The story of corn is anything but corny. It's a tale that highlights resilience, survival, and a remarkable genetic trait that has allowed maize to adapt and thrive across diverse climates for thousands of years. This incredible journey, driven by what scientists believe to be a genetic process known as selfish inheritance, has not only fed countless generations in the Americas but also offers insights that may extend beyond agriculture.
Around 9,000 years ago, maize was first domesticated by Indigenous peoples in the lowlands of Mexico. About 5,000 years later, it crossed paths with a different type of maize from the highlands known as teosinte mexicana. This interbreeding led to a remarkable adaptability to cold climates, allowing maize to spread rapidly across the continent. The speed of this adaptation has puzzled scientists for years.
Rob Martienssen, a geneticist at Cold Spring Harbor Laboratory (CSHL) and co-author of a new study published in the journal *Nature*, explains, "Civilizations depend on the domestication of staple crops like corn, and like humans, crops often undergo multiple rounds of evolution. Modern maize, for instance, interbred with teosinte mexicana only 4,000 years ago and gained multiple beneficial genes in a very short time."
The Role of RNA Interference
Martienssen's research focuses on RNA interference, a process where small RNAs silence genes and manipulate how they are expressed. The study was sparked by an intriguing observation from University of Wisconsin geneticist Jerry Kermicle. While crossing semi-sterile teosinte hybrids with traditional maize, Kermicle noticed unusual behavior in the offspring. Normally, such hybrids would become completely sterile or fertile over time, but in this case, the resulting offspring remained semi-sterile when crossed with maize. This unusual inheritance pattern hinted at a deeper, perhaps selfish, genetic mechanism at play.
Unraveling Selfish Inheritance
Previously, selfish genetic systems, also known as gene drives, were thought to be rare in nature. However, the discovery of such a system in maize suggests that it may have played a significant role in the rapid evolution of the crop. In their study, Martienssen and CSHL graduate student Ben Berube sequenced the genomes of hundreds of pollen grains from the semi-sterile offspring. They found that specific segments of the teosinte genome, located on chromosomes 5 and 6, were always inherited. This pointed to the presence of a "selfish" genetic system they named the Teosinte Pollen Drive.
The team discovered that a gene called Dicer-like 2 on chromosome 5 was responsible for producing small RNAs that were consistently present in the semi-sterile hybrids but absent in traditional maize. This selfish genetic system effectively eliminated competing pollen grains lacking the gene drive, ensuring that certain traits were passed on more frequently through males than females.
Martienssen noted, "The most surprising finding was that modern maize had become ‘immune’ to the selfish inheritance, by mutation of a single gene in pollen that could be traced from wild grasses, through tropical maize and popcorns, all the way to modern maize. This suggests the selfish genetic system was active over a long period of time."
The Broader Implications
The study's findings have broader implications for both agriculture and our understanding of domestication in all living things. If teosinte mexicana is what Martienssen calls "the Neanderthal of maize," for its ability to interbreed, this discovery may be a missing link that explains how corn was able to thrive across the Americas. It also sheds light on why small RNAs are so prevalent in plant and animal sperm cells, including those of humans.
"Basic discoveries in plant biology can have implications far beyond agriculture," says Martienssen. "The small RNAs we describe in maize pollen are powerful agents of inheritance and evolution, and similar small RNAs are found in sperm cells from animals, including our own. It is possible they played similar roles in animal domestication, and perhaps even in human evolution."
In conclusion, the story of corn is not just a tale of survival but also a testament to the complex and fascinating ways in which life adapts and evolves. The insights gained from this study may unlock new understanding in genetics, agriculture, and even human history.