Mechanisms of genetic variation in fungi
CALS Impact Statement
Biological control of chestnut blight with viruses is constrained by the genetic diversity of the chestnut blight fungus, Cryphonectria parasitica. Viruses can be transmitted between genetically identical strains of the fungus, but are inhibited by a self/nonself recognition system, called vegetative incompatibility. Variation in vegetative incompatibility, and hence a deterrent to virus transmission and biological control, is maintained by sexual recombination in most populations. In some populations, however, a considerable amount of reproduction occurs asexually, resulting in clonal populations with little genetic variation. Recombination in fungi without sexual reproduction, a process called parasexuality, has been described for many fungi in the laboratory, but not in nature. Parasexuality is initiated by the fusion of cells from genetically different individuals such that different nuclei inhabit the same cells. This fusion is controlled by the same self/nonself recognition system that controls virus transmission. We have documented the occurrence of parasexuality in C. parasitica from clonal populations in Wisconsin, and most recently in Macedonia. In both of these populations, we have shown that cellular fusion and parasexual recombination occurred between strains that are vegetatively incompatible, contradicting the prevailing model for self/nonself recognition mechanisms in fungi.
Two factors motivated this work: the biological control of chestnut blight with viruses, and the basic curiosity that a nonstandard mechanism for genetic recombination in fungi (parasexuality) occurs in nature, even though it has not been shown to occur previously in nature. Understanding how fungal populations generate and maintain genetic diversity is relevant to biological control with viruses, therefore, we wanted to know whether the chestnut blight fungus could increase its genetic diversity without sexual recombination.
We sampled the chestnut blight fungus in populations in Macedonia because we previously showed that they have relatively little genetic diversity and no sexual reproduction. One population in particular that we sampled had two vegetative compatibility types. With large samples of fungi, we used molecular genetic markers to determine genetic fingerprints. One strain of fungus isolated from nature had genetic markers from both vegetative compatibility types. Closer analysis of this strain showed that it contained a mixture of genetically different nuclei, and that some nuclei had fused. Finally, we showed that recombination had occurred to produce novel genotypes (or fingerprints) of the fungus. Therefore, we have demonstrated that parasexuality occurred in natural population of C. parasitica.
The impact of these results is primarily for basic science, not in any direct economic impact. Parasexuality has been known only as a laboratory phenomenon for more than 50 years, and we have now shown definitively that it can occur in nature. This result is all the more significant because we showed that recombination occurred between strains that are vegetatively incompatible. Previously it was thought that vegetative incompatibility, by definition, precluded parasexuality. Our results contradict this simple model and show that vegetative incompatibility is not an absolute barrier to this nonstandard mechanism of recombination.
Kiril Sotirovski (St. Cyril and Methodius University)
Thomas Kubisiak (Southern Institute for Forest Genetics)