobacterial symb ionts of Azolla spp. coevolved into distinct genetic groups with their hosts.