Nuclear and cytoplasmic effects of ultraviolet irradiation inNeurospora crassa

Summary

• 1. Macroconidia of two strains ofNeurospora crassa were given repeated exposures to ultraviolet light. Three levels of exposure, 10 seconds, 50 seconds and 90 seconds were used. At each treatment these exposures killed approximately 0%, 90% and 99% of the macroconidia respectively.

• 2. There was no significant change in viability or survival rate of the macroconidia after 6 successive exposures each of 10 seconds duration. After 7 successive exposures of 50 seconds there was a significant overall decrease in viability and in one of the strains a significant decrease in survival rate after irradiation. After 7 successive exposures to 90 seconds there was a significant decrease both in viability and survival rate.

• 3. After 5 successive exposures to ultraviolet, reciprocal crosses were made between all six irradiated lines and the original strains. Germination tests on the ascospores from these crosses provided estimates of the degree of ascospore lethality induced by the ultraviolet. In the 50-second lines an ascospore lethality of about 20% could be attributed to radiation damage to the nuclear material. In the 90-second lines about 30% ascospore lethality was due to radiation damage to the nuclear material. In addition about 20% ascospore lethality could be attributed to radiation induced cytoplasmic damage in one of the lines.

• 4. Some unexpected features of the results are discussed. An explanation is suggested for the observed increase in ultraviolet sensitivity of the irradiated strains.