Construction of a Linkage Map with AFLPs to Identify Genes for Bacterial Leaf Spot Resistance in Lettuce (Lactuca sativa L.)

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 disease in lettuce is caused by Xanthomonas campestris pv. vitians. Severe outbreaks of the disease have occurred in major lettuce production areas in the United States since 1990s, and also have been reported in other countries. The authors have developed a F_2:3 segregation population consisting of 163 individual families, and a genetic linkage map is being developed with AFLP markers. A screening test of 64 MseI+3/EcoRI+3 primer combinations among 2 parents, 3 resistant and 3 susceptible F₂ progenies revealed polymorphic markers in 60 combinations. To date, 16 of the 60 combinations have been tested in 62 F₂ progenies and the parents, generating 124 polymorphic markers. On average, 7.75 polymorphisms were detected for each primer combination. This result complies with the previous report that the number of polymorphic bands obtained using three selective nucleotide primers was small in lettuce. Genetic study of disease resistance and identification of the linked markers are undertaken.