Résumé
Sexual reproduction may be cryptic or facultative in fungi and therefore
difficult to detect. Magnaporthe oryzae, which causes blast, the most
damaging fungal disease of rice, is thought to originate from South-East
Asia. It reproduces asexually in all rice-growing regions. Sexual
reproduction has been suspected in limited areas of South-East Asia, but
has never been demonstrated in contemporary populations. We characterized
several M. oryzae populations worldwide both biologically and genetically,
to identify candidate populations for sexual reproduction. The sexual
cycle of M. oryzae requires two strains of opposite mating types, at least
one of which is female-fertile, to come into contact. In one Chinese
population, the two mating types were found to be present at similar
frequencies and almost all strains were female-fertile. Compatible strains
from this population completed the sexual cycle in vitro and produced
viable progenies. Genotypic richness and linkage disequilibrium data also
supported the existence of sexual reproduction in this population. We
resampled this population the following year, and the data obtained
confirmed the presence of all the biological and genetic characteristics
of sexual reproduction. In particular, a considerable genetic reshuffling
of alleles was observed between the two years. Computer simulations
confirmed that the observed genetic characteristics were unlikely to have
arisen in the absence of recombination. We therefore concluded that a
contemporary population of M. oryzae, pathogenic on rice, reproduces
sexually in natura in South-East Asia. Our findings provide evidence for
the loss of sexual reproduction by a fungal plant pathogen outside its
centre of origin.