Localization of a Novel Gene for Resistance to Fusarium Root Rot Race 2 in Lettuce, Lactuca sativa L.

Root rot disease in lettuce is induced by Fusarium oxysporum f. sp. lactucae, which is classified into three races and race 2 among them is now spreading in the lettuce field in Japan. We have been attempting to develop SCAR markers closely linked to the resistant genes against root rot race 2 for the convenience of marker assisted selection in lettuce breeding. We used 90 plants of F₂ generation from a cross between a strain ‘PI0735’ and a cultivar ‘Patriot’, which are resistant and susceptible to root rot race 2, respectively. Seventeen AFLP markers were found closely linking to the resistant gene by bulk method using 4096 possible combinations of primers. Among them, five SCAR markers, having map distances of 1.2 cM to 2.5 cM from the resistant gene, were developed through conversion of AFLP markers. Then we attempted to localize the resistant gene more precisely and developed primers based on the information on the sequence data from LETTUCE GENOME RESOURCES (https://lgr.genomecenter.ucdavis.edu/), using 191 F₂plants from a cross ‘2008-11”(resistant) x ‘Shinano Hope’ (susceptible). As a result, it was found that the resistant gene was located between the newly developed markers with the range of 1 Mbp on the linkage group 7. It was also clarified that the gene was different from that reported previously by Aruga et al. (2012). Therefore, it is clear that at least two different genes for resistance against race 2 are present in the lettuce genome.