Marker-Assisted Selection for Disease Resistance in Lettuce - Translational Genomics for Crop Breeding: Biotic Stress - page 268

1997). China is the largest producer of stem let-
tuce, which is consumed raw, cooked, pickled,
dried, or stir-fried, in soups or as a sauce veg-
etable (Mou 2008; Simoons 1991). In Egypt,
oilseed lettuce is used to produce oil (Lebeda
et al. 2007; Ryder 1986). From the main lettuce
producing regions, the largest per capita produc-
tion in 2010 (FAOSTAT 2012) was in Spain (17.5
kg), followed by Italy (13.9 kg), and the USA
(13.1 kg).
Cultivated lettuce is a self-fertilizing diploid
(2 n 18 chromosomes) species from

the family of Compositae ( Asteraceae ). Lettuce
cultivars are usually classified into seven hor-
ticultural types based on head and leaf shape,
size, and texture, and stem length (Figure 14.1):
crisphead, butterhead, romaine (cos), leaf (cut-
ting), Latin (grasse), stem (stalk), and oilseed
(de Vries and van Raamsdonk 1994; de Vries
1997; Ryder 1999; Lebeda et al. 2007; Krıstkova
et al 2008; Mou 2008). The crisphead lettuce
produces a firm spherical head. This type is fre-
quently split into two subtypes - iceberg, with
a dense head, and Batavia, with a less dense
and smaller head. Butterhead lettuce produces
a smaller, less compact head than the crisphead
lettuce. Romaine type lettuce tends to have an
upright stature and forms a loaf-shaped head.
Leaf lettuce forms a bunch or rosette of leaves.
This type shows a large phenotypic variation in
leaf size, shape, color, and margin. Latin let-
tuce somewhat resembles romaine lettuce, but
the leaves are shorter and less crispy. Stem lettuce
produces a long, erect stem, with either narrow
or broad leaves. Oilseed lettuce to some degree
resembles stem lettuce with its long stem and
narrow, elongated leaves. The stem of this type
is frequently branching and the seeds are larger
than those formed on other types of lettuce.
Lettuce suffers from several economically
important diseases. To introgress novel resis-
tance genes into the cultivated lettuce, crosses
between L. sativa and its wild relatives are
carried out. Of the approximately 100 species
belonging to the genus Lactuca (Ferakova 1977)

= 2 x =

Fig. 14.1. Drawing of seven lettuce types: (1) butterhead,
(2) crisphead, (3) Latin, (4) leaf, (5) stem, (6) romaine (de
Vries and van Raamsdonk 1994), and (7) oilseed (illustration
by Nina Simkova).

only a few species are completely or par-
tially sexually compatible with cultivated lettuce.
Prickly lettuce ( L. serriola ) belongs to the pri-
mary gene pool that is closely related to culti-
vated lettuce and does not show any crossing
barrier; willowleaf lettuce ( L. saligna ) belongs
to the secondary gene pool that exchanges genes
with L. sativa to a limited degree; while bitter let-
tuce ( L. virosa ) belongs to the tertiary gene pool
that is difficult to cross with L. sativa . Combined