GENETICS OF HOST-SPECIFICITY IN EPICHLOE-TYPHINA

Epichloe typhina perennially and systemically infects grass plants, ca using choke disease in which maturation of host inflorescences is supp ressed. In seedling-inoculation tests, isolate E8 from perennial ryegr ass established and maintained infection in this host but not in orcha rdgrass. In contrast, isolates E469, E2466, and E2467 from orchardgras s varied in infection frequency and stability in orchardgrass, but all were unable to establish stable infections in perennial ryegrass. To investigate the ge netics of host specificity, isolate E8 was crossed with each of the isolates from orchardgrass. Seedlings of parental hos t species were inoculated with F-1 progeny, and the frequencies of see dling infection and stability in adult plants were assessed. In the E8 x E2466 cross, the F-1 progeny exhibited a wide range of infection fr equency and stability in each parental host. In crosses E8 x E469 and E8 x E2467, where the orchardgrass-derived parents infected 5 to 13% o f inoculated perennial ryegrass seedlings, the distributions of infect ion frequencies for the F-1 progeny were skewed toward levels comparab le to that of the parent from perennial ryegrass. In all crosses, most progeny had low frequencies of infection in orchardgrass. However, tr ansgression was evident in a cross of E8 with E469, an isolate that in fected orchardgrass seedlings at a low frequency (2 to 3%). The E8 x E 469 cross had a few F-1 progeny that infected orchardgrass at high eff iciency (up to 81%). A Spearman rank correlation applied to the E8 x E 2466 progeny indicated a significant negative correlation between infe ction frequencies in perennial ryegrass and orchardgrass. Also, there was a significant correlation of infection frequency and stability in perennial ryegrass but not in orchardgrass. To test whether only a few genes governed infection frequency in perennial ryegrass, an E8 x E24 66 F-1 progeny (designated E386.04), which had intermediate compatibil ity with this host, was backcrossed to Es. The progeny of this backcro ss exhibited a distribution of infection frequencies in perennial ryeg rass between that of E386.04 and the backcross parent, suggesting that multiple genes may determine compatibility at the seedling infection stage. The results of these experiments indicated multiple genetic det erminants of compatibility or incompatibility with each host, with int ermediate or high heritability.