36. Induction of diploids from haploid rice plants by X-ray irradiation

It is well known in rice that diploid plants can be obtained by vegetative propagation of haploid plants, although the frequency of occurrence of diploids is very low. Diploid plants were obtained with a high frequency by an acute X- ray irradiation of haploid plants in the vegetative stage, which had occurred spontaneously in a Japanese cultivar, Norin 8 (Futsuhara and Hattori 1979). We call these induced diploids "doubled haploid".

On the basis of spikelet sterility and grain size taken as criteria, the doubled haploid plants were classifiable into three types: DH-1 type with normally sized fertile spikelets, DH-2 with normally sized sterile spikelets, and DH-3 with small but fertile spikelets (the grain size similar to that of haploid plants). Their frequencies were 63.9% for DH-1, 8.3% for DH-2, and 27.8% for DH-3 types. Photographs of panicles taken with transmitting light are shown in Fig. 1, which indicates the chimeric structure of the panicles and spikelets. Probably, the induced diploids would become the DH-3, DH-2 and DH-1 types respectively, depending on whether the duplication of chromosome complement in the haploid cells takes place in the embryo (germ cells) originating from the corpus (inside of apical meristem; DH-3), or in the glume originating from the tunica (outside layers of meristem; DH-2), or in both (DH- 1). On this assumption, the DH-2 and DH-3 types may be considered as periclinal chimeras composed of 2n glume and n embryo (DH-2), and of n glume and 2n embryo (DH-3), respectively. On the other hand, most of "doubled haploid" plants with partly sterile spikelets seemed to be either sectorial or mericlinal chimera with respect to chromosome number. The selfed progenies of both DH-1 and DH-3 types were of DH-1 type.

In general, mutations induced in haploid plants are detectable in the first generation of treatment (M\1\). Consequently, an acute irradiation of haploid plants would be one of the most effective methods to induce mutants, as is the case with chronic irradiation of haploid plants described by Tanaka (1970). In addition, the induction of diploids by irradiation of haploid plants will be useful in studying the chimeric structure of induced mutants.

Fig. 1. Photographs of panicles taken with transmitting light, showing the chimeric structure of "doubled haploid" plants.

Futsuhara, Y. and K. Hattori, 1979. Induction of somatic mutations in rice by irradiation. Jpn. J. Breed. 29, Supplement 2: 90-91. (in Japanese)

Tanaka, S., 1970. Radiation-induced dihaploid lines derived from a haploid rice plant. Gamma Field Symp. 9:43-56.