1) National Institute of Genetics, Misima, 411 Japan
2) National Institute of Aerobiological Resources, Tsukuba, 305 Japan
A. Certation
Up to now, ten loci related to certation (gametic effect, or gametophyte genes) heve been reported (ga-1 to ga-10, Table 1). As for their linkage relationships, recombination values were calculated from the distorted segregation ratios of
Table 1. List of genes responsible for certation
______________________________________________________________
Gene Linkage Chromo- Reference
group some
______________________________________________________________
ga-1 I 6 Iwata et al. (1964)
ga-2 XI 5 Nakagahra (1972); Nakagahra
et al. (1972)
ga-3 XI 5 do.
ga-4 I 6 Mori et al. (1973);
Nakagahra et al. (1974)
ga-5 I 6 Mori et al. (1973)
ga-6 II 11 Maekawa et al. (1981a)
ga-7 III 3 Maekawa et al. (1981b)
ga-8 III 3 Nakagahra (1981)
ga-9 III 3 Maekawa et al. (1981b)
ga-10(t) II 11 Kinoshita and Takamure (1984)
______________________________________________________________
marker genes. Since symbols ga-1 to ga-10(t) have been assigned to the ten
genes so far reported, new genes for gametic effects should be symbolized as
ga-11, ga-12 and so on.If the certation occurs between viable pollen grains due to a difference in the rate of pollen tube elongation or pollen competition after pollination, it can be distinguished from sterility. Otherwise, it may be regarded as a kind of pollen sterility in hybrids. Tests of pollen germination and pollen tube elongation in vitro will be useful to examine the certation.
Oka (1974) demonstrated that several sets of duplicate genes were responsible for gametophytic F1 sterility. In certain combinations, they may bring about certation not causing sterility. It remains unknown whether the certation genes and gametophytic sterility genes are independent or related.
B. Sterility
Hybrid sterility is commonly observed in F1 and F2 plants from distantly related crosses. Three types of gene action, gametophytic (for F1 sterility), sporo-phytic (for F2 sterility), and sporo-gametopytic interacting (F1 sterility), have been known besides cytoplasmic-genic male sterility (Sano et al. 1979).
So far, four cases of gametophytic F1 sterility were reported by Oka (1953, 1957a, 1974), in each of which the sterility was controlled by a set of duplicate genes whose double recessive combination inhibits the development of gametes carrying it. Some of the genes affect the development of both pollen and embryo sac, but others affect only pollen development. The loci of some of the duplicate genes were detected as shown in Table 2. The gametophytic sterility genes cause segregation distortion for genes linked with them. The sterility is eliminated from the hybrid population rapidly with inbreeding generations.
Three sets of duplicate genes which control F2 sterility have been reported as given in Table 2 (Oka and Doida 1962; Oka 1978; Yokoo 1984). These are the cases of sporophytic sterility, in which the development of gametes is affected by certain recessive homozygous combinations of the genes through a sporophytic effect. Plants with varying levels of sterility are found in F1 and later generations, and true-breeding semisterile lines can be selected from the selfed progeny of partially sterile segregants. Sporophytic sterility is characterized by a large variance of fertility among individuals or spikelets having the same genotype.
Moreover, hybrid sterility due to sporo-gametophytic interaction of genes at one locus was detected in hybrids between 0. sativa and 0. glaberrima (S-1 to S-4, Sano et al. 1979; Sano 1983, 1985, 1986) and in varietal hybrids of 0. sativa (S-5, Ikehashi and Araki 1986). Kitamura (1962) and Oka (1964) also reported smilar cases.
Recently, Ikehashi and Araki (1986) pointed out that genic analysis was insufficient in some cases of gametophytic sterilities, as their data from three -variety crosses were explainable by allelic interaction at S-5. For further studies, five
Table 2. Cases of F1 and F2 sterilities reported, their
genetic control and gene symbols
______________________________________________________________
Genera- Linkage Chromo-
Gene tion Cross group some Reference
observed
______________________________________________________________
Gametophytic F1 sterility (duplicate genes)
s-a. 1 F1 sativaXsativa I 6 Oka(1953,1957a)
s-a-2 _ _
s-c-1 F1 do. I 6 Oka (1957a)
s-c-2 II 11
s-d-1 F1 sativaXsativa I 6 Oka (1974)
s-d-2 - -
s-e-1 F1 sativaXsativa XI 5 Oka (1974)
s-e-1 II 11
Sporophytic F2 sterility (duplicate genes)
S-A-1 F2 sativaXsativa I 6 Oka&Doida(1962)
S-A-2 _ _
S-B-1 F2 sativaXsativa _ _ Oka (1978)
S-B-2 I 6
Undecided F2 sativaXsativa _ _ Yokoo (1984)
Sterility due to sporo-gametophytic interaction
Sa-1&S-1 F1 sativaXglaberrima I 6 Sano et al(1979)
Sa-2&S-2 F1 sativaxglaberrima - - do.
Sa-3&S-3 F1 sativaXglaberrima VIII 9 Sano (1983)
Sa-4&S-4 F1 sativaXglaberrima - - Sano (1985)
Sn-5,Si
-5&Sj-5 F1 sativaXsativa I 6 Ikehashi& Araki
(1984,19-6)
_______________________________________________________________
isogenic F1-sterile lines raised by Dr. Oka are available (maintained at
National Institute of Genetics, Japan, RGN 1, p. 40-41).C. Weakness, chlorosis and inviability
Several kinds of hybrid weakness, hybrid chlorosis and hybrid inviability have been found in F1 and F2 plants from distantly related crosses, although the incidence is less frequent than that of F1 and F2 sterilities. Three cases of F1 weakness, a case of F2 weakness, a case of F2 chlorosis, and a case of F1 inviability have so far been reported as listed in Table 3. The genes for F1 weakness are complementary dominant, whereas those for F2 weakness and chlorosis are duplicate or complementary recessive and their double recessive homozygotes express the phenotype resulting in a 15: 1 F2 ratio. In some cases, plants homozygous-recessive for one locus and heterozygous for the other locus also show weakness giving rise to a 11 : 5 ratio (similarly as F2 sterility).
The genes for F1 weakness are symbolized as Hwa-1, Hwa-2 and so on, where "h" indicates hybrid, "w" indicates weakness, and the third letter designates
Table 3. List of occurence of hybrid weakness and chlorosis in
rice, and their gene symbols assigned
________________________________________________________________
Phenomenon Gener- Recommended Previous Cross Reference
ation gene gene combi-
symbols symbols nation
________________________________________________________________
Weakness F1 Hwa-1 L-l-a An IndicaX Oka (1957b)
(L/a) Indica cvs.
Hwa-2 L-L-b
(L/b)
F1 Undecided W-1 breviligulataX Chu&Oka
glaberriuma (1972)
W-2
F1 Hwc-1 L-2-a An JaponicaX Amemiya&
(L/r1) Japonica cv. Akemine
(1963);
Hwc-2 L-2-b Sato&Hay-
(L/r2) ashi(1983)
F2 hwb-1 w'-a Ac451XAc521 Oka(1957b)
hwb-2 w`-b
F2 hwd-1 _ A Japanese cv. Okuno
XThai waxy cv. (1985)
hwd-2 _
F2 Undecided _ W593XTaichung Sato
65 unpubl.
F2 Undecided _ Burmese cv.X Sato
Taichung 65 unpubl.
F2 Undecided _ AkihikariX Toriyama
Milyang 23 (per.comm.)
Chlorosis F2 hca-1 _ A Japanese cv. Sato et
XJaponica cv. al.(1984)
hca-2 _
Inviabil- F1 Undecided D-1 sativaXlong- Chu&Oka
ity istaminata (1970)
D-2
_________________________________________________________________
a particular set of genes in alphabetical order. Their recessive alleles not
causing weakness may by denoted by Hwa-l+ and Hwa-2+, or simply by +-1 and
+-2. Similarly, the genes for F2 weakness which are recessive are symbolized
as hwb-1 and hwb-2, etc. Their dominant alleles not causing weakness may be
denoted by hwb-l+ and hwb-2+ or simply by +-1 and +-2. In case of chlorosis,
"c" is used in place of "w" for weakness.A crossing barrier was observed between 0. longistaminata and its related taxa with the A genome. This was due to a set of complementary genes which caused inviability of young F1 zygotes (Chu and Oka 1970). Gene symbols are tentatively assigned as D-1 and D-2. The complementary dominant lethals disturbed tissue differentiation in the endosperm and had an incomplete penetrance.
References
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_____ and _____, 1972. The distribution and effects of genes causing F1 weakness in Oryza breviligulata and 0. glaberrima. Genetics 70: 163-173.
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_____ and ______, 1986. Genetics of F1 sterility in remote crosses of rice. In Rice Genetics, pp. 119-130. IRRI, Manila.
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_____, 1964. Considerations on the genetic basis of intervarietal sterility in Oryza sativa. In Rice Genetics and Cytogenetics, p. 158-174. Ed. by IRRI. Elsevier, Amsterdam.
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