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Surprises from a wild cousin

By crossing wild and cultivated potatoes....


November 14, 2007
By Carolyn King

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6aBy crossing wild and cultivated potatoes, researchers are developing lines
with strong resistance to the Colorado potato beetle and good processing characteristics.

A breeding program that is crossing wild and cultivated potatoes is already
producing hybrids with resistance to the notorious Colorado potato beetle and
enhanced processing quality. With a few more years of breeding and evaluation,
researchers could have a variety with plenty to offer to Canada's potato growers.

Dr. David De Koeyer and Dr. Yvan Pelletier, both from Agriculture and Agri-Food
Canada's Potato Research Centre in Fredericton, New Brunswick, have teamed up
on this breeding program. They are working with Solanum oplocense, a wild potato
species that originates in northwest Argentina and southern Bolivia.

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A key goal of the program is to develop a variety resistant to the Colorado
potato beetle, the most significant insect pest of potatoes in Canada. According
to some estimates, about 10 percent of potato production costs in Canada are
related to control measures and yield losses due to the beetle, says Pelletier.

He explains that this formidable pest has successfully followed the potato
around the world. "The Colorado potato beetle is very adaptable to different
environmental situations. It originated in Mexico and was first described in
the southern US where it is very warm, but the beetles survive very well under
our conditions. They do not have a lot of natural enemies, the population has
high fecundity and good survival during the winter, and they like the potato
plant which is good for them, so their populations just explode. They can multiply
by a 100-fold within a generation. So they are very, very successful. On top
of that, they have been able to find a way to survive, detoxify or become resistant
to the insecticides we've used against them so far, except for imidacloprid
(the active ingredient in Admire)."

Pelletier adds, "Although there are some signs that resistance to imidacloprid
might be coming, so far it is still used widely in Canada and I haven't heard
of any application failures. But we know that genetically the beetle can become
resistant to imidacloprid."

Pelletier and De Koeyer bring complementary skills and perspectives to this
breeding program. Pelletier, an entomologist, focusses on beetle/plant interactions,
evaluating which breeding lines have the best resistance to the beetle, why
the plant is resistant and whether the beetle can adapt to that resistance.
De Koeyer, a plant geneticist who is part of the research centre's potato breeding
team, focusses on breeding and evaluating the hybrid potato lines for processing
and agronomic characteristics. The researchers bring these two viewpoints together
to decide which plants to use as the parents for the next generation of hybrids.

The breeding program began in 1998 under the direction of Dr. Henry De Jong
who is now retired. The first generation of crosses between Solanum oplocense
and cultivated varieties (Solanum tuberosum) had some promising qualities.
De Koeyer says, "Almost all of the progeny were Colorado potato beetle-resistant,
but they did not have very good agronomic traits. They were quite late, and
they didn't produce very good tubers."

Next, in collaboration with Dr. Daniel Ronis of McCain Produce, the researchers
crossed five different first generation hybrids that showed excellent resistance
to the beetle with 10 cultivated potato breeding lines and varieties. From 2002
to 2005, they evaluated this second generation extensively.

About 25 percent of the second generation lines maintained a high level of
resistance to the beetle. The resistant lines were evaluated for tuber size
and shape and for processing characteristics. Many of the resistant progeny
had high specific gravity, which is preferred for French fry and chip processing,
and some had excellent chip colour after four months of cold storage.

In spring 2005, the researchers crossed the most promising hybrids with another
set of cultivated varieties to continue to improve maturity and other agronomic
traits while retaining resistance to the beetle and processing quality.

With a touch of luck
Along with their complementary skills and a lot of painstaking work, De Koeyer
and Pelletier and their teams of technicians have had some lucky surprises in
this breeding program. Perhaps the best piece of luck was the choice of Solanum
oplocense
.

Their program is the first time that Solanum oplocense has been used
to obtain Colorado potato beetle resistance. Pelletier says, "If you had
asked people who specialize in all of the species in the potato family, they
wouldn't have come up with oplocense as a first choice for resistance to the
Colorado potato beetle, but it has turned out to be very good."

Another happy surprise was that this wild species has enhanced the processing
quality of the hybrid lines. Pelletier says, "The quality of the tubers
from the hybrids with oplocense – the ability to store the tubers in cold
storage and to produce very nice fries – was a total surprise."

This research could help to improve the economic and environmental sustainability
of potato production in Canada, notes De Koeyer. A resistant variety could provide
a very important tool to add to the mix of practices that producers can adopt
to control the Colorado potato beetle. Such a variety could reduce insecticide
costs for conventional growers, and it might be suitable for the organic and
natural food niche markets. As well, by reducing insecticide use, it could help
to reduce environmental concerns for the potato industry.

And there is a further aspect to sustainability considerations: the flexibility
to adapt to various challenges and needs that comes with greater genetic diversity.
De Koeyer explains, "Typically potato has a relatively narrow genetic base
compared to other crops that we grow. So there is a requirement for sustainability
of production to introduce new genetic material into new varieties, not only
for Colorado potato beetle resistance, but also the quality attributes that
seem to be coming from wild species. For me that's the most exciting part of
the research – that it's a really nice illustration of the use of beneficial
diversity from wild sources."

Solanum oplocense is not the only wild potato species being used in
the breeding programs at the Potato Research Centre. Pelletier says, "Currently,
I am working with seven different wild species with regard to the potato beetle,
and we are working with another four or five species in a project with De Koeyer,
Robert Coffin from Cavendish Farms and a colleague from France on aphid resistance."

Pelletier and De Koeyer hope to have a Colorado potato beetle-resistant variety
available for growers, possibly by 2015. De Koeyer says, "I'm optimistic
that in the end we will be able to offer to the industry a potato which has
very good quality attributes and the required agronomic traits in combination
with Colorado potato beetle resistance, which I think will be an attractive
package for growers to consider."

Plants can use many methods to defend themselves against
attack. So how do you figure out how a particular plant species, like
Solanum oplocense, stops a particular pest, like the Colorado potato
beetle, from feasting on its foliage?

"We just ask the beetles why they don't like it,"
laughs Pelletier. "But seriously, that's basically it – although
we haven't been successful in training them to talk back! You know that
communication is 60 percent non-verbal, so we bank on that 60 percent.
Either in the field or in the lab, we put the beetles on the plant and
we observe what they do."

He says, "There are two moments in their life cycle
when the beetles make a decision about whether they'll stay on the plant
and use it, or leave. One is when they first contact the plants as adults
and they decide, 'I'm going to eat that plant and lay eggs on it, or
I'm out of there'. The other is when they emerge as first instar larvae.
The first instar larva is not very mobile and usually is very reluctant
to leave the plant, while the adult can fly quite a distance."

The reactions at each of these moments can be quite different.
Pelletier says, "For example, we are also working with another
wild potato species called Solanum tarijense, where the adult
beetles don't want to spend any time on the plant. You put them there,
and they fly off within two minutes. So they don't lay eggs on it. But
when we put the larvae on it, they eat it as if it were a cultivated
potato plant."

The researchers observe such things as how much the beetles
walk and fly, how much they feed on the plant, whether they get sick
after eating it, whether they reproduce on it, how well they grow if
they feed on it, and how fast the beetle population grows. Then the
researchers use those observations to try to determine how the plant
is defending itself.

"With oplocense, we think that the foliage has a
very strong taste the beetles really don't like so they stop feeding
after a while," says Pelletier. That, of course, is just an educated
guess, but that will have to do until the beetles start talking back.
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