Raleigh, North Carolina
August 6, 2009There's no
"silver bullet" gene or gene region that controls so-called
complex traits in maize, commonly known as corn.
Instead, in two research papers published this week in the
journal Science, North Carolina
State University crop scientists and colleagues show that
lots of small changes in a number of gene regions affect complex
traits – like flowering time or reproductive ability – in corn.
Finding out more about the mechanisms behind complex traits like
flowering time – as well as even more difficult-to-map traits
like yield or drought tolerance, for example – has the potential
to help plant breeders build the best traits into tomorrow's
corn plants, says Dr. Jim Holland, NC State professor of crop
science, research geneticist for the U.S. Department of
Agriculture - Agriculture
Research Service (USDA-ARS) and one of the lead authors of
the Science papers.
Holland and Dr. Major Goodman, NC State professor of crop
science, joined with researchers from Cornell University, the
University of Missouri and other institutions to assemble a set
of genetic maize varieties called the maize nested association
mapping population. They found a number of chromosomal regions –
called quantitative trait loci (QTL) – affecting flowering time
in corn.
Identifying QTLs can help scientists get closer to figuring out
the actual genes involved in certain traits. Holland likened it
to looking for a specific house in a large city, with the QTL
providing the correct street, but not necessarily the right
house.
The scientists found that an average of 29 to 56 QTLs affected
flowering time; the effects of these QTLs were small.
That finding contrasts with studies of Arabidopsis, or mustard
weed, the ubiquitous lab rat of the plant world. In that plant,
small numbers of QTLs have large effects on genetic variance.
The scientists also studied more than 1,100 marker genes that
characterize genetic inheritance. In other words, the
researchers wanted to know if genes from one parent are
inherited more frequently than genes from another parent.
While they predicted that more genes from one parent would be
inherited, the study showed that, for the vast majority of the
genome, each parent contributed about half. But subtle
deviations from this were often observed, indicating that many
genes had small effects on reproductive success.
Holland says that the nested association mapping population will
be a resource for scientists to both build a better corn plant
and to show how changes in the genome produce differences in
individual plant families. That, in turn, will help scientists
make more accurate predictions about complex traits.
"These findings will be a big help in the future," Holland says.
"We can now take a complicated trait, identify gene regions
involved in the trait, and then use that information in breeding
to ensure the best combinations of genes from different sources
or varieties."
The research was funded by the National Science Foundation and
the USDA-ARS.
"The Genetic
Architecture of Maize Flowering Time"
James Holland and Major Goodman, North Carolina State
University; et al
Published: Aug. 7, 2009, in
Science
Abstract
Flowering time is a
complex trait that controls adaptation of plants to
their local environment in the outcrossing species Zea
mays (maize).We dissected variation for flowering time
with a set of 5,000 recombinant inbred lines (maize
Nested Association Mapping population, NAM). Nearly a
million plants were assayed in eight environments but
showed no evidence for any single large effect
quantitative trait loci (QTLs). Instead, we identified
evidence for numerous small-effect QTLs shared among
families; however, allelic effects differ across founder
lines. We identified no individual QTLs at which allelic
effects are determined by geographic origin or large
effects for epistasis or environmental interactions.
Thus, a simple additive model accurately predicts
flowering time for maize, in contrast to the genetic
architecture observed in the selfing plant species, rice
and Arabidopsis.
"Genetic Properties of
the Maize Nested Association Mapping Population"
James Holland and Major Goodman, North Carolina State
University; et al
Published: Aug. 7, 2009, in
Science
Abstract:
Maize genetic diversity
has been used to understand the molecular basis of
phenotypic variation and to improve agricultural
efficiency and sustainability. We crossed 25 diverse
inbred maize lines to the B73 reference line, capturing
a total of 136,000 recombination events. Variation for
recombination frequencies was observed among families,
influenced by local (cis) genetic variation. We
identified evidence for numerous minor single-locus
effects but little two-locus linkage disequilibrium or
segregation distortion, which indicated a limited role
for genes with large effects and epistatic interactions
on fitness. We observed excess residual heterozygosity
in pericentromeric regions, which suggested that
selection in inbred lines has been less efficient in
these regions because of reduced recombination
frequency. This implies that pericentromeric regions may
contribute disproportionally to heterosis.
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