Merida, Mexico
June 20, 2008
Source:
American Society of Plant
Biologists
Domesticated tomatoes can be up to
1000 times larger than their wild relatives. How did they get so
big? In general, domesticated food plants have larger fruits,
heads of grain, tubers, etc, because this is one of the
characteristics that early hunter-gatherers chose when foraging
for food. In addition to size, tomatoes have been bred for
shape, texture, flavor, shelf-life, and nutrient composition,
but it has been difficult to study these traits in tomatoes,
because many of them are the result of many genes acting
together. These genes are often located in close proximity on
chromosomal regions called loci, and regions with groups of
genes that influence a particular trait are called quantitative
trait loci (QTLs). When a trait is influenced by one gene, it is
much simpler to study, but quantitative traits, like skin and
eye color in humans or fruit size in tomatoes, cannot be easily
defined just by crossing different individuals. Now, with genome
sequencing and genomics tools, chromosomal regions with QTLs can
be mapped and cloned more easily than in the past. These genomic
maps can also be compared across plant genomes to identify
similar genes in other species. With this knowledge, breeders
can improve tomato varieties as well as other less well known
food plants in the family Solanaceae.
Dr. Steven D. Tanksley and his colleagues, Bin Cong and Luz S.
Barrero, are studying QTLs that influence fruit size. Dr.
Barrero, of the Corporación Colombiana de Investigación
Agropecuaria (CORPOICA), Colombia, will be presenting this work
at a symposium on the Biology of Solanaceous Species at the
annual meeting of the American Society of Plant Biologists in
Mérida, Mexico (June 29, 8:30 AM).
Tomato (Solanum lycopersicum) is a member of the Solanaceae or
nightshade family, which also includes potato, eggplant,
tobacco, and chili peppers. The center of origin and diversity
of tomato and other solanaceous species is in the northern
Andes, where endemic wild populations of these species still
grow. Tanksley and his colleagues have been employing the data
emerging from the International Tomato Genome Sequencing Project
as well as the tools of structural genomics to clone and
characterize the major gene and QTL responsible for extreme
fruit size during tomato domestication—fas.
The first QTL, fw2.2, was the first ever cloned in plants and
may have been the site of one of the earliest mutations in
tomato that led to its selection by humans and subsequent
domestication. The size of tomato fruit can vary up to 30% as a
result of variation at this locus alone. Cloning and sequencing
of this locus reveals that the wild type protein codes for a
repressor of cell division. When the control sequence is
mutated, the repressor protein is not expressed or only very
little, leading to higher cell division during fruit development
and, consequently, larger fruits.
However, fw2.2 and associated genes related to cell-cycle
control and cell division are not solely responsible for extreme
fruit size. Two other loci-- locule-number and fasciated (fas)--
influence fruit size indirectly by affecting the number of
carpels, the female parts of the flower that will become seed
chambers in the fruit. Most wild tomatoes have only 2-4 locules
(ovary chambers) while domesticated varieties can have 8 or
more, and it appears that increase in locule number can increase
fruit size by 50%. The data indicate that, of the two loci, fas
has the larger effect. Tanksley and his colleagues used
positional cloning to isolate the fas locus.
Sequencing suggested that the fas gene encodes a protein
(YABBY-like transcription factor) that controls transcription of
DNA into RNA as the first step of gene expression. It also
revealed that there were no changes in the protein coding region
of the gene but rather the mutation consisted of an insertion in
the first intron, which is a non-coding sequence embedded within
the protein coding sequence. Although introns are not part of a
gene’s protein code and are removed from the RNA sequence before
translation into proteins, they are nevertheless structurally
and functionally important, as demonstrated in this locus. The
presence of an insertion in this intron reduces expression of
the fas gene. The scientists looked at where and when the gene
is expressed and found it dramatically reduced in developing
flower buds in plants with high locule numbers.
Further comparisons of this locus across different tomato
cultivars, including wild varieties, which turned out not to
contain the mutation, suggests the mutation occurred relatively
recently in tomato domestication and spread rapidly throughout
modern tomatoes as a result of selection for extreme fruit size.
Comparative genomics tools are being applied in both well-known
and obscure solanaceous species. Conservation of genes and loci
across a number of these species suggests that the knowledge
gained from these efforts can also be applied in crop and yield
improvement for other members of the Solanaceae. |
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