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Fatty Acid Composition of Symbiotic Cyanobacteria from Different Host Plant (Azolla) Species: Evidence for Coevolution of Host and Symbiont

¹ Department of Biochemistry and Microbiology, Rutgers-The State University of New Jersey, New Brunswick, New Jersey 08903-0231
² Agricultural Research Service, U.S. Department of Agriculture, Philadelphia, Pennsylvania 19118

^* Corresponding author. Present address: Department of Biological Sciences, Botswana University, Private Bag 0022, Gaborone, Botswana. Phone: 267-351151, ext. 2604. Fax: 267-356591. Electronic mail address: caudales{at}motswedi.ub.bw.

ABSTRACT

The total cellular fatty acid contents of 40 recently isolated cyanobacterial symbionts obtained from seven species of Azolla host plants were determined by gas-liquid chromatography-mass spectroscopy. A total of 63 fatty acids belonging to seven distinct chemical classes were identified. Fatty acid compositions varied among the cyanobacteria depending 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 acids, the ratios of unsaturated fatty acids to saturated fatty acids, and the total percentages of straight-chain even-carbonnumber fatty acids, unsaturated fatty acids, and branched-chain unsaturated fatty acids. The results of an analysis of variance suggested statistical regression for the total percentages of these fatty acids and chemical classes according to the following linear alignment of cyanobacteria by host: Azolla filiculoides, Azolla microphylla, Azolla caroliniana, Azolla mexicana, Azolla rubra, Azolla nilotica, and Azolla pinnata (including 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. mexicana, and A. caroliniana constituted one cluster, and the symbionts obtained from A. rubra, A. nilotica, and A. pinnata constituted a second cluster. A minor dichotomy separated the A. filiculoides symbionts from the other members of the first cluster. The clustering of Azolla cyanobacterial symbionts based on the results of our fatty acid analysis correlates remarkably well with the taxonomic grouping of the American Azolla species. This correlation suggests that the cyanobacterial symbionts of Azolla spp. coevolved into distinct genetic groups with their hosts.

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