Spontaneous Auxotrophic Mutations in Verticillium

Abstract

Spontaneous auxotrophs were sought among conidia of various Verticillium species using the glycerol enrichment technique developed earlier. Seven of 11 isolates of V. dahliae had a high frequency (1-3.4 x 10-4) of spontaneous auxotrophs, and at least 98% of these required nicotinic acid (nic). Other spontaneous auxotrophs required methionine, arginine, adenine, or pyridoxine. Two other isolates of V. dahliae were already nic when tested. The remaining two V. dahliae isolates yielded no auxotrophs. Five isolates of V. albo-atrum had only low levels (4 x 10-6) of auxotrophs, and an isolate of V. nigrescens yielded no auxotrophs. All of the nic variants of V. dahliae were functionally identical in complementation tests. Heterokaryon and limited parasexual analysis suggested that they were all due to mutations in the nucleus at a site called nic-B. The rate of mutation per cell generation at this site was 3 x 10-5. The frequency of nic variants was enhanced by 2-aminopurine, caffeine, 5-fluorodeoxyuridine, and proflavin. The nic variants never reverted to prototrophy, even after ultraviolet light or 2-aminopurine treatment. The mechanism of mutation at nic-B is not known; frameshift and base-substitution mutations and even deletion may be involved.