Principles of Plant Genetics and Breeding

SEXUAL HYBRIDIZATION AND WIDE CROSSES IN PLANT BREEDING 175
Another means of circumventing reproductive
barriers is to bypass them completely by the use of
somatic hybridization. This technique involves the
isolation of potato cells from leaf tissue of parental
clones, the enzymatic digestion of their cell walls to
form protoplasts, and the fusion (using chemicals or
electric currents) of parental protoplasts. Fused protoplasts
are then cultured on medium whereby they
re-form a cell wall and are allowed to divide to form
undifferentiated tissue called callus. Calli are then
placed on culture media that promotes cell differentiation
and the formation of plants. These hybrid plants
can then be excised from the calli, induced to form
roots, and can then be grown as a normal plant in field
or greenhouse environments.
Using somatic hybridization, Novy and Helgeson
(1994a) successfully generated hybrids between a S.
etuberosum clone from virus-resistant PI 245939 and
a subsp. tuberosum dihaploid × S. berthaultii hybrid
clone (2n = 2x = 24). The trispecies hybrids, based on
cytological and molecular analyses, were at or near
Figure 1 Flower and foliage of S. etuberosum. Considered a
the expected 2n = 4x = 48. Somatic hybrids had very
weed in Chile, this wild relative of potato has desirable genetic vigorous foliar growth in the field with limited tuber-
resistances to viruses and insects that plague cultivated potato.
ization (Figure 2); poor tuber type and yield was not
unexpected in that half the genome of the somatic
hybrids was from non-tuber-bearing S. etuberosum.
Backcrossing of somatic hybrids to potato cultivars was undertaken to improve tuberization and yield. Crosses using somatic
hybrids as the male parent yielded few berries and no seeds. Stylar analyses showed blockage of somatic hybrid pollen tuber
growth generally occurred in the upper third of Gp. tuberosum styles. Pollen tube blockage of cultivated potato was not observed
in the styles of somatic hybrids: 503 pollinations produced 99 berries containing 24 seeds. Five of the seed germinated to produce
viable BC1 progenies that were at or near the tetraploid level (48–49 chromosomes).
The five progenies obtained had much improved tuberization relative to the somatic hybrid parent, while still retaining 11–13 S.
etuberosum chromosomes. One of the five progenies produced an average of six seeds per berry when crossed to cultivated potato.
Viable BC2 progenies were obtained from this seed. Tubers of BC2 , now looking like those of cultivated potato, are shown in Figure 2.
Virus and green peach aphid resistances of somatic hybrids and their progeny
Novy and Helgeson (1994b) analyzed the fusion parents, their somatic hybrids, and the sexual progeny of the somatic hybrids for
resistance to PVY following their mechanical inoculation in the greenhouse over a 2-year period. The S. etuberosum fusion parent
was highly resistant to PVY infection whereas the tuberosum–berthaultii fusion parent was highly susceptible. Three somatic
hybrids analyzed in this study did not show the high level of resistance found in the S. etuberosum parent; however they were
significantly more resistant than the cultivars “Katahdin” (moderate field resistance to PVY) and “Atlantic” (PVY susceptible). Five
progenies of the somatic hybrids also were analyzed in this study. Three displayed PVY resistance comparable to the somatic
hybrid parents, whereas the remaining two were more susceptible with absorbance means comparable to the potato varieties
included in the study.
Solanum etuberosum also had been identified as having resistance to PLRV and green peach aphid. Resistance to green peach
aphid (Myzus persicae) can aid in decreasing the transmission of viruses by decreasing aphid population size and subsequent
opportunities for virus transmission. However, green peach aphid resistance alone is not adequate to confer the necessary level of
resistance needed by the industry. This is especially true in the case of PVY, which can be quickly transmitted by the stylar probings
of many different aphid species – species that may not include potato as a primary host and therefore will not be adversely
impacted by host plant resistance.
A combination of green peach aphid and PVY/PLRV resistances is the most effective means to reduce virus infection and
spread. Novy et al. (2002) evaluated five BC 2 progenies of the S. etuberosum somatic hybrids (the recurrent parents being potato
varieties) for green peach aphid, PLRV, and PVY resistance. Virus resistances were evaluated in both open field and field cage
trials; aphid resistance was evaluated in the field and greenhouse.
The authors identified resistance to green peach aphid in all S. etuberosum-derived BC 2 progeny. Resistance was characterized
by reduced adult body size and fecundity. One BC 2 individual also exhibited reduced nymph survival. Prolonged development
from nymph to adult also appeared to contribute to reduced aphid populations on the BC 2 relative to susceptible checks.