Ⅰ. HIGHLIGHTS OF THE YEARRIKEN Accel. Prog. Rep. 56 (2023)
RIKEN
Accel. Prog. Rep. 56 (2023)

Screening for high-growth mutants in sporophytes of Undaria
pinnatiﬁda using heavy-ion beam irradiation†
Y. Sato,∗1,∗2 T. Hirano,∗1,∗3 Y. Hayashi,∗1 N. Fukunishi,∗1 S. Kawano,∗4 and T. Abe∗1Undaria pinnatiﬁda (Wakame) is primarily cultivated as a food resource on Japan’s Sanriku coast,
but production yield is declining due to environmental changes and a decrease in the number of producers. To meet domestic market demand, excellent strainsthat improve productivity per producer must be developed. Until now, there have been no results of mutation breeding using heavy ion beams in brown macroalgae. Our previous study developed a technique for producing mutants in the gametophyte from zoospores obtained by irradiance of heavy-ion beams on the sporophylls.1) However, this method limits the time for obtaining mature sporophylls. Therefore, in this study,
gametophytes and young sporophytes of this alga wereirradiated with a heavy-ion beam, and the subsequentculture was conducted using a tank system (CFCS)2)
to select elite cultivars without aquaculture on the sea.
The mother plant for the irradiation was collected from Hirota Bay, Iwate Prefecture. According to the method described in previous studies,1,2)
the zoospores were induced and cultured to gametophytes and sporophytes. Gametophytes, and sporophytes were irradiated with C ions (135 MeV/nucleon,LET, 30.0 keV/µm) at a dose of 0.5–25 Gy or Arions (95 MeV/nucleon LET, 284 keV/µm) at a doseof 0.2–10 Gy. The sporophytes obtained from theseions irradiated gametophytes, and the irradiated sporophytes were cultivated in a 7 L plastic tank for about
two weeks and then transferred to a CFCS for cultivation. UV-sterilized seawater was used throughout thecultivation. The seawater temperature and light intensity were set at 10◦ C and 100 µmol photons m−2 s−1 ,respectively. On the 100th day after the start of cultivation, mutant candidates (M1 ) were selected basedon a total length of 100 cm or more and an individual weight of 40 g or more. Cultivation was continued
until sporophylls were formed and zoospores could becollected. The sporophytes obtained by crossing maleand female gametophytes derived from the individual
zoospore were cultured for 20 days, and the total length
was measured. Subsequently, the culture of M2 wascontinued, and those with a weight twice that of the
non-irradiated group, at 100 days after germination,
were selected as excellent mutant candidates (M2 ). M2
was then cultured until sporophylls were formed, released zoospores, and obtained gametophytes.
The survival rate of irradiated individuals varied de†∗1∗2∗3∗4Condensed from the article in Cytologia 86, 291 (2021)
RIKEN Nishina Center
Riken Food Co., Ltd.
Faculty of Agriculture, University Miyazaki
Future Center, Tokyo University

pending on the irradiated generation, ions, and dose.
The decreasing rate was more pronounced in the sporophyte than in the gametophyte at higher doses. Although the survival rates of female gametophytes irradiated with heavy-ion beams with C and Ar ionsdecreased with increasing dose, those of male gametophytes did not decrease after both irradiations. These
diﬀerences in sensitivity to ions may be inﬂuenced by
diﬀerences in haploid and diploid stages or diﬀerencesin tissue structure.
In the M1 generation, 18 individuals were selectedfrom 210 sporophytes obtained from irradiated gametophytes, and 30 were selected from 297 irradiated sporophytes. As a result of cultivating the sporophyte obtained by self-crossing of these 48 individuals and conﬁrming their growth, a total of four lines was obtainedfor particularly large individuals compared to others; 3lines [1302G-R68 (C ions, 5 Gy), 1302G-B60 (Ar ions,0.2 Gy), 1302G-W40 (Ar ions, 2.5 Gy)] from M1 candidate of irradiated gametophytes, and one line [1302SR40 (C ions, 2 Gy)] from the M1 candidate of the irradiated sporophyte (Fig. 1). Among these four lines,
the weight of the largest individuals on the 100th dayof cultivation was two to three times that of the nonirradiated group, so they were selected as M2 individuals. In addition, a sensory test was conducted after
cutting oﬀ a part of the blade and boiling it; the tasteand texture of M2 candidates did not diﬀer from thoseof the non-irradiated individuals. As a result of cultivating the next-generation sporophytes from the gametophytes obtained from M2 , all four lines showed earlier
growth in size than non-irradiated individuals, similar
to the M2 generation. These lines are stored as M3 , conﬁrming the ﬁxation of the mutation. In the future, thesehigh-growth mutants can be used for food production
and biomass utilization, including feed and fertilizer.

Fig. 1. Sporophytes in selected M2 lines (3 lines out of total
4 lines) containing high-growth mutants 100 days aftergermination. (a): 1302G-B60 line, (b): 1302G-W40 line,(c) 1302S-R40 line. Bars = 10 cm.

References
1) T. Hirano et al., Phycological Res. 68, 63 (2020).
2) Y. Sato et al., J. Appl. Phycol. 29, 1683 (2017).

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