Are diploid potatoes in your future?

What is all the fuss about? Perhaps, you have been hearing about diploid potatoes and how they are going to change the industry. Are diploid potatoes really the wave of the future?

It is important to tell you up front that these new potatoes will look, taste and cook like the potatoes we grow now. To see what we mean, see if you can pick out the diploid potatoes in the accompanying figure.

Diploid potatoes will have benefits to the potato industry that come straight from their DNA. We expect that diploid hybrid potatoes will be able to change with the times more quickly. As your needs evolve, so will new potato varieties.

Let’s say you have a large toolshed. It is big enough that, when you buy a new tool, you can keep the old one, even if it is broken. You never know if you can repurpose it. A potato plant is like that. The potato varieties you grow have four copies of every gene. The genes are like tools. They enable the plant to do everything from capturing sunlight to building tubers and defending itself against pathogens. Since the potato plant has four sets of every tool, it can afford to hold onto the old dysfunctional ones. Only one of the four needs to be able to do its job. The accumulation of these poorly functioning and broken tools is what a geneticist calls genetic load. Potato has a high genetic load.

Now, let’s say your neighbor also has a large toolshed full of both useful and broken tools. He is going to help you to build a pole barn. You recruit a few family friends, Fred and Joe, to do the manual labor. Fred and Joe don’t know anything about tools, but they are good at following directions. You send Fred to your toolshed and Joe to your neighbor’s shed to get shovels to dig the footings. Fred comes back with a spade and a snow shovel, while Joe returns with a garden trowel and a spade with a broken handle. Among those four tools, only one will do the job. Next, you send them for a saw so you can begin to cut two-by-fours. They return with a hack saw, a hand saw, a broken hand saw and a bow saw.  They don’t bring the circular saw that you really need. As you build the pole barn, the tools selected randomly by Fred and Joe sometimes get the job done well, and sometimes are really inefficient. This process is similar to what happens when a potato breeder crosses two varieties in an attempt to create an improved variety. Two genes from each parent come together randomly in the offspring. Since many of the genes are broken or less than fully functional, the chance of assembling a better set of genes in the offspring is slim. The new plant created by crossing two varieties only rarely has an improved set of genes (tools) to build its leaves and tubers.

To continue the metaphor, assume a tornado has blown away your toolshed and your neighbor’s, along with your new pole barn. You both rebuild, starting with smaller tool sheds equipped with new, mostly power, tools. Even though the new sheds are smaller, there is plenty of room because you aren’t storing broken or inefficient tools. Now, you have to rebuild your pole barn. Once again, you enlist the help of Fred and Joe. You send them to get a saw and Fred brings a circular saw from your shed, while Joe brings back a table saw from the neighbor’s. Fred and Joe haven’t gotten any smarter. Its just that the odds of them randomly grabbing a useful tool have improved substantially. You now have power drills and your neighbor has air hammers. Together, with a better set of tools, you build a pole barn that is better than the original one in less time.

This process of improving the tool set and building a better pole barn is analogous to diploid potato breeding. First, we are reducing the genetic load in potato by making diploids — plants with only two copies of every gene instead of four copies. There is less room for defective genes. Then, through breeding, we clear out the dysfunctional and broken genes, making sure that both copies of every gene are able to do the job the potato plant needs them to do.

Plants that carry two identical copies of each gene are called inbred lines. We don’t need to assemble a perfect set of genes in any single inbred line. Instead, we will identify plants that complement each other. One inbred line may carry genes for fast emergence, Verticillium wilt resistance and resistance to cold-induced sweetening. Another may carry genes for PVY resistance, mid-season maturity and fast tuber bulking.

When they are crossed with each other, they produce a hybrid plant with all six traits.

The beauty of creating diploid inbred lines is that breeders can improve existing lines by adding new traits. For example, we can add a gene for late blight resistance to one of the lines. Then, the hybrid will be identical to the previous one, except that it is now resistant to late blight. When you grow the new variety, you will not have a steep learning curve. You will already know how to grow it based on the previous version of that hybrid.

The other important outcome of creating inbred lines is that we can now think about growing some of our potatoes from true seeds. At this point, we envision that the seed industry will plant hybrid seeds instead of tissue culture plants to generate mini-tubers in a greenhouse. From there, the system will be identical to the current one. Since we can more quickly and easily produce large numbers of true seeds than tissue culture plantlets, we expect that the seed industry will be able to quickly ramp up production of diploid hybrids in response to grower demands.

Potential challenges

First of all, most traits of interest to you are controlled by many genes that act together. We can’t simply plug in a gene for high yield or good storage quality. However, we are making this transition at a time when both DNA sequencing and computer processing technologies are advancing at unprecedented speeds. We can most effectively make use of these technologies in diploid potatoes. This will allow us to find combinations of genes responsible for complex traits such as yield.

A second challenge is simply one of competing resources. Potato breeders are among the hardest-working and most resourceful people you will meet. They have to be in order to create successful new varieties despite nearly impossible odds. How can they add diploid breeding to their already full plates?

We need to find ways to support diploid breeding without pulling resources away from current breeding programs.

Finally, the true seed approach is an exciting new road to travel, but we have much to learn before it can be fully implemented. We need to learn how to produce large numbers of seeds cheaply, and then how to get them to germinate quickly and uniformly. This will require many science experiments, which we love to do. We will also take advantage of seed technology knowledge from other crops. We look forward to making new connections with scientists outside the potato world who routinely use seeds for the propagation of new varieties.

This is truly an exciting time for the potato industry.

Paul Bethke is a molecular biologist and Shelley Jansky a research geneticist for the USDA’s Agricultural Research Service. Both are University of Wisconsin-Madison’s Department of Horticulture professors.