Marker-Assisted Selection for Disease Resistance in Lettuce - Translational Genomics for Crop Breeding: Biotic Stress - page 267

Chapter 14

Marker-Assisted Selection for Disease
Resistance in Lettuce

I.Simko

Abstract

Lettuce ( Lactuca sativa L.) is the most popular leafy vegetable cultivated mainly in moderate climates.
Consumers demand lettuce with good appearance and free of disease. Improved disease resistance
of new cultivars is achieved by combining desirable genes (or alleles) from existing cultivars or by
introgressing novel resistance genes from wild Lactuca species sexually compatible with cultivated
lettuce. Development of elite lettuce cultivars is a lengthy process that involves cross-pollination, sev-
eral rounds of selection, development of homozygous genotypes, and testing of material performance.
Use of molecular markers linked to the disease resistance genes allows for rapid and frequently more
accurate selection of desirable genotypes than phenotype-based selection. This marker-assisted selec-
tion (MAS) is used by both public and private lettuce-breeding sectors. At the present time, MAS for
disease resistance in lettuce is limited to simply inherited traits. Assays have been developed to detect
genes and alleles for resistance to downy mildew, corky root, lettuce mosaic, and lettuce dieback
(a description of three assays based on the high-resolution DNA melting approach is provided in
this chapter). Resistance genes against Verticillium wilt, turnip mosaic, root downy mildew, powdery
mildew, big-vein, Fusarium wilt, and anthracnose have been mapped on the molecular linkage map of
lettuce, and assays for MAS are under development for several of these genes. The lettuce genome has
been sequenced recently, paving the way to speedier mapping, cloning, and functionally validating
genes for disease resistance and for more efficient development of molecular markers used in MAS.

the total, followed by North and Central Amer-
ica (19%), and Europe (13%). Substantially less
lettuce was grown in South America, Africa,
and Oceania, each producing around 1% of the
world's total. The largest producer of lettuce as a
salad crop is the United States, where production
is centered in California (70-75% of the lettuce
grown in the U.S.) and Arizona (18-20%) (Ryder

Introduction

Lettuce ( Lactuca sativa L.) is commercially pro-
duced in many countries around the world, mak-
ing it the most popular vegetable from the group
of leafy vegetables. The total world production
of lettuce in 2010 was more than 23 million tons
(FAOSTAT 2012). Asia produced about 65% of