Fitness Characters in Parasexual Recombinants of the Rice Blast Fungus, Pyricularia oryzae

We examined the fitness characters in parasexual recombinants derived from cocultures of two isolates, NAO-02 (race 133.1) and TH77-1 (race 047.0), of the rice blast fungus, Pyricularia oryzae, in liquid yeast extract medium. The cocultured isolates were transferred to oatmeal agar to produce a conidial inoculum. Then, the conidial suspension was sprayed on the japonica rice cultivar Akiyutaka, which has two resistance genes to rice blast: Pik and Piz. Three pathogenic variants were isolated from the typical leaf blast lesions that were subsequently produced. We used randomly amplified polymorphic DNA (RAPD) to examine whether the variants were derived from parasexual recombination between the parents. Eleven RAPDs indicated that the variants possessed genomic DNA from both parents. After inoculating Akiyutaka 7 times in succession, the variants maintained their original pathogenicity. The parasexual recombinants produced more conidia and larger lesions on the host than did parent TH77-1, but fewer conidia and smaller lesions than parent NAO-02. Disease development on plants inoculated with either variant or a parental isolate was also midway between the disease caused by each of the parents in a field containing a single cultivar, but the variant caused the most severe disease in a plot with a mixture of cultivars. These results suggest that parasexual recombination produces variation in fitness. Discipline: Plant disease Additional key words: aggressiveness, Magnaporthe oryzae JARQ 41 (2), 123 – 131 (2007) http://www.jircas.affrc.go.jp Present address: 3 Environmental Biofunction Division, National Institute for Agro-Environmental Sciences (Tsukuba, Ibaraki 305–8604, Japan) *Corresponding author: e-mail [email protected] Received 7 September 2006; accepted 13 October 2006.