Characterisation, evolutionary trends and mapping of putative resistance and defence genes in Actinidia (kiwifruit)

Abstract

A virulent strain of Pseudomonas syringae actinidae has invaded kiwifruit (genus Actinidia) orchards in New Zealand, making the study of Actinidia resistance genes critical. Examination of expressed sequence tag (EST) libraries constructed in various species of Actinidia showed that, based on homologies to Arabidopsis, many families of resistance genes were represented. The mapping of these genes to establish their chromosomal positions on the Actinidia chinensis genetic linkage map was facilitated by designing 71 primer pairs to the EST sequences. The genetic markers mapped across 22 of the linkage groups, both as individuals and as members of a cluster. The clusters were generally homologous in nature, though heterologous examples were present. A sample of genes representing both the basal defence and the resistance-gene mediated defence pathways, here termed collectively ‘resistance genes’, was characterised. Chromosomal sites of resistance gene markers representing a range of such genes have been identified and can be considered during the selection of parents in breeding programmes.