March 2020 - 22

Researchers track ancestry of tomato to help growers
By Merritt Melancon
University of Georgia
The path from wild weed to the
carefully cultivated vegetables that
fill our refrigerators is not always a
straightforward tale of domestication.
Different cultures have different
priorities and growing conditions, and
sometimes crops are domesticated more
than once.
Recently a large team of molecular
biologists and computational data
experts, led by University of Georgia
horticulture Professor Esther van
der Knaap, has unraveled part of the
twisted history of the tomato.
Anthropologists and geneticists have
long known that modern tomatoes were
cultivated from their blueberry-sized
wild ancestors in South and Central
America several thousand years ago. But
a recent National Science Foundationfunded
study found two domestication
paths - one that occurred in Central
and South America thousands of years
ago in Ecuador and one that occurred
thousands of years later in Mesoamerica,
or modern-day Mexico.
The researchers were surprised
to find that the commonly accepted
wild ancestor, called Solanum
pimpinellifolium, may actually not be
the wild ancestor of the modern tomato.
Instead, S. lycopersicum cerasiforme
evolved 76,000 years ago and became
domesticated in Ecuador many years
later. The second Mesoamerican
domestication created the tomato that
has spread around the world today.
In the most recent published study
funded by van der Knaap's NSF grant -
which was published online in Molecular
Biology and Evolution - the team was
able to document tomato's history by
analyzing the genomes of multiple
ancestral tomato varieties. What
was also surprising is that tomato
underwent a dedomestication step
" For grain crops, like corn and rice,
selections for grain size were very
important. Larger was better because
it meant more calories, but now people
don't eat vegetables for caloric intake
but more for the overall nutritional
qualities, " van der Knaap said. " For
tomato, we think the weight was very
important, but who knows what the
people before us thought? "
So, while we enjoy a juicy slice of
tomato on a sandwich, other cultures
may have valued more tart fruit for
their cuisine or medicinal purposes.
One culture's trash could be another's
treasure, and researchers are currently
looking for discarded genetic treasure
among the wild populations identified
in this study.
These traits, which could include
Professor Esther van der Knaap, who works at the University of Georgia College of Agricultural
and Environmental Sciences Department of Horticulture and Institute of Plant Breeding,
Genetics and Genomics, is part of the team that is unlocking the history of ancient tomatoes to
breed a more sustainable future for modern crops. Photo: University of Georgia
as fruit size became smaller while
migrating to Mesoamerica. This
suggests a reduction in selection
pressures or that tomato became a feral,
weedy type that was not tended well.
Eventually, tomato was domesticated
further into the large types we are
familiar with today.
The work involved researchers at
four universities and was spearheaded
by Professor Ana Caicedo and
postdoctoral researcher Hamid Razifard
from the University of Massachusetts
Amherst Department of Biology.
The researchers documented
23,797,503 polymorphic sequences
and compared them to the standard
genome for domesticated tomato.
Comparing where the mutations
occurred in the genome with the
samples' geographic locations enabled
researchers to track the evolutionary
history of today's tomato.
By tracing the history of the tomato,
scientists are able to gain insight into
the evolution of modern crop plants and
their intersection with early cultures,
learning more about the genetics that
may make crops more sustainable and
productive in the future.
Each time the tomato was grown,
early farmers selected for desired
traits. However, it is not well known
whether fruit weight and flavor were
as important 10,000 years ago as they
are today. It is possible that the early
farmers left some traits behind in the
unselected population. Some ancient
farmers may have selected for traits that
we might find distasteful today.
improved flavor profile, fruit weight,
water efficiency, and disease- and
pest-resistance, can be bred back into
modern tomato lines to help farmers
grow better, healthier tomatoes with
fewer inputs.
" The tomatoes we studied are
genetically very diverse, " van der Knaap
said, describing the wild ancestor
populations. " There were some useful
(traits) that were left behind for fruit
weight and possibly for the flavor of the
tomato. Why they were left behind, I
don't know. But those are clearly alleles
that we could use to produce better
modern tomatoes. "
Van der Knaap's lab has spent the
past three to four years evaluating
individual plants from these wild
populations in field tests in Blairsville,
Georgia, at the Georgia Mountain
Research and Education Center and
in Lyons, Georgia, at the Vidalia
Onion and Vegetable Research Center.
In both locations, researchers have
been working to pinpoint parts of the
genomes present in these semi-wild
plants that may help to create a new
generation of improved tomatoes. VGN
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March 2020

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