National Institute of Genetics, Misima, 411 Japan
Even though no meiotic irregularities are detected, hybrid sterility often
occurs among the hybrids of Oryza species. An example is found in the hybrid
between the two cultivated rice species, O. glaberrima and O. sativa. The
hybrid is highly male sterile although their chromosomes pair normally during
meiosis (Morinaga and Kuriyama 1957). One possible explanation for this
situation might be cryptic chromosomal rearrangements. To distinguish the two
taxa which are reproductively isolated, the genome symbol was recommended as AA
for O. sativa and as AgAg for O. glaberrima (Anon. 1964). The presence of
small rearrangements of chromosomal segments was supported from a tendency of
preferential pairing observed in the tetraploid hybrid between the two species
(Oka 1968). A series of experiments are under way in order to get insights into
cryptic rearrangements, if any, from a change in the gene order.
Induction of mutant genes was attempted in O. glaberrima with EMS treatments (Sano 1977, 1979). Out of 96 mutants, wx, d\1\, lg\1\, bc\1\, d\28\ and gl\1\ were identified after testcrossing with strains of O. sativa carrying the corresponding markers. The complementation test does not give information about their chromosomal locations. To compare linkage relations of genes in the two species, additional gene markers are needed in O. glaberrima.
Another method to compare chromosomal location of genes is to transfer dominant genes from O. glaberrima into O. sativa and investigate their chromosomal location in O. sativa background. It should be noted that recessive genes cannot be used for this purpose. Wxa has been introduced from O. glaberrima into O. sativa (T65wx, a near isogenic line of Taichung 65 with wx) by successive backcrosses (Sano et al. 1985), but the chromosomal location was not studied. Three other dominant genes were recently introduced from O. glaberrima into O. sativa by using T65wx as the recurrent parent. In each generation of backcrossing, plants showing photosensitivity, apiculus coloration and brown pericarp were used for next backcross. Those introduced traits segregated into a 3:1 ratio in B\n\F\2\ indicating that each trait is controlled by a single dominant gene. The introduced dominant genes from O. glaberrima were tentatively assumed to be Se-1, A and Rc since the genotype of the recurrent parent was wx se-1 C a rc. True breeding lines expressing those traits were selected from BC\11\F\3\ (photosensitivity) and BC\8\F\3\ (apiculus coloration and brown pericarp) as near isogenic lines carrying an alien gene.
The four isogenic lines obtained so far were crossed with O. sativa strains carrying the corresponding dominant markers. The F\2\ populations of these crosses gave no recessive segregants indicating that the introduced genes of Wxa, Se-1, A and Rc are located at the same loci as those in O. sativa. This suggests that gene arrangement is similar in the two species. However, a question arises as to if cryptic rearrangments exist near the introduced genes. If chromosomal rearrangements like small translocations are involved in the introduced segments, the near isogenic lines are expected to give infertility in a cross with O. sativa. The near isogenic lines having Se-1, A and Rc gave fully fertile plants in crosses with O. sativa strains. Infertility was observed when the line having Wxa was crossed with O. sativa strains. However, the infertility was proved to result from an allelic interaction of sterility factor (S\1\/S\1\a) which was closely linked with wx. The segregation patterns for sterility was different from that expected when small interchanges existed (Sano 1986). The results suggest that there are no rearrangements such as translocations near the introduced genes.
References
Anonymous, 1964. Recommendation of the Committee on genome symbols for Oryza species. In IRRI (ed.), Rice Genetics and Cytogenetics, pp. 253-254. Elsevier, Amsterdam.
Morinaga, T. and H. Kuriyama, 1957. Cytogenetical studies on Oryza sativa L. IX. The F\1\ hybrids of O. sativa L. and O. glaberrima Steud. Jpn. J. Breed. 7: 57-65.
Oka, H.I., 1968. Preferntial pairing of chromosomes in a tetraploid hybrid between Oryza glaberrima and O. sativa. Can. J. Genet. Cytol. 10: 527-535.
Sano, Y., 1977. Selection of induced mutants in Oryza glaberrima, Ann. Rep. Nat. Inst. Genet. 27: 101-102.
Sano, Y., 1979. Gene markers obtained in Oryza glaberrima. Ann. Rep. Nat. Inst. Genet. 29: 97-98.
Sano, Y., 1986. Sterility barriers between Oryza sativa and O. glaberrima. In IRRI (ed.), Rice Genetics, pp. 109-118. IRRI, Manila.
Sano, Y., M. Katsumata and K. Okuno, 1985. Genetic studies of speciation in cultivated rice. 5. Inter- and intra-specific differentiation in the waxy gene expression of rice. Euphytica 35: 1-9.