Tomosaburo Yabuno

Triple fusion of the primary endosperm nucleus as a cause of interspecific cross-incompatibility in Avena

SummaryUsing five diploid, two tetraploid and three hexaploid Avena species (x=7) 78 reciprocal crosses were made. Of these 74 were successfull, though the frequency of seed set varied greatly. The development of hybrid seeds differed widely between cross combinations, especially in reciprocal crosses. Hybrid seeds were grouped into four types, (1) normal kernels, well developed and germinative (designated as D+); (2) shrivelled-empty kernels, which do not germinate (E−); (3) small viable kernels (Rd+) and (4) small inviable kernels (Rd−). Some rare intermediate types were found in certain crosses.The results are well interpreted in terms of a hypothesis of polar-nuclei activation in which the strength of the activating action of the male nucleus, and the reaction of the female nucleus at double fertilization are expressed as ‘activating value(AV)’ and ‘response value(RV)’, respectively. The degree of seed failure is closely related to the activation index(AI) of the polar nuclei, AV/2RV (or × 100). In a selfed plant the activation index is always 1/2=0.5 (or50%), and this usually results in the formation of normal seeds. If AI deviates from 50% the endosperm often stops developing or degenerates.Based on the degree of abnormality of hybrid seeds, activating values of the ten species were arbitrarily assigned from 0.4 in A. ventricosa (2x) to 3.0 in A.sterilis (6x). For comparison A.strigosa (2x) was used as a standard and set at 1.0. Among the 74 succesful interspecific crosses the AI of the polar nuclei varied widely, from 7 to 375% Generally speaking, AI values of less than 20, 20–30, 30–80, and more than 80% show the R-, Rd+, D+ and E-kernel types, respectively. In other words, if the AV of the pollen parent is < 40 or >160% that of the maternal parent hybrid seeds fail to develop. Thus in double fertilization of Avena or probably angiosperms, the triple fusion of two polar nuclei with a secondary male nucleus is apparently a sexual isolation mechanism, being a barrier to interspecific hybridization.

A gametocidal factor of Oryza glaberrima Steud. in O. sativa L.

SummaryRecurrent backcrossing has been carried out with a view to transfer a gene for non-glutinous endosperm from two strains of O. glaberrima (Wxg/Wxg) to glutinous japonica and indica varieties (wx/wx) of Oryza sativa. In the course of backcrosses Wxg/wx segregants were crossed with each of the two glutinous varieties of sativa as the respective recurrent male parent. The wx/wx and Wxg/wx segregants in the successive generations were consistently fully fertile and semi-sterile, respectively. The semi-sterility of Wxg/wx plants was attributable to abortion of most of the pollen grains carrying the gene wx. The nucleus but not cytoplasm was related to the semi-sterility. The Wxg/Wx plants having the gene for non-glutinous endosperm of a glaberrima strain and a japonica variety of sativa were also semi-sterile. Both wx- and Wx-megaspores in the plants heterozygous for the gene Wxgwere deleteriously affected. The results could be explained by assuming that a factor tightly linked with the gene Wxgof glaberrima sterilizes gametes not carrying it in the heterozygotes and that the gametocidal action is exerted when combined with the sativa nucleus by the recurrent backcross method.

The transfer of a gene for glutinous endosperm to Oryza glaberrima Steud. From a japonica variety of O. sativa L.

SummaryIn common rice, Oryza sativa L. (n=12), the gene Am for ‘non-glutinous’ is dominant over the gene am for ‘glutinous’. In African rice, O. glaberrima Steud. (n=12), no spontaneous glutinous strain has been found, but recently a glutinous strain of glaberrima was induced by EMS-treatment.The interspecific cytoplasm substitution line with sativa cytoplasm and glaberrima nucleus is male sterile. It has been confirmed that the complete restoration of pollen fertility in this male sterile line is attributed to a single dominant nuclear gene Rfj.Trial to transfer gene am from sativa to glaberrima was commenced with backcrosses of the F1 hybrid (glutinous sativa cv. Iwai-mochi ♀ × glaberrima ♂) to glaberrima type plants of the substitution line homozygous for Rfj,using the latter as the pollen parent. At the B1 step, highly fertile glaberrima type Am/am plants were obtained. Thereafter plants of this type were backcrossed to normal glaberrima as the recurrent pollen parent to complete the nuclear substitution. It was confirmed that the EMS-induced glutinous character of glaberrima was a monogenic recessive and that the same gene controls the expression of glutinous character in the different rice species, sativa and glaberrima.

Genetic studies on the interspecific cytoplasm substitution lines of an Asian perennial strain of Oryza rufipogon Griff. and O. glaberrima Steud.

SummaryIt is shown that the restorer gene Rfjextracted from the Japanese rice variety Akebono is effective on pollen restoration in the cytoplasm substitution line having the nucleus of Oryza glaberrima and japonica or indica cytoplasm of O. sativa, and is of the sporophytic type.The Asian perennial type of the wild rice species O. rufipogon is considered to be the progenitor of O. sativa. Two substitution lines having the cytoplasm of a perennial strain of O. rufipogon from Sri Lanka and the nucleus of O. glaberrima with or without the gene Rfjin homozygous condition have been bred by means of successive backcrosses. These lines have now reached the BC5 generation. Plants of the lines resemble morphologically the recurrent parent, but do not show pollen restoration, indicating that the cytoplasm of the rufipogon strain induced male sterility and that the gene Rfjdoes not act as the restorer.