Sacristan, Soledad*, Ridout, CJ; Vigouroux, M; Chartrain, L; Brown, JKM.
Diversity and evolution of avirulence genes in powdery mildew fungi. In: International Meeting on "Population and Evolutionary Biology of Fungal Symbionts", Ascona, Switzerland, 2007. AB-25. Click here to download the presentation on pdf format.

The fungus Blumeria graminis f.sp. hordei (Bgh) is a biotrophic pathogen, causal agent of powdery mildew of barley, a highly damaging disease of crops. Interactions between it and its host are governed in a gene-for-gene manner, such that a barley plant is only resistant to Bgh genotypes with the corresponding avirulence (Avr) gene. Plant resistance (R) genes are rapidly overcome by parasite mutants that evade recognition without apparent loss of parasite fitness.

Over 25 independent AVR genes have been described in Bgh isolates. Recently, the genes AVRa10 and AVRk1 have been isolated in our lab. They act as elicitors in incompatible reactions, but also as effectors, enhancing the infectivity of the fungus in compatible reactions. AVRa10 and AVRk1 are homologues and belong to a large family with an estimated number of at least 30 members in the genome of Bgh. These findings support the hypothesis that the mildew fungus has a repertoire of avirulence genes, which may function as effectors and contribute to parasite virulence. Multiple copies of AVR-effector genes might enable populations of Bgh to rapidly overcome host R genes whilst maintaining virulence.

We have analysed the genomic and expressed AVR-like sequences in different isolates and tested if they cosegregate with avirulence phenotypes. The analysis of the sequences has given us information about the functional constraints for the evolution of these genes: ie. identifying the portions of the genes that are conserved, and therefore probably essential for the function of the Avr genes, and the regions which are subject to diversifying selection, and so likely to be involved in specific interactions with host genotypes.

Homologous sequences are also present in related formae speciales (ff. spp) of B. graminis infecting other cereals. Our preliminary results indicate that they are not present in more distant ff. spp from wild grasses, neither in other powdery mildew species. The phylogeny of these sequences matches with the phylogeny of the ITS region of the ff. spp. The possible implication of this gene family in the specialization of B. graminis is discussed in this communication.

Keywords: avirulence, gene families, gene-for-gene interactions, specialisation, powdery mildew

*Institution: Universidad Politecnica de Madrid, Spain

E-mail: soledad.sacristan@upm.es