A Novel Pathway to Produce Color Mutants by Short-Wavelength Ultraviolet Ray Irradiation on the Germinating Conchospore in Pyropia haitanensis

Abstract

Ultraviolet mutagenesis is a safe and efficient method to induce mutations in laver. It has the advantages of non-pollution, high efficiency, easy operation, and low cost. It has been primarily used for mutagenesis of the filament, protoplast, or blade of cultivated Pyropia species, but is rarely used to induce genetic mutants from germinating conchospore. The germination period of conchospores is the period of meiosis in Pyropia haitanensis. The four progeny cells of germinating conchospores are linearly arranged forming a meiotic tetrad. The tetrad cells that undergo genetic recombination can determine the developmental pattern and segregation of traits in the thallus. In this study, short-wave ultraviolet (UV-C) irradiation with different doses (50, 100, 200, 300, 400, 500, and 600 J/m2) was used to induce color mutants during the germination of conchospores in P. haitanensis. The results showed that low-dose irradiation (50 J/m2) promoted the germination of conchospores, while irradiation doses above 100 J/m2 inhibited the germination and growth of tetrad germlings. Therefore, low-doses of UV-C irradiation were used in the production to promote the germination of conchospores and improve the production efficiency of laver. In the dose range of 50–400 J/m2, the frequency of color chimeras increased with increasing irradiation intensity. When the dose was 300 J/m2 and 400 J/m2, and the pigmentation mutation rate was 15.22% and 17.18%, respectively, and the death rate of conchospores was 51.70% and 61.00%, respectively. In the dose range of 50–500 J/m2, with the increase in UV-C irradiation dose, the proportion of 2-color sectored chimera showed a trend of first decreasing and then increasing, and the proportion of 3- and 4-color sectored chimera showed a trend of first increasing and then decreasing. Among them, the regenerated color chimeras that appear were generally 2- and 3-color sectored chimera, yet the proportion of 4-color sectored chimera was the least. When the irradiation intensity reached 500 J/m2, the majority of the conchospores died, and the death rate was 83.98%, and the frequency of color mutants was significantly lower than that of 300 J/m2 and 400 J/m2 dose groups. These results indicate that the mutagenesis effect was the best when the dose was 300 or 400 J/m2, which was convenient to obtain abundant genetic recombination and genetic variation in progeny cells. In addition, UV-C irradiation also had a significant effect on the early development of conchospores and phenotypes of pigmentation mutant arranged in tetrad germlings, which was mainly manifested in the large number of color-sectored blades developed from the irradiated conchospores. Simultaneously, UV-C irradiation retarded the development of cells at the top of conchospore germlings, inhibited the development of cells at the middle base toward the poles, and increased the lateral development, resulting in the decrease in blade aspect ratios. Somatic cell germlings with single colored pigmentation were also isolated from color sectored chimeras by enzymatic hydrolysis. The color species of the five monochromatic mutants obtained were basically consistent with the pigment variant sectors observed on color-sectored thallus, indicating that the obtained color mutants were derived from a single mutant cell on the maternal color-sectored thallus. In conclusion, UV-C irradiations can effectively mutate the conchospores of P. haitanensis, and appropriate irradiation doses (300 or 400 J/m2) can obtain a certain number of color-sectored thallus. This study provides a novel pathway for the preparation of artificial color mutants and mutation breeding in P. haitanensis.

Keywords

• pyropia haitanensis
• conchospores
• short-wavelength ultraviolet
• color mutants
• isolation