Mutation breeding and released cultivars in Japan In a 2007 search regarding the number of induced mutation varieties in the IAEA database, China is first in the number of described induced mutation varieties at 638; India is second listing 272 varieties; and Japan is third with 233 varieties. The total number of mutant cultivars, including direct-use mutant cultivars and indirect-use cultivars, exceed these totals. A selection of mutant cultivars developed in Japan, including the economic impact of these cultivars, and their characteristics are reviewed here.
The number of cultivars developed by mutation breeding Figure 2 shows the number of direct-use and indirect-use (hybrid) mutant cultivars registered in Japan in each five-year period from 1960 to 2005. The number of direct-use cultivars had been rapidly increasing until 1995, when 67 cultivars were registered in five years (about 13 cultivars per year). This number fell from 2001 to 2005, with only 41 cultivars being registered (about 8 cultivars per year). The number of indirect-use cultivars primarily generated in rice has steadily increased over time and 68 cultivars were registered from 2001 to 2005. This number can be increased if agronomically useful, direct-use cultivars, such as “Reimei” with the sd1 dwarf gene for rice are developed. Two hundred and forty two direct-use mutant cultivars comprising 61 species generated through irradiation utilizing Gamma-ray, X-ray and ion beams, chemical mutagenesis and in vitro culture (somaclonal variation), have been registered and released in Japan (Fig. 3). More than 61% of these were induced by Gamma-ray irradiation and those induced by somaclonal variation and chemical mutagen, not including those with double chromosome numbers through colchicine treatment, are 15.7% and 6.6%, respectively. Recently, the development of mutant flower cultivars, generated by ion beam irradiation, has been a growing area of mutation induction in Japan.