Principles of Plant Genetics and Breeding

SEXUAL HYBRIDIZATION AND WIDE CROSSES IN PLANT BREEDING 177
Insecticide use has been the primary means of
controlling wireworm damage. However, insecticides
currently used for wireworm control may lose their
registration in the future. Genetic resistance to wireworm
could be an important component of an integrated
pest management (IPM) program, and it was
decided to evaluate the progeny of S. etuberosum for
wireworm resistance.
In collaboration with Dr Juan Alvarez, an entomologist
with the University of Idaho, one BC 1 and four
BC 2 clones derived from the S. etuberosum somatic
hybrids were evaluated for wireworm resistance; comparisons
of these clones were made relative to a susceptible
cultivar, “Russet Burbank”. An additional
treatment also was included in the evaluation; susceptible
“Russet Burbank” was treated with Genesis®, an
insecticide commonly used for wireworm control.
Four of the five backcross clones had a percentage of
damaged tubers comparable to or lower than that
observed with the use of Genesis®. Wireworm entry
points or “holes” per tuber among the five clones were
comparable to the numbers observed with the use of
Genesis®.
The resistance of two of the five clones was
attributable to high levels of certain chemical compounds
naturally produced in the tuber called glycoalkaloids.
Total tuber glycoalkaloid concentrations
need to be less than 20 mg/100 g tuber fresh weight
for safe consumption by humans – these two highly
resistant clones had levels ≥ 47 mg/100 g. However, the remaining three clones had acceptable total tuber glycoalkaloid levels of
≤ 13 mg/100 g; all three, relative to susceptible “Russet Burbank”, had reduced wireworm entry damage and two of the three
had a reduced percentage of wireworm-damaged tubers. These data indicate that high total tuber glycoalkaloid levels are not
necessary for conferring wireworm resistance – an important finding if wireworm-resistant potato cultivars with acceptable
glycoalkaloid levels are to be developed.
References
etb txb SH Kat Atl BC1l--------BC2--------------l
Figure 3 Segregation of a RFLP (restriction fragment length
polymorphism) specific to S. etuberosum; the RFLP probe used
was TG65. The arrow indicates the RFLP unique to S.
etuberosum that also is present in the somatic hybrid (SH), BC 1 ,
and two of six BC 2 . This molecular marker is not present in the
S. tuberosum × S. berthaultii fusion parent (txb) or the potato
cultivars “Katahdin” (Kat) or “Atlantic” (Atl) that were used as
parents in the generation of the backcross progeny. Numbers
on the side are approximations of the RFLP fragment sizes.
Correll, D.S. 1962. The potato and its wild relatives. Texas Research Foundation, Renner, TX, 606 pp.
Novy, R.G., and J.P. Helgeson. 1994a. Somatic hybrids between Solanum etuberosum and diploid, tuber bearing Solanum
clones. Theor. Appl. Genet. 89:775–782.
Novy, R.G., and J.P. Helgeson. 1994b. Resistance to potato virus Y in somatic hybrid between Solanum etuberosum and
S. tuberosum × S. berthaultii hybrid. Theor. Appl. Genet. 89:783–786.
Novy, R.G., A. Nasruddin, D.W. Ragsdale, and E.B. Radcliffe. 2002. Genetic resistances to potato leafroll virus, potato virus Y,
and green peach aphid in progeny of Solanum etuberosum. Am. J. Potato Res. 79:9–18.
Overcoming barriers to fertilization
1 Conduct reciprocal crosses. Generally, it is recommended
to use the parent with the larger chromosome
number as the female in a wide cross for a
higher success rate. This is because some crosses are
successful only in one direction. Hence, where there
is no previous information about crossing behavior, it
is best to cross in both directions.
2 Shorten the length of the style. The pollen tube of a
short-styled species may not be able to grow through
a long style to reach the ovary. Thus, shortening a
long style may improve the chance of a short pollen
tube reaching the ovary. This technique has been successfully
tried in corn.
3 Apply growth regulators. Chemical treatment of
the pistil with growth-promoting substances (e.g.,
naphthalene acetic acid, gibberellic acid) tends to