Gene Cloning and Characterization - Genetics, Genomics and Breeding of Crop Plants - page 184

In the future, isolation of these fertility restorer genes from sunflower
will hopefully help to understand the different molecular mechanisms behind
cytoplasmic male sterility and its fertility restoration. Comparison of different
restorer genes within sunflower will be of special interest.
6.2.2 Resistance to Downy Mildew
Downy mildew caused by Plasmopara halstedii (Farl.) Berl. et de Toni, is one
of the most widely occurring important pathogens of sunflower (Sackston
1992). It is an obligate parasite and specific to sunflower. A number of
physiological races have been reported that can be distinguished by their
differential virulence on sunflower genotypes (Gulya et al. 1991a, b).
Resistance to downy mildew, which is controlled by several single dominant
genes named Pl , has been detected in cultivated sunflower (Vranceanu 1970)
or introduced from wild Helianthus species (Miller and Gulya 1991). Some
of them provide resistance to a single race of downy mildew, whereas others
impart resistance to two or more races (Miller 1992). So far, 11 Pl genes
named as Pl 1 to Pl 11 have been identified conferring resistance to different
races of Plasmapara halstedii (Zimmer and Kinman 1972; Vranceanu et al.
1981; Miller and Gulya 1991; Rahim et al. 2002).
For marker-assisted selection, several molecular markers have been
identified and mapped to the different genomic region containing Pl loci
(Bert et al. 1991; Gentzbittel et al. 1995; Mouzeyar et al. 1995; Roeckel-Drevet
et al. 1996; Brahm et al. 2000; Gedil et al. 2001a, b; Radwan et al. 2003;
Radwan et al. 2004). Chapter 4 gives a detailed overview about the marker
analyses performed with regard to downy mildew resistance genes. Most of
the research groups also intend to use these markers for map-based cloning
approaches. Marker analyses were also the first to reveal that Pl 1 , Pl 2 and Pl 6
cluster together (Mouzeyar et al. 1995; Roeckel-Drevet et al. 1996; Vear et al.
1997). These three downy mildew resistance gene were located on LG 1 of
the RFLP consensus map (Gentzbittel et al. 1995) corresponding to LG 8 of
the public SSR map (Yu et al. 2003). In addition, the Pl 5 locus and the Pl 8
locus have been mapped in the same region of LG 6 of the RFLP consensus
map (Radwan et al. 2003; Slaubaugh et al. 2003) corresponding to the lower
end of LG 13 of the public SSR map (Yu et al. 2003).
Wild species have played a major role in introducing resistance genes
for downy mildew into the cultivated sunflower. For example, the downy
mildew resistance gene Pl Arg on LG 1, Pl 7 on LG 8 and Pl 8 on LG 13 were
introgressed from wild species (Miller et al. 2002; Slabaugh et al. 2003;
Dußle et al. 2004). However, introgression from other species can reduce
recombination frequencies, which might cause problems in a map-based
cloning approach.