Agriculture Reference
In-Depth Information
cultivar with him, as allowed by the seedsman.
The cultivar Burbank soon became popular in
California and from there spread to other western
potato production states with a russet- skinned
genetic sport from Montana, becoming the cultivar
Russet Burbank, which is still the predominant
cultivar grown in the USA (Bethke
et al
., 2014).
Breeders of the 19th century, such as
Goodrich and Burbank, have left a lasting leg-
acy. Their cultivars are present in the pedigrees
of most modern-day potato cultivars—in fact,
prior to the release of the cultivar Katahdin by
the US Department of Agriculture in 1932, all
potato cultivars in the USA had previously been
developed by such private breeders (Hoopes and
Plaisted, 1987). The extent of progress in po-
tato breeding since the first fledgling attempts
by Knight and Knott in the early 19th century is
exemplified in the
World Catalogue of Potato Var-
ieties
(Pieterse and Hils, 2009), with over 4500
cultivars listed.
that there were more certified seed acres of
strain selections of Norkotah being grown in
the USA in 2009 than of the original Norkotah
cultivar.
Although strain or line selection is prefer-
able from the standpoint of maintaining the de-
sirable attributes of a cultivar while improving
the less desirable attributes, it requires that the
trait for improvement be readily identifiable, as is
the case with the initial selection of giant hill
variants of Norkotah having more robust vine
growth to withstand hail and early dying disease
better, or tuber skin variants such as the Russet
Burbank originating from the white-skinned
Burbank. Otherwise, it literally can be like look-
ing for “a needle in a haystack” to identify the
infrequent genetic variant of a cultivar.
Traditional potato breeding focuses on sex-
ual rather than asexual reproduction in the
earliest stage of the breeding process. Superior
parents are identified and subsequently hybrid-
ized, allowing for the production of botanical seed,
also called “true potato seed”, as distinguished
from “potato seed”—a tuber piece with an eye
(sprout). Each seedling that arises from the ger-
minated true potato seed represents a unique in-
dividual with new combinations of genes. The
process of selection and evaluation then begins,
and
10
or more years after the seedling is first
produced, a new cultivar may be released. Add-
itional background and greater detail regarding
the sexual component of potato breeding is pre-
sented later in this chapter in the section on
traditional breeding methodologies. Also detai-
led in this section is how asexual reproduction
is again employed by breeders to maintain the
unique combination of genes of superior seed-
lings for their release as potato cultivars.
16.2 Attributes of Potato and Their
Significance for the Breeding
of Potato
Every crop has unique attributes that must be
taken into consideration by breeders in formu-
lating the approach to be taken for its genetic en-
hancement. This section details the important
characteristics of potato that guide the approach
taken by breeders for its genetic improvement.
Dual reproduction (sexual
and asexual)
Potato cultivars have “fixed genotypes” in that
they are propagated via asexual means, specific-
ally by the cutting of tubers into seed pieces,
which are planted in the field. Thus, plants of
any given cultivar are genetically identical to
one another, with the exception of occasional
genetic variants. Selections can be made from
these genetic variants, as was the case with
strain (line) selections of Russet Norkotah (Nor-
kotah) made by the state breeding programs of
Colorado and Texas (Miller
et al
., 1999). Strain
selections of Norkotah have met with commer-
cial success. Janksy and Miller (2010) reported
Tetraploid genetics
Cultivated potato, found in fields worldwide, is
an autotetraploid with four sets of
12
chromo-
somes (2
n
=
4×
= 48), allowing the pairing of
up to four homologous chromosomes during
meiosis. At the tetraploid level, a locus can be
represented by up to four different alleles (vari-
ants) of a gene, whereas a diploid would have a
maximum of two divergent alleles. More com-
plex interactions between genes at different loci
(inter-allelic), as well as between up to four genes
