Mapping and Tagging of Simply Inherited Traits - Genetics, Genomics and Breeding of Crop Plants - page 122

Pl 1 locus was located between SUN017 (5.6 cM) and SUN124 (7.1 cM), thus
the first markers were available for marker-assisted selection (MAS), but
they were not linked closely enough for molecular cloning of Pl 1 . Roeckel-
Drevet et al. (1996) and Vear et al. (1997) used BSA in order to map Pl 6 and Pl 2
in the F 2 progeny of H52 x HA335 ( Pl 6 ) and GH x PAC2 ( Pl 2 ). Pl 6 (4.8%
recombination) as well as Pl 2 (8.3% recombination) were found to be closely
linked with the RFLP marker SUN124, previously mapped closely to Pl 1 .
Vear et al. (1997) additionally investigated the segregation pattern of F 3 and
F 4 progenies of the cross HA335 ( Pl 6 ) x H52 after infection with five downy
mildew races (100, 300, 700, 703, 710). The results indicated that Pl 6 could
be separated at least into two parts, one giving resistance to race 100 and
300 and the other giving resistance to races 700, 703 and 710. This was the
first report that a Pl locus ( Pl 6 ) is not a single independent gene, conferring
resistance to many downy mildew races, but rather a cluster of genes, each
providing resistance to one, or a few, downy mildew races. Brahm et al.
(2000) were in search of markers for the Pl 2 locus and used two sets of near-
isogenic lines (AS110/AS110 Pl 2 , S1358/S1358 Pl 2 ) and bulks of a segregating
F 2 population (cmsHA89 x AS110 Pl 2 ) to identify nine random amplified
polymorphic DNA (RAPD) and two amplified fragment length
polymorphism (AFLP) markers. The most closely linked markers were
OPAA14 750 , OPAC20 831 , and E35M48-3 within about 2 cM distance from the
Pl 2 locus; all markers were linked in coupling phase. Markers in repulsion
phase are also available so that they can be used in combination to select
against heterozygous genotypes in breeding programs. The markers for the
Pl 2 locus can also be useful for Pl 6 , because Pl 2 is a part of the complex Pl 6
locus (Vear et al. 1997; Brahm et al. 2000).
A second linkage group carrying genes controlling resistance to downy
mildew was identified with molecular mapping of Pl 5 and segregation
studies in testcrosses between different sources of downy mildew resistance
(Bert et al. 2001). Pl 5 showed linkage with ten AFLPs, two RFLPs, and Rf1 , a
fertility restorer locus, in the F 2 progeny of XRQ ( Pl 5 ) x PSC8 ( Pl 2 ). The two
RFLP markers as well as the Rf1 gene were previously mapped on group 6
of the the CARTISOL map (Gentzbittel et al. 1995), which is identical with
LG 13 of the public map. In the testcross XRQ ( Pl 5 ) x Ha338 ( Pl 7 ) segregation
showed a pattern of two independent genes each conferring resistance to
races 100, 710, 703, but for the cross between XRQ ( Pl 5 ) x RHA340 ( Pl 8 ) no
segregation occurred. The authors considered that Pl 5 and Pl 8 may be closely
linked, but that the two genes are not identical because of their non-identical
race resistance patterns (Bert et al. 2001). Radwan et al. (2003) confirmed the
position of Pl 5 on group 6 of the RFLP map of Gentzbittel et al. (1999) and
mapped Pl 8 in addition on the same linkage group.
Vear et al. (2003) investigated the segregation pattern of progenies, which
originated from different crosses between lines with different Pl loci [HA335