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markers for the restorer gene Rf1 of sunflower hybrids based on PET1
cytoplasm. Using an F 2 population (RHA325xHA342), a linkage map around
the Rf1 gene was constructed using AFLP and RAPD markers. Markers
tightly linked to the gene Rf1 were identified, cloned and sequenced (Kusterer
et al. 2002, 2004a). The cloned markers were used directly as probes for
colony hybridization against high-density filters of two BAC libraries
(RHA325 and HA383) or their sequences were used to design overgo probes
for hybridizations and primers to screen the BAC library by 3D-PCR of BAC
pools. Several BAC clones could be identified. Rf1 was finally mapped to
LG 13 in the consensus map of Tang et al. (2002) using SSR markers (Kusterer
et al. 2004b, 2005). Later on, BAC fingerprinting using different restriction
enzymes was performed to develop contigs. BAC ends were cloned and
sequenced to develop new overgo probes for chromosome walking (Hamrit
et al. 2008). However, no closed contig around the restorer gene Rf1 could be
developed so far.
The isolation of fertility restorer genes in petunia (Bentolila et al. 2002),
rice (Akagi et al. 2004; Komori et al. 2004; Wang et al. 2006), radish (Brown
et al. 2003; Desloire et al. 2003; Imai et al. 2003; Kazama and Toriyama 2003;
Koizuka et al. 2003) as well as in sorghum (Klein et al. 2005) have shown
that these restorer genes belong to a class of pentatricopeptide repeat (PPR)
proteins, which show characteristic tandem repeats of 35 amino acids. These
proteins represent a large family of proteins that are assumed to play a role
in RNA processing and translation within organelles (Aubourg et al. 2000;
Lurin et al. 2004). A combination of candidate gene approach and map-
based cloning might allow isolation of the restorer gene Rf1 in sunflower in
the future.
Meanwhile, more than 70 new CMS sources have been described in
sunflower (Serieys 1999) that might be used in the future to broaden the
base of hybrid production. New restorers have been identified for some of
the new CMS sources (Horn and Friedt 1997). However, molecular
characterization of 28 new CMS cytoplasms (Horn and Friedt 1999)
demonstrated that cytoplasms with very different origins show a
considerable similarity, which would not be expected based on their pedigree
(Horn 2002). Molecular markers have been identified for isolation of other
restorer genes in sunflower by map-based cloning. The restorer gene Rf_PEF1 ,
restoring pollen fertility in hybrids based on one of the new CMS sources,
PEF1, could be linked to AFLP markers (Schnabel et al. 2008). In addition,
SSR marker analyses demonstrated that Rf_PEF1 is not located on LG 13 as
Rf1 . Feng and Jan (2008) identified a new dominant restorer gene Rf4 , which
restores pollen fertility in the presence of CMS GIG2. The Rf4 gene was
mapped to LG 3 with SSR markers and RFLP-derived STS-markers, and is
about 0.9 cM apart from the SSR marker ORS1114 based on a segregating
population of 933 individuals.
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