Brachiaria breakthrough: scientists home-in on apomixis
The US corn industry underwent a massive transformation during the 20th century. The introduction of hybrid seeds allowed farmers to grow row after row of uniform plants, which produced large quantities of grain.
The seeds, which were produced by selectively breeding distantly related parents, captured what scientists call hybrid vigor – the ideal mix of superior genes, which allow for healthier, higher-yielding plants.
The only drawback for farmers was that they had to buy new seed each year. Breeders were only able to guarantee hybrid vigor in one generation of plants.
When hybrid plants mate, the previously-superior mixture of genes becomes jumbled. Some plants grow tall, others short; some have lots of grain, others very little; but the bottom line is that desirable uniformity is lost and yields, overall, decrease.
Some plants, however, don’t face this problem. Many tropical forage grasses, including Brachiaria, reproduce by apomixis.
Derived from the Latin roots apo (away) and mixis (mixing), apomixis is when plants clone themselves through their seeds and skip sex all together.
The advantage of apomixis is that when scientists find an outstanding plant with the right combination of desirable traits, they can continue to produce it faithfully through seeds over many generations without losing that all-important hybrid vigor.
Since the late 1980s, CIAT scientists led by John Miles have been breeding Brachiaria with the goal of developing superior apomictic hybrids for the tropical forage market. Brachiaria has a number of advantages over other forage grasses: highly nutritious, it can help farmers increase the productivity of their cattle, while also capturing carbon dioxide and restoring poor soils – particularly when used in silvopastoral systems. Some varieties are also drought resilient, meaning they can help farmers respond to climate change.
Steady advances to improve brachiaria have been made over the years using classical breeding methods. Recently, however, CIAT forage breeder Margaret Worthington has been looking to accelerate these gains through modern molecular breeding strategies.
With funding from the CGIAR Research Program on Livestock and Fish, Worthington and her team have developed the world’s first genetic maps of Brachiaria. These enabled them to identify the genetic region responsible for controlling apomixis – a major advancement for the forage breeding community that could help speed up Brachiaria breeding, and which has implications for several other crops. The details have just been published in Genetics.
“The complex genomics of Brachiaria and its ‘orphan crop’ status have prevented us from using modern molecular breeding strategies until recently,” she said.
Using the new maps and building on the research of Peggy Ozias-Akins and Joann Conner at the University of Georgia, Worthington and her team discovered that a gene believed to play a role in apomixis in a similar plant was also likely responsible for apomixis in Brachiaria.
“We found that a molecular marker from within an apomixis gene in the related forage genus Pennisetum was also perfect predictor of apomixis in Brachiaria,” Worthington said.
“This new molecular marker will allow Brachiaria breeders around the world to test thousands of seedlings for apomixis with a turnaround of just a few weeks, rather than waiting years to determine the reproductive mode of these plants using traditional methods.”
The discovery opens the door for further collaborative research within the forage breeding community, to better understand the genetics responsible for apomixis.
Looking ahead, Worthington says that she also has hopes that other key genes for apomixis can be identified and introduced in other crops.
“The dream is to develop an apomictic hybrid maize plant someday so that smallholder farmers can save their seed and capture the benefits of hybrid vigor over many generations without having to buy new seed each year,” she says.
In the meantime, several opportunities for further research exist, including comparative genomics of the region controlling apomixis in other grasses and mapping other gene sequences that affect the expression of apomixis in Brachiaria.
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