Professor Emeritus. Hokkaido University. Sapporo. 060 Japan
Male sterility is an important character for hybrid seed production on a large scale. Information on genes responsible for various types of male sterility in rice is summarized as shown in Tables, 1, 2 and 3, which were partly transposed from the data of rice genes mentioned in Rice Genetics Newsletter Vol. 12 (Kinoshita 1995).
I. Cytoplasmic male sterility (CMS)
CMS was first reported by Katsuo and Mizushima (1958). They introduced nuclear genome of Fujisaka 5 into the cytoplasm of 'Chinese wild rice' (Oryza f. spontanea=0. rufipogon) by successive backcrosses. In hybrid rice breeding 'Chinsurah boro' cytoplasm or BT cytoplasm designated as [cms-bo] is extensively used for japonica hybrid rice development. The mode of inheritance of fertility restoring genes and molecular nature of mtDNA have been studied as a basis for breeding (Shinjyo 1975: Kadowaki et al. 1986, 1988, 1990-, Shikanai et al. 1989; Yamato et al. 1992 etc.). On the other hand, the cytoplasm of a male sterile line, 'wild abortive' designated as [cms-WA] is also successfully used for indica hybrid rice development (Lin and Yuan 1980). There are several cyto-plasmic sources as mentioned by Virmani and Shinjyo (1988). Virmani et al. (1989) demonstrated the cytoplasmic diversity among five cytoplasms, cms-bo, cms-WA, cms-GAM, cms-ARC and cms-sp depending on the spikelet fertilities of the progenies from test-crosses with a set of 28 cultivars. As to the utilization of wild Oryza species, Shinjyo (1990) classified at least four kinds of new cytoplasms derived from UR-lines belonging to 0. rufipogon depending on the interaction with different genotypes of Rf1 alleles. Ling et al. (1989) also found a CMS line, 54257 among somaclonal variants regenerated from anther culture and the mutant CMS was controlled by a cytoplasmic factor designated as [cms-54257]. Following this, Dalmacio et al. (1992, 1995) produced a CMS line, IR66707A as a new source in the combination between the cytoplasm from (O. perennis Acc104823 and the nuclear genome of IR64. Nagamine et al. (1995) also selected a new cytoplasmic source from boro varieties introduced from Bangladesh. The cytoplasm was different from [cms-boro] using test-crosses with several restorers and molecular structure of mtDNA, and was named as [cms-Khiaboro]. These new CMS systems are useful to enlarge the cytoplasmic diversity in hybrid rice breeding.
Many kinds of somatic and cytoplasmic hybrids were produced by protoplast fusion using Oryza species (Kyozuka et al. 1989). Transfer of [cms-bo] cytoplasm was achieved by donor-recipient method (Aviv et al. 1984) in japonica varieties. It is effectively used not only to shorten the time for breeding of CMS lines, but also to elucidate the genetic nature of CMS. Later it was demonstrated that a new chimeric gene, rps1 is also responsible for CMS in mtDNA besides atp6 (Kinoshita and Mori 1996).It is generally accepted that Rf followed by a numeral is used to designate the fertility restoring genes. Hitherto Rfl and Rf2 located on chromosomes 10 and 2 respectively are used for the restorer genes for [cms-bo] and [cms-ld] (Table 2). It is indicated that two dominant restorer genes have additive effects for the restoration of fertility in [cm^-W^]. The two genes are located on chromosomes I and 7 respectively (Zhang and Lu 1996; Zhang et al. 1994, 1997). These genes were named as Rf3 and Rf4 by Zhang et al. ( 1997) replacing Rf2(R2) and Rf1 (R1) which were formerly used in several papers (Bharaj et al. 1991.1995: Teng and Shen 1994; Zhang et al. 1994). Teng and Shen ( 1994) showed that there is no evidence for interaction and pleiotropic effects between the fertility restoring genes for [cms-bo]and [cms-WA]. In addition, a male fertile revertant was induced with Co60 gamma ray treatment from a CMS line, 11-32A having [cms-WA] and the mutant acted as a fertility restoring gene (tentatively named as Rf5(t)) and is different from Rf3 and Rf4 present in IR24 and Minghui 63 (Shen et al. 1993a, 1996a, b). Fine mapping of these genes is progressing by using RFLP markers around OPB07-640 (chromosome 1) which are closely linked with the gene (Shen et al. 1996a). DNA markers closely linked to Rf-genes provide useful tools in marker-aided selection for the sterile maintainer and fertility restoring genes (Akagi et al. 1996).
Induction of partial male fertility in [cms-bo]Taichung 65 was controlled by the gene designated Ifr (Sano and Eiguchi 1991: Sano et al. 1992).
II. Genetic male sterility (GMS)
Usually single recessive genes condition male sterility and number over 70 in rice. As shown in Table 3, a numeral attached to ms were assigned without the coordinated allelism tests by the respective research group. In a few cases, duplicate recessive genes were reported, while no dominant genes have been reported. Type and stage of pollen abortion are various among the mutants (Hu and Rutger 1992; Tamaru 1994).
Environmentally conditioned male sterility (EGMS) can be efficiently used for hybrid seed production and 'two lines method' (EGMS and pollen parent) is gradually replacing 'three lines method' (CMS, maintainer and restorer). Sometimes EGMS are divided into thermosensitive MS (TGMS) and photoperiod-sensitive MS (PGMS or PSMS) depending upon their mode of response.
In the case of TGMS, male sterility is conditioned under high temperature but returns to normal under low temperature. Three single recessive genes, tmsl, tms2 and tms3(t) have been reported (Sun et al. 1989; Yang et al. 1992; Maruyama et al. 1991; Subudhi et al. 1995). Both tms2 and tms3(t) were induced with gamma irradiations, while tmsl occurred as a spontaneous mutant. The TGMS line containing tms3(t) shows complete male sterility at day and night temperatures of 32°C and 24°C, but is partially fertile at 27° and 21°C or 24°C and 18°C in the phytotron (Borkakati and Virmani 1993). In RFLP analysis, tms1 and tms3(t) were located on chromosome 8 and 6 respectively (Wang et al. 1995, 1996:Subudhi et al. 1995, 1996, 1997). A promising TGMS mutant named as 26 Zhaizao S was induced by gamma-rays and the mutant showed low fertility especially at 30°C though there is no inheritance data (Shen et al. 1993b).
In the case of PGMS, a spontaneous mutant was found in the field of Nongken 58. The mutant exhibited various types of pollen abortion under the restricted condition of long day (14hr/day) (Wang et al. 1991). Genetic analysis indicated that at least two recessive genes, pins1 and pms2 are responsible for the trait and are located on chromosomes 7 and 3 respectively (Shao and Tang 1992; Zhang et al. 1993). Main effects of pms1 were about three times larger in the average than that of pms2 showing a considerable interaction between these two loci (Zhang et al. 1993). There is a possibility that one of the two sterility inducing genes from Nong-Ken 58S (NK58s) is allelic to one of the fertility restoring genes for [cms-WA] (Shao and Tang 1995). Similar PGMS mutants were found in progenies of the ethyl methanesulfonate-treated materials of the rice cultivar M-201 (Oard et al. 1991). Pollen fertility increased 3 to 44-fold when the mutant plants were ratooned and grown in a growth chamber with a 12hr daylength. PGMS was governed by one to three recessive genes. From the experiments with red or far-red light treatments, it was found that phytochrome is involved in regulation of pollen fertility (Oard and Hu 1995).
A spontaneous mutant named as open hull male sterile was controlled by a single recessive gene, oms. The pleiotropic effects of mutant gene resulted in lower seed fertility, smaller caryopsis, darker leaf color and good ratooning (Takeda 1987).
In order to promote gene identification and allelism tests, free exchange of the germplasm of male sterile mutants among different research groups is urgently needed.
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