Wednesday, October 28, 2020
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Marker-assisted breeding helps feed a changing world


A generation ago, it might have taken scientists at huge land-grant universities, working with farmers, an entire career to develop new varieties of plants with just the right traits, the Washington Post reports. Today, with the help of huge DNA sequencing machines in sterile labs, what used to take plant breeders decades out in the field can be completed in just a few years, as scientists access huge databases with catalogs of genetic markers from different plant species and associate those markers with desirable or undesirable traits.

Let’s say we want a variety of rice that tastes good and can withstand flooding in India. Once breeders know which gene gives the rice the ability to survive submerged for an entire monsoon season, they can grow tens of thousands of individual plants in a greenhouse and screen the DNA for the flood-resistant gene. They usually collect the DNA by cutting out a few pieces of the plants’ leaves and grinding them up into a chemical soup, which they run through a sequencer.

If they find one that has the right gene—and spreadsheets tell them which individuals are most likely to carry the desired phenotype—they discard the others and breed the good ones. Within a few generations, they have enough rice seeds with the flood-resistant gene to bring it to market.

One of the advantages of screening the DNA for key genetic markers, so called “marker-assisted breeding,” is that the method steers clear of any genetic modification. No scientist ever touches the plant’s genes.

Nature and the process of genetic mutation alone do all the work, so there’s no need for any genetic engineering. Any genetic modification to the plant’s genome would add about 10 years to the time it would take to get government approval. We just don’t have that much time in some cases, and that elevates marker-assisted breeding to a new plateau in the high-tech world of plant biotechnology.

By simply growing tens of thousands of individual plants, there’s a better chance that at least one of them will have the desired phenotype—whether that’s a certain flavor or color, which might be linked to only one gene, or resistance to certain diseases, which might be linked to several different loci in the genome.

Are we reducing genetic variation?

So scientists might throw away hundreds of thousands of individual plants, which didn’t have the right genes, keeping only the one that did. Some scientists are worried that this produces plant lines that are weaker overall due to the presence of only highly selected traits.

The Post quotes Major Goodman, a leading corn expert and geneticist at North Carolina State University, as worrying that the new technology is so precise in finding desired genes that the genetic diversity of the discarded material will be lost. “In the long term,” he said, “it may have a detrimental effect. We are getting advances now that may cost us in the future.”

In other words, by breeding a tomato that tastes better, are we making the tomato plant more susceptible to hotter summers or in some way throwing out all the strength that comes from the diversity of a species gained through genetic variation? By proceeding quickly and throwing out thousands of viable plants, we’re probably thwarting tomato plant evolution to some extent and removing many “wild type” characteristics of the plant species.

But geneticists are smart. Most of them test the final result before bringing any seed line to market. They have to, for a failure would spell disaster for the huge plant companies that conduct the research and buy all those DNA sequencing machines. They’re working against the clock: the world’s population is increasing faster than ever, yet climate changes are also proceeding faster than tomato plants are accustomed to dealing with just by natural selection alone.

Our understanding of plant genetics has to grow if we are to provide an acceptable quality of food crops for a growing population under rapidly changing climate conditions.

Paul Katula
Paul Katula is the executive editor of the Voxitatis Research Foundation, which publishes this blog. For more information, see the About page.

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