EVOLUTIONARY DIVERSIFICATION OF FUNGAL ENDOPHYTES OF TALL FESCUE GRASS BY HYBRIDIZATION WITH EPICHLOE SPECIES

The mutualistic associations of tall fescue (Festuca arundinacea) with seed-borne fungal symbionts (endophytes) are important for fitness of the grass host and its survival under biotic and abiotic stress. The tall fescue endophytes are asexual relatives of biological species (ma ting populations) of genus Epichloe (Clavicipitaceae), sexual fungi th at cause grass choke disease. Isozyme studies have suggested considera ble genetic diversity among endophytes of tall fescue. Phylogenetic re lationships among seven isolates from tall fescue, three from meadow f escue (a probable ancestor of tall fescue), and nine Epichloe isolates from other host species were investigated by comparing sequences of n oncoding segments of the beta-tubulin (tub2) and rRNA (rrn) genes. Whe reas each Epichloe isolate and meadow fescue endophyte had only a sing le tub2 gene, most tall fescue endophytes had two or three distinct tu b2 copies. Phylogenetic analysis of tub2 sequences indicated that the presence of multiple copies in the tall fescue endophytes was a conseq uence of hybridization with Epichloe species. At least three hybridiza tion events account for the distribution and relationships of tub2 gen es. These results suggest that interspecific hybridization is the majo r cause of genetic diversification of the tall fescue endophytes.