4. Gene symbols for gametic effect, sterility and weakness

Yo-Ichiro Sato1, Yoshio Sano1 and Masahiro Nakagahra2

1) National Institute of Genetics, Misima, 411 Japan

2) National Institute of Agrobiological Resources, Tsukuba, 305 Japan

A. Certation

Up to now, ten loci related to certation (gametic effect, or gametophyte genes) have been reported (ga-1 to ga-10, Table 1). As for their linkage relationships, recombination values were calculated from the distorted segregation ratios of 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.

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)  
______________________________________________________________

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 O. sativa and O. glaberrima (S-1 to S-4, Sano et al. 1979; Sano 1983, 1985, 1986) and in varietal hybrids of O. 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 isogenic F1-sterile lines raised by Dr. Oka are available (maintained at National Institute of Genetics, Japan, RGN 1, p. 40-41).

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     sativa X sativa      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     sativa X sativa      I    6   Oka (1974)
s-d-2                                    -    -
s-e-1        F1     sativa X sativa     XI   5    Oka (1974)
s-e-1                                   II  11

Sporophytic F2 sterility (duplicate genes)

S-A-1        F2     sativa X sativa      I    6    Oka&Doida(1962)
S-A-2                                    _    _
S-B-1        F2     sativa X sativa      _    _    Oka (1978)
S-B-2                                    I    6
Undecided    F2     sativa X sativa      _    _    Yokoo (1984)

Sterility due to sporo-gametophytic interaction

Sa-1&S-1     F1     sativa X glaberrima  I    6    Sano et al(1979)
Sa-2&S-2     F1     sativa X glaberrima  -    -    do.
Sa-3&S-3     F1     sativa X glaberrima  VIII 9    Sano (1983)
Sa-4&S-4     F1     sativa X glaberrima  -    -    Sano (1985)
Sn-5,Si    
-5&Sj-5      F1     sativa X sativa        I    6    Ikehashi& Araki   
                                                   (1984,1996)
_______________________________________________________________

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 a particular set of genes in alphabetical order. Their recessive alleles not causing weakness may by denoted by Hwa-1+ 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-1+ 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 O. 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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