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Geneticists Study Origin, Evolution of "Sticky"
study by two North
Carolina State University geneticists traces
the origin and evolution of a genetic mutation
that long ago led to the creation of a type of
rice known as glutinous, or "sticky,"
molecular genetic research leads researchers to
believe that glutinous rice - which differs from
non-glutinous, or common, rice on account of a
mutation in its Waxy gene that suppresses
the formation of a starch called amylose - most
likely originated a single time in Southeast Asia.
Further, DNA evidence - namely the lower-than-expected
genetic variability in the Waxy gene -
suggests that early domesticators of glutinous
rice liked its adhesive quality and wanted to
preserve that particular trait.
tested in iodine, grains of non-glutinous rice
turn black (left), while glutinous rice remains
Digital images are available by contacting NC
State News Services at 919/515-3470.
Michael Purugganan, associate professor of genetics,
and Dr. Kenneth Olsen, post-doctoral research associate
in genetics, publish their findings in the Oct. 23 edition
learn more about the origin and evolution of sticky
rice, the researchers studied 105 glutinous and non-glutinous
samples of rice donated from the multitudinous stock
kept by the International Rice Research Institute in
Los Banos, Philippines.
contains two starches: amylose and amylopectin. Glutinous
rice lacks amylose; in fact, it is the lack of amylose
that gives it its sticky composition. Non-glutinous
rice - what you'd find if you cooked up a name-brand
package of rice from the grocery store, for example
- contains up to 30 percent amylose; the result is rice
with grains that separate.
rice is the staple food in some areas in Southeast Asia,
including parts of Laos, Thailand and Cambodia, the
researchers say. Sticky rice has also migrated north
to become an important part of the diet in places like
China and Japan. Used primarily in a number of desserts
- rice cakes, for example - sticky rice has achieved
important cultural standing in East and Southeast Asia.
Asian folklore diverges on the origin of glutinous rice,
Purugganan says. He found both a Laotian Buddhist legend
charting the existence of glutinous rice to about 1,100
years ago and Chinese folklore that indicated the existence
of glutinous rice more than 2,000 years ago.
no one really knows where glutinous rice came from,
we wanted to find its origin using molecular means,"
Purugganan says. "We also wanted to find out the
number of times the mutation in the Waxy gene
that suppresses amylose, which produces glutinous rice,
arose during rice domestication. And, we wanted to see
if the Waxy gene showed evidence of selection
by early Asian farmers."
genetic sequencing of these samples at NC
State's Genome Research Laboratory, Purugganan and
Olsen assembled a "gene tree," or network
that represents patterns of genetic differences among
the DNA sequences, Olsen explains.
the gene tree, the researchers found that sticky rice's
genetic mutation maps to a single mutation on the gene
tree, suggesting that the mutation occurred a single
time rather than more than once, Olsen says. Looking
at the geographic locations of the rice DNA sequences
that are direct ancestors of the mutation, the researchers
found fairly strong evidence that Southeast Asia was
the geographic origin of sticky rice. This squares with
the fact that sticky rice is a staple in some parts
of Southeast Asia.
type of research really opens up the window of not only
how crops originate, but also how specific features
evolve," Purugganan says. Olsen adds, "This
is one of the first times that anyone has looked within
a crop species at the evolutionary and geographical
origins of important domestication traits in crops."
and Olsen now plan to study other genes involved in
starch synthesis in rice.
funding for the study of the origin and evolution of
glutinous rice was provided by the Alfred P. Sloan Foundation.
to editors: An abstract of the paper follows.
Evidence on the Origin and Evolution of Glutinous Rice"
Authors: Kenneth M. Olsen and Michael D. Purugganan,
North Carolina State University
Published: Oct. 23, 2002, in Genetics
Abstract: Glutinous rice is a major type of cultivated
rice with longstanding cultural importance in Asia.
A mutation in an intron 1 splice donor site of the Waxy
gene is responsible for the change in endosperm starch
leading to the glutinous phenotype. Here we examine
an allele genealogy of the Waxy locus to trace
the evolutionary and geographical origins of this phenotype.
Based on 105 glutinous and non-glutinous landraces from
across Asia, we find evidence that the splice donor
mutation has a single evolutionary origin, and that
it probably arose in Southeast Asia. Nucleotide diversity
measures indicate that the origin of glutinous rice
is associated with reduced genetic variation characteristic
of selection at the Waxy locus; comparison with
an unlinked locus, RGRC2, confirms that this
pattern is specific to Waxy. In addition, we
find that many non-glutinous varieties in Northeast
Asia also carry the splice donor site mutation, suggesting
that partial suppression of this mutation may have played
an important role in the development of Northeast Asia
non-glutinous rice. This study demonstrates the utility
of phylogeographic approaches for understanding trait
diversification in crops, and it contributes to growing
evidence on the importance of modifier loci in the evolution
of domestication traits.