ORIGIN OF A FUNGAL SYMBIONT OF PERENNIAL RYEGRASS BY INTERSPECIFIC HYBRIDIZATION OF A MUTUALIST WITH THE RYEGRASS CHOKE PATHOGEN, EPICHLOE-TYPHINA

Seed-borne fungal symbionts (endophytes) provide many cool-season gras s species with biological protection from biotic and abiotic stresses. The endophytes are asexual, whereas closely related sexual species of genus Epichloe (Clavicipitales) cause grass choke disease. Perennial ryegrass (Lobium perenne) is a host of two endophyte taxa, LpTG-1 (L. perenne endophyte taxonomic grouping one = Acremonium lolii) and LpTG- 2, as well as the choke pathogen, Epichloe typhina (represented by iso late E8). Relationships among these fungi and other Epichloe species w ere investigated by analysis of gene sequences, DNA polymerphisms and allozymes. The results indicate that LpTG-2 is a heteroploid derived f rom an interspecific hybrid. The LpTG-2 isolates had two copies each o f nine out of ten genes analyzed (the exception being the rRNA gene lo cus), and the profiles for seven of these were composites of those fro m E. typhina E8 and A. lolii isolate Lp5. Molecular phylogenetic analy sis grouped the two P-tubulin genes of LpTG-2 into separate clades. On e (tub2-1) was related to that of E. typhina E8, and the other (tub2-2 ) to that of A. lolii. The mitochondrial DNA profile of LpTG-2 was sim ilar to that of A. lolii, but its rRNA gene sequence grouped it with E . typhina E8. A proposed model for the evolution of LpTG-2 involves in fection of a L. perenne-il. lolii symbiotum by E. typhina, followed by hybridization of the two fungi. Such interspecific hybridization may be a common and important mechanism for genetic variation in Epichloe endophytes.