Agriculture Reference
In-Depth Information
Consortium, 2011), thereby facilitating the con-
tinued genetic improvement of potato. Readers
are encouraged to see Veilleux and Bolu-
arte-Medina (Chapter
17,
this volume) for more
detail regarding the importance of molecular
breeding and how sequencing of the potato gen-
ome can facilitate breeding.
appeared to be a recent phenomenon, with the
only meaningful introgression prior to 1966 oc-
curring from the use of
S. demissum
for its late
blight resistance in the late 1930s and 1940s.
Why the relatively sparse utilization of the
diversity of wild potato relatives by potato breed-
ers? Bradshaw (2007a,b) identified ploidy dif-
ferences (the majority of wild species being
diploid), endosperm balance number (EBN) mis-
matches, pollen-pistil incompatibilities, and
nuclear-cytoplasmic male sterilities as contrib-
uting factors. However, Bradshaw also stated that
the use of chromosome doubling techniques or
unreduced 2
n
gametes from wild species, or the
development of haploids (dihaploids) of culti-
vated potato following crosses with
Solanum
phureja
as the male parent could allow for the
successful utilization of virtually any potato
species by breeders. Somatic hybridization or cell
fusion is also another tool for overcoming hy-
bridization barriers and allowing the introgression
of desirable traits, such as virus, fungal, nema-
tode, and insect resistances, from wild potato
species into cultivated potato (Novy and Helgeson,
1994a,b; Brown
et al
., 1995, 2006; Novy
et al
.,
2002a, 2007; Thompson
et al
., 2007; Zhang
et al
.,
2007; Butler
et al.
, 2011).
The primary barrier to the utilization of wild
species is often the undesirable attributes that ac-
company the desired genes of interest. For ex-
ample, late blight resistance is often associated
with later maturity, which is not desirable in the
development of early- to mid-season potato culti-
vars with late blight resistance (Nowicki
et al
.,
2012). Market classes of potato, especially for pro-
cessing, have very specific commercial requirements
with respect to tuber size and length, starch and
sugar concentrations, and extended tuber dor-
mancy, to allow for processing from long-term stor-
age. The use of wild species tends to cause substan-
tial deviations in attributes important to the potato
industry, and as stated by Brown (2011): “The in-
convenient accompaniment of introducing exotic
genetic variation is that the breeding products are
often outside of the targeted market niche.”
While wild species use by breeders can be
problematic, continued genetic enhancement of
potato requires their successful utilization. An
example is the use of the wild species
Solanum
etuberosum
and
Solanum berthaultii
. Both species
have desirable traits, with multiple virus resist-
ances identified in
S. etuberosum
—a diploid,
Diversity of wild relatives
Early potato breeding efforts were highlighted by
the importation and utilization of new sources
of germplasm such as Rough Purple Chile. Ger-
mplasm with desirable traits not found in culti-
vated potato are as important to contemporary
potato breeders as they were to Chauncy Goodrich
over
161
years ago. Fortunately, potato has a
diversity of wild relatives with genetic traits
important to the continued enhancement of
cultivated potato. Hawkes (1990) identified
217
potato relatives, but more recent taxonomical
assessments have reduced that number to ap-
proximately
100
(Spooner, 2009). These wild
relatives have a wide geographic distribution,
stretching from south-western USA to the central
regions of Chile and Argentina (Hijmans and
Spooner, 2001). Collections or gene banks of
these wild potato species exist in the USA, the UK,
Peru, the Netherlands, Germany, Argentina, and
Russia. These germplasm collections truly repre-
sent banks in which genes are deposited, to be
withdrawn to address emerging problems in
potato production. These gene banks also play a
vital role in conserving species diversity, as in-
creasing human encroachment on native habitats
is associated with the loss of unique genotypes.
Unfortunately, the successful utilization of
wild potato species for the development of potato
cultivars has been infrequent. Love (1999)
identified
11
species and subspecies in the pedi-
grees of prominent North American cultivars—
approximately 11% of the known wild relatives
of cultivated potato. While the numbers of wild
relatives used successfully in the development of
potato cultivars is low,
34
of
44
(77%) potato
cultivars have “exotic germplasm” in their pedi-
grees—defined by Love as the previously men-
tioned
11
wild species, four South and Central
American cultivars, and three accessions de-
rived from
Solanum demissum
. Love stated that
the use of exotic germplasm in potato breeding
