Mapping of the Bacterial Leaf Spot Resistance Gene in Lettuce PI358001-1

Lettuce is a diploid (2n=18) with a genome size of 2639 Mbp, and belongs to the family of Compositae.  It is one of the top ten most valuable crops in the United States with an annual value of over $2 billion.  Bacterial leaf spot (BLS) is a major disease of lettuce and is caused by Xanthomonas campestris pv. vitians. Severe outbreaks of the disease have occurred in the areas of lettuce production in the United States since 1990s, and also have been reported in other lettuce production countries in the world.  Leaf lettuce PI358001-1 has been characterized as a highly resistant accession to BLS. A F_2:3 segregating population with 162 families has been developed from the cross PI358001-1 (resistant leaf lettuce accession) × Tall Guzman (a susceptible Romaine lettuce variety).  The segregation ratio for the F_2:3 families was showed to be 32 homozygous resistant: 82 heterozygous: 48 homozygous susceptible by seedling inoculation with BLS L7 strain, fitting to 1:2:1 (Χ²=3.19, p=0.20).  The results indicate that a dominant gene controls the resistance in PI358001-1.  A genetic linkage map with AFLP and CAPs markers is under development.  Based on testing 64 MseI+3/EcoRI+3 primer combinations among 62 F₂ progenies and 2 parents, a total of 368 polymorphic markers were scored.  On average, 5.75 polymorphisms were detected for each combination. The AFLP results comply with the previous report that the polymorphism was relatively low in lettuce.