INHERITED AND ENVIRONMENTALLY-INDUCED DIFFERENCES IN MUTATION FREQUENCIES BETWEEN WILD STRAINS OF SORDARIA-FIMICOLA FROM EVOLUTION CANYON

We have studied whether there is natural genetic variation for mutatio n frequencies, and whether any such variation is environment-related. Mutation frequencies differed significantly between wild strains of th e fungus Sordaria fimicola isolated from a harsher or a milder microsc ale environment in ''Evolution Canyon,'' Israel. Strains from the hars her, drier, south-facing slope had higher frequencies of new spontaneo us mutations and of accumulated mutations than strains from the milder , lusher, north-facing slope. Collective total mutation frequencies ov er many loci for ascospore pigmentation were 2.3, 3.5 and 4.4% for thr ee strains from the south-facing slope, and 0.9, 1.1, 1.2, 1.3 and 1.3 % for five strains from the north-facing slope. Some of this between-s lope difference was inherited through two generations of selfing, with average spontaneous mutation frequencies of 1.9% for south-facing slo pe strains and 0.8% for north-facing slope strains. The remainder was caused by different frequencies of mutations arising in the original e nvironments. There was also significant heritable genetic variation in mutation frequencies within slopes. Similar between-slope differences were found for ascospore germination-resistance to acriflavine, with much higher frequencies in strains from the south-facing slope. Such i nherited variation provides a basis for natural selection for optimum mutation rates in each environment.