Principles of Plant Genetics and Breeding

174 CHAPTER 10
Richard Novy
Industry highlights
The use of the wild potato species, Solanum etuberosum, in
developing virus- and insect-resistant potato varieties
USDA-ARS, Aberdeen, ID 83210, USA
Historical background
Oftentimes referred to as the “Irish” potato, Solanum tuberosum subsp. tuberosum might more aptly be termed the “Inca” potato.
The origin of the fourth most widely grown cultivated crop in the world is thought to be the Central Andes region of South
America, with the possibility of independent domestication in Chile as well. Potato was an important food crop for the Incas, but
it is unlikely that they were the civilization responsible for its domestication. Potato food remnants have been found in preceramic
archeological sites in South America that date to over 5,000 years ago, indicating that the potato truly is an ancient food crop.
The Spanish and English are thought to have brought this New World crop back to Europe in the late 16th century. Adapted to
form tubers under the short day conditions near the equator (approximately 12 hours), potatoes did not successfully produce
tubers in most northern latitudes prior to being killed by freezing temperatures in the fall. The exceptions were the milder climates
of Spain, Italy, southern France, and Ireland, where the potato was maintained as a botanical oddity in private and botanical gardens.
Over the course of 150–200 years, potato clones (with the help of man) were identified and propagated that formed tubers
under the longer day lengths of the northern latitudes. This environmental adaptation allowed for the expansion and adoption of
potato as a food crop in Europe, and eventually throughout the world.
Viruses of potato
Accompanying the introduction of the potato to Europe were pathogens that had coevolved with the crop, most notable being
potato viruses X and Y (PVX and PVY) and potato leafroll virus (PLRV). These viruses are transmitted from an infected plant to the
tubers it produces. When virus-infected tubers are cut and used to establish the potato crop the following growing season (asexual
propagation), plants developing from the tuber seed are infected with the virus as well. Symptoms of virus infection in potato
include stunting, chlorosis/necrosis of leaf tissue, and, in the case of PLRV (as indicated by the name) rolling or cupping of leaves.
Total yield in a growing season can be reduced by as much as 80% if virus-infected seed is used. Transmission of PVY and PLRV
from infected to healthy plants is mediated by aphids, most notably green peach aphid; PVX is transmitted to healthy plants via
mechanical contact with an infected plant or with PVX-contaminated field equipment. The detrimental impact of viruses on
potato was termed “degeneration” or “running out” by early growers of potato who did not yet know of the existence of viruses.
Potato varieties with resistances to viruses can be effective in reducing crop losses. Cultivated potato is fortunate in having
> 200 wild Solanum relatives, many of which have been identified as virus resistant. In the United States, potato species collected
in Mexico and Central and South America are maintained at the Potato Genebank in Sturgeon Bay, Wisconsin. This species
collection has been systematically screened for resistance to the major pests and diseases of potato. Many species have
been identified with high levels of resistance to PVX and PVY, with a lesser number identified as having desirable levels of PLRV
resistance.
Ideally, from the standpoint of a potato breeder, it is desirable to work with species that have a high level of resistance to all
three viruses. A search of the Potato Genebank collection – consisting of 5,634 introductions representing 168 species – identified
only one introduction (PI 245939) of the wild potato species, S. etuberosum, as having a high level of resistance to PVY, PVX, and
PLRV. This accession also was identified as having resistance to green peach aphid – a primary insect vector of PVY and PLRV.
The introgression of these multiple virus and insect vector resistances from S. etuberosum into cultivated potato will be the focus
of the remainder of this box.
Solanum etuberosum: its use in the genetic improvement of potato
Solanum etuberosum is a wild potato species endemic to Chile. Its natural habitat is among rocks on slopes with seepage, or
along streams. It is generally found in the open, or in the shade of trees and shrubs (Correll 1962). It is notable for its large, deep
purple flowers (Figure 1). The attractiveness and abundance of its flowers and its striking foliage led a taxonomist in 1835 to propose
that it be grown as a hardy perennial for ornamental purposes in England (Correll 1962). It also is notable among wild potato
species in that it does not form tubers.
A diploid (2n = 2x = 24) species, S. etuberosum does not cross readily with either tetraploid (2n = 4x = 48) or dihaploid (2n =
2x = 24) forms of cultivated potato. Bridging species and ploidy manipulations have been used by breeders in synthesizing
etuberosum–tuberosum hybrids with limited success.