COEVOLUTION BY COMMON DESCENT OF FUNGAL SYMBIONTS (EPICHLOE SPP) AND GRASS HOSTS

Epichloe species are ascomycetous fungi (family Clavicipitaceae) that are ecologically obligate symbionts of grasses. Because they can enhan ce host fitness by protection from biotic and abiotic stresses, but ca n also reduce host seed production, these symbionts span a continuum f rom antagonistic (highly pathogenic) to mutualistic. Their mutualistic or antagonistic effects are directly related to the relative importan ce of their sexual and asexual life cycles. The sexual cycle of the fu ngus occurs only on ''choked'' tillers on which no seeds are produced, so the more antagonistic Epichloe species permit almost no host seed production and only transmit horizontally (contagiously). Other Epichl oe species are called ''pleiotropic symbionts'' because they have both mutualistic and antagonistic effects, being transmitted both vertical ly (in host seeds) and horizontally. The possibility of coevolution by common descent of Epichloe and grass species was addressed by surveyi ng grasses for pleiotropic or antagonistic symbionts, identifying the biological species of Epichloe associated with each host, conducting m olecular phylogenetic analysis of the symbionts based on noncoding por tions of nuclear genes for beta-tubulin and rRNA, and comparing gene t rees with each other and with the established molecular phylogeny of t he host tribes. A total of nine confirmed or likely biological species of Epichloe were identified, eight of which were specialized to group s of related host species or genera within a single tribe. Five of the Epichloe species were pleiotropic whereas the other four species were antagonistic. Phylogenetic analysis indicated that the five pleiotrop ic species and two of the four antagonistic species coevolved by commo n descent with their hosts. Of the two species for which common descen t was not evident, one had a broad host range and was paraphyletic to a pleiotropic species, and the other had discordant gene phylogenies s uggestive of a hybrid origin. These results suggested that common desc ent is more likely in pleotropic than in antagonistic symbioses of gra sses with these fungi.