Genetics of Fertility and Mating Type in Magnaporthe Grisea

Abstract

Blast disease of rice (Oryza sativa L.) caused by Magnaporthe grisea (anamorph = Pyricularia grisea) is one of the principal disease of rice. M. grisea includes a large number of monocots among its host range, however individual isolates are specific to only one or few hosts species. Sexual fertility has repeatedly been reported in rice pathogen from mountain regions of South and East Asia. In the Indian Himalayas where the rice and non-rice crops are grown in adjoining fields or some times mixed together the population of M. grisea was observed to be highly diverse, suggesting sexual recombination and random mating in pathogen population. The likelihood of sexual recombination occurring in a population is a function of coexisting biological competence (fertility) and potential, and can be assessed by the presence of both the mating types, and fertility. Our present observation regarding genetics of fertility and host range in M. grisea indicates that, the isolates from Foxtail millet(Setaria italica) are highly compatible and hermaphrodite and mated with most of the field isolates tested. High compatibility and fertility was observed in non-rice isolates but was low in non-rice × rice crosses. Rice isolates did not mate within themselves. The extent of gene flow between resident M. grisea from rice and non-rice host can be resolved by the distribution and frequencies of these rare alleles. The pattern of fertility and mating type distribution in the two typical rice-derived and setaria-derived isolates from the collection of Himalayan isolates revealed that Foxtail millet(Setaria italica) isolate III 202 was identified as highly compatible, hermaphrodite isolate. Progenies from non-rice × rice crosses produced very few fertile combinations on backcrossing, while the frequency was higher with the non-rice × non-rice derived progenies. The behavior of III 202 both as male and female depending upon opposite strengthens the view that M. grisea is self-sterile hermaphrodite. Genetic ratio for fertility suggests that a single gene did not control it, and a large number of genes functions to affect the mating competence. Fertility segregated independent of mating type. All the progenies did not cross with standard tester isolate III 202, although some of them were compatible within themselves. Some progenies of non-rice crosses were able to show compatible reactions with both the parents, thus suggesting the presence of both mating types, which in normal case any one of them is present in the isolates. The mating type identity for such progenies remains obscure in the absence for the evidence of dual maters, which has not so far been reported in nature. Higher frequency of male progenies indicates the dominance of male fertile parent. Asci with abnormal or aborted ascospores were frequently observed in non-rice x rice crosses. Two-point inoculation technique in test tubes was evaluated as more efficient over widely used three point inoculation technique and a more efficient technique of single ascospore isolation was standardised.