Influence of UV-light on gene-conversion in Neurospora.

1. Crosses between wild type (dark ascospores) and the asco-mutant (light ascospores) were set up and the spores collected as unordered octads. The analysis of 1317 such octads revealed 69 with abnormal segregation of the asco-marker (5.2%). If crosses were irradiated with UV at a dose equivalent to 66% conidial killing, an increase of the number of aberrant octads was found. This increase was most pronounced if the crosses were exposed to the irradiation from one day before to 2 days after plasmogamy, when up to 8.7% aberrant octads were obtained. No effect was observed from 4 days after plasmogamy until the end of the sexual cycle. 2. The relative frequency of the 8 possible types of aberrant octads was analyzed separately for experiments without and for experiments with UV-irradiation. The distribution of these frequencies was the same for unirradiated and irradiated crosses, a finding which suggests that the conversion mechanism acting spontaneously and that acting upon UV-irradiation may be identical. In both cases, there was a strong bias in favour of octads of the 2 : 6 type (giving wild spores first) as compared to such of the 6 : 2 type, also the frequency of the 0 : 8 type was higher than that of the 8 : 0 type. Hence, the conversion mechanism in this cross "prefers" to act from the wildtype to the mutant allele.