Expansion of Gene Families Encoding Disease Resistance Related Proteins in the Lactuca Clade within the Compositae

Disease resistance is one of the most important agronomic traits in lettuce due to the negative effect of pathogen infections on quality and yield.  Consequently breeding for resistance to pathogens is a high priority for breeders.  The majority of resistance genes are nucleotide binding-leucine rich repeat (NLR) encoding genes that tend to be grouped together in major resistance clusters (MRCs) along with other genes not involved in defense responses.  Analysis of these clusters, spanning several megabases, is challenging due the large number and size of NLRs and the high levels of sequence similarity within families.  Sequencing the genomes of two Lactuca and two Chicorium species in the Compositae family has enabled studies of the organization as well as the changes in gene content and evolution of MRCs during speciation, domestication, and crop improvement.   Comparative analysis within and across these genomes has revealed higher numbers of NLR-encoding genes in Lactuca sativa.  The expansion is due to an increase in the number of genes per cluster rather than in the number of clusters per se.  This may reflect breeding efforts to introgress genes for resistance to different diseases, particularly downy mildew, from multiple cultivated and wild accessions.  Further analyses of the divergence and evolutionary relationships within and between these multigene families are underway including additional wild accessions and cultivars to understand the overall diversity of NLR-encoding genes and their genomic architecture.