Germplasm resistant to potato beetle and late blight
By Helen McMenamin
A new technique has produced hybrids of wild Mexican and commercial potato cultivars.
At last! Scientists have developed a potato line that does not need to be sprayed
against two of the worst pests of the crop around the world. Neither Colorado
potato beetle nor late blight has any effect on the plants.
Scientists have known for years that Mexican wild type potatoes are resistant
to common pests of commercial lines. But, transferring that resistance into
breeding lines has defeated them as the two types of potato refuse to pollinate
Now, Qin Chen, a molecular cell biologist with Agriculture Canada at the Lethbridge
Research Centre, has developed a hybrid of a Mexican wild type and cultivated
potato that carries resistance to both late blight and Colorado potato beetle.
"We were very lucky to find both characteristics in a single plant,"
says Chen. "We have resistance to late blight and Colorado potato beetle
that's expressed as strongly in the hybrids as in the Mexican line.
"Hybrids have been made before, but in most cases their traits were intermediate
between those of their parents. The appearance of most of our new hybrid plants
is intermediate between the parents, but more like the cultivated potato; a
few look more like their Mexican parent. Our hybrid plants were unharmed by
both late blight and the beetle, just like the Mexican wild parent."
In Chen's tests, leaves of commercial potato are completely killed by the late
blight fungus. Beside them, hybrid leaves look green and healthy and appear
to be growing despite inoculation with the fungus. When potato beetles were
confined with leaves from a hybrid plant and a commercial variety, the commercial
leaf was completely eaten in a few hours, but the hybrid leaf was untouched.
Transferring traits like resistance to pests and disease and tolerance of poor
environments from the Mexican ancestors of our modern varieties back into commercial
lines has been a challenge for breeders for many years. Mexican wild types are
different species from the familiar potatoes and from each other and have only
half as many chromosomes. They do not hybridize with commercial types naturally
or with any conventional breeding techniques.
Chen's approach is to make somatic hybrids, daughter plants originating from
parental leaf tissue. This has worked better than developing plants from pollen
grains or ova, the parental cells used in some other breeding work.
To make the hybrids, Chen digests young leaves from Mexican and commercial
potato plants in enzyme solutions to separate the cells and strip off the cell
walls. Hybrid cells are formed in a process called electrofusion. He applies
an electric current to a mixture of cells from conventional and wild potatoes
in a special solution that causes the cells to fuse and form hybrid cells.
Given the right nutrient media and conditions, hybrid cells grow and multiply.
Eventually, some form a tiny clump of tissue called a microcallus.
After six to eight weeks, the microcalli are transferred from liquid culture
media to solid nutrient plates where they grow into calli. A callus can eventually
form a shoot that Chen and his technicians place in an individual tube of growing
medium to develop into a plantlet that can be grown in the greenhouse.
Not all the fused cells grow and multiply successfully. The commercial potato
line has four sets of chromosomes and the wild Mexican potatoes have two sets
and much smaller cells. Functioning plants can develop from calli with six or
eight sets of chromosomes. From over 900 calli with 72 to 96 chromosomes (six
to eight sets of 12), only about 150 develop into plants.
To ensure the plants are hybrids, Chen checks the plantlets' chromosomes, using
fluorescent dyes to identify the chromosomes from each parent. He also uses
DNA markers to track particular traits.
The hybrid plants do not have the traits needed in a commercial variety.
"This is the first germplasm resistant to both late blight and Colorado
potato beetle," says Chen. "The hybrid plants have these two resistances,
but we still need to combine its pest resistance with commercial traits like
high yield and eating and cooking quality. Breeders could use this hybrid germplasm
to develop new lines for growers." -30-