High monospore-producing mutants obtained by treatment with MNNG in Porphyra yezoensis Ueda (Bangiales, Rhodophyta)

Two high monospore-producing pigmentation mutants were obtained by treatment with MNNG in Porphyra yezoensis Ueda. The mutants produced many monospores in young gametophytic blades (1 month old) and old large blades (3 months old). Monospore production of the mutants was affected by the culture conditions. The higher the temperature and/or the light intensity, the more the number of monospores released. When the conchospores and monospores of the mutants were in a monoculture at 15 and 20 C, they did not develop into large blades because their germlings repeatedly released many monospores. However, they developed into large blades when they were in a co-culture with the large gametophytic blades of the wild-type or other pigmentation mutants. One high monospore-producing red mutant (rm-1) was genetically characterized by crossing with the wild-type which does not easily release monospores. In the F-1 gametophytic blades from heterozygous conchocelis produced in the crosses, there were unsectored blades (2 types) and sectored blades (6 types) consisting of 2, 3 or 4 sectors having both parental color phenotypes. Sectors of both parental colors appeared in the F1 sectored blades in the proportion 1W:1.03R, indicating that the mutant rm-l has a mutation for its color phenotype. In the 4-week-old F-1 blades, high monospore production occurred only in the unsectored red blades and the red sectors of the sectored blades. These results indicate that the tendency of high monospore production is associated with the color phenotype of the mutant, suggesting that high monospore production of the mutant rm-1 is controlled by gene(s), which is closely linked with the gene for the mutant color.