Biology Reference
In-Depth Information
The group of exploited wild species was led by
the more often used S. demissum (16 centers) ,
S. bulbocastanum (13) , S. microdontum (9) ,S.
stoloniferum (8) , S. andigena (6) , S. berthaultii
(6) , S. hougasii (6) , S. phureja (6), and S. verru-
cosum (6). The rest of the group was represented
by 25 wild species. Among them were S. cha-
coense (5), S. pinnatisectum (4) , S. vernei (4), S.
iopetalum (3), S. acaule (2), S. circaeifolium (2),
S. fendlerii (2), S. papita (2) S. polytrichon (2), S.
stenotomum (2), S. sucrense (2), and S. tarijense
(2). An additional thirteen wild species were
investigated only in single centers (Zimnoch-
Guzowska and Flis 2002).
R1
The first genetic maps of potato were constructed
of restriction fragment length polymorphism
(RFLP) markers, as was the map that located
the first late blight resistance gene, R1 , to potato
chromosome V between the markers GP21 and
GP179 (Leonards-Schippers et al.1992). Apart
from the application of molecular markers,
the use of potato dihaploids, which made
genetic analyses and mapping feasible, was
a breakthrough in the research on Rpi genes
(Leonards-Schippers et al.1992). Later, the posi-
tion of the R1 gene was shown to overlap with the
position of a quantitative trait locus (QTL) for
late blight resistance (Leonards-Schippers et al.
1994; Sandbrink et al. 2000) and in other studies,
with a QTL for foliage and tuber resistance to P.
infestans as well as a QTL for foliage maturity
(Oberhagemann et al. 1999; Collins et al. 1999;
Bradshaw et al. 2004; Sliwka et al. 2007). More
detailed studies devoted to the relation between
maturity and late blight resistance also support
the importance of the R1 region for both traits but
have not revealed so far if it is a pleiotropic effect
of the same genes or rather a result of different
genes genetically linked (Visker et al. 2003; Bor-
mann et al. 2004). Meta-QTL for both traits were
also detected in this region (Danan et al. 2011).
The R1 gene was also the first Rpi gene that
was cloned from a diploid potato line (Ballvora
et al. 2002). This was done by positional cloning
combined with a candidate gene approach. The
R1 gene was shown to belong to the R gene fam-
ily with characteristic domain composition con-
taining a coiled coil (CC), a nucleotide-binding
site (NBS), and a leucine-rich repeat domain
(LRR). The functional R1 gene was present
as a large insertion only in a resistance allele
(Ballvora et al. 2002). However, the complex
nature of the R1 gene cluster was described in
three genomes of allohexaploid S. demissum ,
where numerous and diverse R1 homologues,
including an R1 sequence identical to the one
mentioned above (Ballvora et al. 2002), formed
three NBS-LRR families (Kuang et al. 2005).
R genes
In the 1990s, the first major genes encoding
resistance (R genes) to late blight were located
on potato genetic maps. They all originated
from S. demissum , and although they no longer
provide effective resistance in the field, there
has been renewed interest in vertical resistance.
Many scientists started the search within the
collections of wild potato relatives for new R
genes, potentially more durable ones than those
from S. demissum . Updated, detailed data on
R genes mapped and cloned and implications
of the mode of operation of R genes in potato
are described in this section. R genes for late
blight resistance (resistance to P. infestans , Rpi
genes) that have been mapped so far are summa-
rized in Figure 14.1. It was postulated that the R
genes not only for late blight resistance but also
encoding resistance to other diseases and even
pests tend to be located together, in several seg-
ments of the potato genome called resistance hot
spots (Gebhardt and Valkonen 2001). Later, after
many more resistance genes were identified and
mapped, this postulate remained valid, especially
for the Rpi genes that, although from different
Solanum species, are all located in ten hot spots
or clusters on eight potato chromosomes (Fig-
ure 12.1). These clusters usually contain genes
of high sequence similarity.
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