Identification of an ancestral resistance gene cluster involved in the coevolution process between Phaseolus vulgaris and its fungal pathogen Colletotrichum lindemuthianum

The recent cloning of plant resistance (R) genes and the sequencing of resi stance gene clusters have shed light on the molecular evolution of R genes. However, up to now, no attempt has been made to correlate this molecular e volution with the host-pathogen coevolution process at the population level . Cross-inoculations were carried out between 26 strains of the fungal path ogen Colletotrichum lindemuthianum and 48 Phaseolus vulgaris plants collect ed in the three centers of diversity of the host species. A high level of d iversity for resistance against the pathogen was revealed. Most of the resi stance specificities were overcome in sympatric situations, indicating an a daptation of the pathogen to the local host. In contrast, plants were gener ally resistant to allopatric strains, suggesting that R genes that were eff icient against exotic strains but had been overcome locally were maintained in the plant genome. These results indicated that coevolution processes be tween the two protagonists led to a differentiation for resistance in the t hree centers of diversity of the host. To improve our understanding of the molecular evolution of these different specificities, a recombinant inbred (RI) population derived from two representative genotypes of the Andean (Ja loEEP558) and Mesoamerican (BAT93) gene pools was used to map anthracnose s pecificities. A gene cluster comprising both Andean (Co=y; Co-z) and Mesoam erican (Co-9) host resistance specificities was identified, suggesting that this locus existed prior to the separation of the two major gene pools of I! vulgaris. Molecular analysis revealed a high level of complexity at this locus. It harbors 11 restriction fragment length polymorphisms when R gene analog (RGA) clones are used. The relationship between the coevolution pro cess and diversification of resistance specificities at resistance gene clu sters is discussed.