In hybrid breeding programs involving cytoplasmic male sterility systems,
restorers are routinely identified by testcrossing prospective lines with
available CMS lines and evaluating F1s for pollen and spikelet
fertility. In IRRI's hybrid rice breeding program, hundreds of testcrosses
are made and evaluated every season and lines showing >-80% pollen
and spikelet fertility are designated as restorers. Using this protocol,
up to 40% restorer frequency has been observed among elite indica lines.
The majority of the elite lines are found as partial or incomplete restorers
which cannot be utilized as parents to make experimental rice hybrids.
Fertility restoration for 'WA' cytoplasm has been found to be controlled
by two Rf genes, one of which is stronger than the other (Govinda
Raj and Virmani, 1988; Bharaj, et al., 1991). Chromosomal locations
for such genes have been reported on chromosomes 7 and 10 (Bharaj et
al., 1995), chromosome 1 (Zhang et al., 1997), four QTLs on
chromosomes 2, 3, 4, and 5 (Zhu et al., 1996), two QTLs on chromosome
10 (Tan, 1998) and four QTLs on chromosomes 1, 7, 10 and 11 (Zhuang and
Zheng, 2000). Govinda Raj and Virmani (1988) also provided evidence to
indicate the presence of different sets of Rf genes in different
restorer lines. Perhaps, complex inheritance and involvement of more than
one gene discouraged hybrid rice breeders to use marker aided selection
for fertility restoration.
We targeted the major 'WA' restorer gene located on chromosome 1 previously
tagged with the PCR marker RG140STS. Polymorphism between restorer and
non-restorer is achieved after digestion with PvuII (Figure 1).
A total of 227 lines from the Source Nursery (2000 wet season) were analyzed
by PCR and F1s from testcrosses with IR68897A and IR70369A
(with 'WA' cytoplasm) were grown and evaluated for pollen and spikelet
fertility in the Testcross
Nursery (2001 dry season) at IRRI. Of the 126 lines identified
by PCR as putative restorers, 105 (83%) exhibited restorer ability with
one or both testers (Table 1). Among the 101 lines identified as putative
non-restorers, only 12 (12%) were found to be restorers by testcross method.
Considering the fact that we targeted only one of the two genes which
control restoration ability for 'WA' cytoplasm in rice, these results
are very encouraging. They demonstrated for the first time the use of
molecular markers for selecting restorers lines. Indirectly, these results
also confirmed that the Rf gene targeted for marker aided selection
in this study was a strong one. With the tagging of other Rf genes
or QTLs using appropriate molecular markers, the efficiency of marker
aided selection for restorer genes can be further improved. It should
then be possible to routinely use molecular markers to select putative
restorers of 'WA' cytoplasm among a set of elite lines. These putative
restorers can then be testcrossed with appropriate CMS lines to confirm
their fertility restoration and check the magnitude of heterosis. This
approach should considerably reduce the amount of work done
in making and evaluating testcrosses to identify effective restorer lines
in a hybrid rice breeding program.
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