Researchers found little genetic variation for grain yield among intermediate-maturity stiff-stalk lines, potentially limiting breeders’ ability to develop productive hybrids for future growing conditions.
A new study led by University of Illinois Urbana-Champaign researchers has identified critically low genetic variation in one of the primary breeding stocks used to develop U.S. corn hybrids.
The finding applies specifically to grain yield in intermediate-maturity hybrids, which dominate commercial production in the U.S. and account for nearly one-third of global corn production.
Researchers warn that the lack of diversity could limit breeders’ ability to develop high-yielding hybrids capable of performing under changing climatic and environmental conditions.
“Our study reveals a significant genetic weakness that threatens maize breeders’ ability to develop high-yielding hybrids, especially in the context of climatic challenges projected for the U.S. Corn Belt,” said lead author Jenifer Camila Godoy dos Santos, a postdoctoral researcher in the Department of Crop Sciences in the College of Agricultural, Consumer and Environmental Sciences at the University of Illinois.
Why Genetic Diversity Matters
Modern corn breeding depends heavily on heterosis, also known as hybrid vigour. When breeders cross parents from genetically distinct groups, the resulting hybrids can significantly outperform either parent in the field, according to a press release.
For decades, breeders have developed new corn hybrids by crossing parents from two principal heterotic groups: stiff stalk, or SS, and non-stiff stalk, or NSS. The genetic differences between these groups help produce high-performing hybrids.
Within each group, however, the inbred lines are increasingly similar.
Over time, breeders have repeatedly selected and reused the strongest-performing lines as parents, gradually reducing the genetic variation available within each heterotic group. As that variation declines, crosses between the groups may become less likely to produce hybrids with significant yield gains.
Godoy dos Santos and her collaborators suspected that genetic variation was declining but did not know the extent of the loss or which traits were most affected.
Intermediate-Maturity Lines Raise Concerns
The researchers evaluated the genetic variation underlying several plant traits in 13 SS and 28 NSS inbred lines representing early-, intermediate- and late-maturing types. They examined variation associated with grain yield, plant height and the number of days to silking and anthesis.
Their most significant finding was a lack of genetic variability related to yield among intermediate-maturity SS lines. This maturity group is the most commonly grown across the U.S. Corn Belt.
According to the researchers, the limited diversity could prevent this important heterotic group from contributing effectively to new hybrids capable of meeting future yield goals.
The results were more encouraging elsewhere in the breeding pool. The team found substantial genetic variability across the measured traits in early- and late-maturing inbreds from both the SS and NSS groups. Intermediate-maturity NSS inbreds also retained sufficient variation for grain yield.
A Decade of Collaborative Research
The project grew out of a collaboration that began more than a decade ago.
In 2013, university corn breeders across the United States recognized that sharing breeding material and testing it across multiple environments could strengthen their research. That effort led to the creation of the Genomes to Fields initiative.
Along with exchanging breeding materials for testing at locations across the country, participating researchers began examining some of the most difficult issues facing the corn industry.
“In 2015 or 2016, we started asking whether we still have genetic variation in our breeding germplasm. That’s important because if we don’t have variation, then we don’t make progress. Without variation, we can’t adapt our germplasm to changing growing conditions, climate change, or diseases that might come up. And most importantly, we can’t produce hybrids that are high-yielding in these new environments,” said Martin Bohn, corn breeder and crop sciences professor at the University of Illinois.
To characterize the remaining genetic diversity, researchers crossed SS and NSS inbred lines and grew 162 single-cross hybrids at more than 30 locations in the United States and Canada. They collected phenotypic data across maturity groups before turning the dataset over to Godoy dos Santos and Alex Lipka, a statistical geneticist and professor of biometry at the University of Illinois.
Broadening the Breeding Pool
The researchers said breeders should take deliberate steps to reintroduce genetic variation into commercial breeding pools.
“Our analysis can be distilled to a very powerful take-home message about the depletion of the genetic variability in an important group of corn: We are lining ourselves up for an emergency,” Lipka said. “The fact that we’ve observed insignificant genetic variability for yield in this heterotic group should be viewed as a canary in a coal mine. We have to do something about it.”
Bohn pointed to the USDA Agricultural Research Service’s Germplasm Enhancement of Maize program as one possible source of greater diversity. The publicly funded program strategically introduces germplasm from around the world into elite heterotic groups.
Although the findings represent a warning for the industry, the researchers emphasized that breeders still have opportunities to broaden the genetic base of future hybrids.
“While this study is a warning sign, it does not signify the end of maize breeding, which has been making tremendous progress for more than 100 years,” Godoy dos Santos said. “Our study reminds us of the importance of protecting and broadening the genetic diversity available to breeders. By introducing new sources of genetic diversity, breeding programs can continue developing better maize hybrids for future generations.”
