Biology Reference
In-Depth Information
ease resistance, fruit quality, and yield (Foolad
2007a).
Introduction
Tomato (
Solanum lycopersicum
L.) is an eco-
nomically and nutritionally important crop
grown throughout the world, and has been used
as a model organism for genetics, genomics and
physiological studies for the past 70 years. Many
factors have contributed to its suitability for
both basic and applied research; this includes
ease of culture, short life cycle, photoperiod
insensitivity, high self fertility and homozygos-
ity, great reproductive potential, ease of con-
trolled pollination and hybridization, amenabil-
ity to asexual propagation and whole plant regen-
eration, and availability of a wide array of mutant
and genetic stocks (
http://tgrc.ucdavis.edu/
;
http://www.sgn.cornell.edu/
). I
n addition,
S.
lycopersicum
(hereafter, “tomato”) is a diploid
species with a rather small genome (
Brief History of Tomato
The
Solanum
genus is thought to have evolved
approximately 12 million years ago (Mya), with
the tomato clade (section Lycopersicon) radi-
ating from the potato clade (section Solanum)
at about 7 Mya (Wikstr om et al. 2001). Since
then, tomato species have evolved to inhabit
a vast array of elevations, soil types, and cli-
mates (Taylor 1986). In fact, although a tropical
plant, tomatoes are now grown in some form
in every region of the world, from the tropics
to within a few degrees of the Arctic Circle
(Foolad 2007a).
Solanum
sect. Lycopersicon is
monophyletic, consists of 13 different species,
and has been thoroughly reviewed (Labate et al.
2007). The present-day cultivated tomato is the
product of more than six centuries of domestica-
tion and selective breeding, presumably initiated
by Mayan agriculturalists (Kalloo 1991; Peralta
and Spooner 2005). The cultivated tomato is
thought to have originated either in Mexico or
Peru, and an ensuing controversy over the exact
geographic origin has persisted since the late
19
th
century (Peralta and Spooner 2005). How-
ever, despite the wide distribution of the genus in
the Andean region, Mexico has been considered
the most likely center of domestication (Rick
1976b). Regardless of origin and place of domes-
tication, natives were already cultivating tomato
when European explorers arrived in the Ameri-
cas in the late 15
th
century, and subsequently the
Spaniards introduced the crop to the Old World
early in the 16
th
century (Rick 1976b; 1978;
Peralta and Spooner 2005). Europeans were ini-
tially suspicious of tomato due to its morpholog-
ical resemblance to poisonous nightshade, thus
preventing widespread use of the tomato as a
food until the late 19
th
century (Rick 1978).
Commercial production of tomato in the U.S.
began in 1847 at Lafayette College, in Easton,
Pennsylvania, leading to a major vegetable pro-
duction industry in the mid 20
th
century (Foolad
0.95 pg/
1C, 950 Mbp) that lacks extensive gene dupli-
cation. Throughout the past 70 years, modu-
lating tomato fruit quality to suit grower and
end-user specifications has been a focal point in
many tomato breeding programs (Foolad 2007a).
As with nearly all crops in the United States,
practical tomato breeding was originally led
primarily by researchers at public institutions,
and breeding efforts and various germplasm
collection expeditions by C.M. Rick and oth-
ers have formed the basis of the present-day
tomato cultigen. Because of the justified focus
on tomato by early genetics and molecular biol-
ogy researchers, there have also been many
widely-heralded discoveries using the tomato
system, including but not limited to identifi-
cation and development of some of the earli-
est plant molecular markers and genetic maps
(Tanksley et al. 1982; Tanksley and Orton
1983), and fine mapping and cloning of the
first plant disease resistance gene,
Pto
(Mar-
tin et al. 1993a; Martin et al. 1993b), and the
first QTL,
fw2.2
(Frary et al. 2000). Tomato
has also been an excellent system for breeding
purposes, evidenced by the continual introgres-
sion of desirable genes and phenotypes from
wild species and improvement of the crop dis-
∼
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