Spontaneous variability of single isolates of Phytophthora infestans. I. Cultural variation

Abstract

The variability in culture of mycelial isolates of P. infestans was studied by examining the variation among single zoospore, single sporangum, and single hyphal tip subcultures. Extensive variation in rate of growth and sporangium production on artificial medium was detected among the single zoospore progenies of 3 mass cultures. Differences in colony morphology and viability of zoospores were also apparent but were not studied in detail. Subcultures established by single sporangia or single hyphal tips were much more uniform than zoospore cultures, although significant differences in growth rate could still be detected. While, with continued culture, some of the single zoospore variants tended to revert to their parental type, others showed a remarkable degree of stability. Isolates established from single zoospores gave rise to as much variation in their asexual progenies as the original mass cultures. This persistence of variation was also observed with isolates whose derivation included 2 successive single zoospore propagations. Selection for high and low growth rate among the zoospore progeny of a single mass culture rapidly led to the production of populations of zoospore cultures with different mean growth rates. Such response to selection implies the existence of a genetic mechanism which allows the transmission of phenotypic characters from one asexual generation to the next. The origin of variation among single zoospore cultures is discussed with reference to 5 different asexual mechanisms of variation. Variation may be the result of cytoplasmic differences present in the original mass isolates, although the possibility of other mechanisms can not be completely dismissed. A review of the literature suggests that asexual variation associated with zoospore propagation is widespread in the genus Phytophthora.