Biology and genetics of the chestnut blight fungus, Cryphonectria parasitica
1) Cloning of vegetative incompatibility (vic) genes
Vegetative incompatibility in C. parasitica is controlled by at least six unlinked vegetative incompatibility (vic) genes, each with two alleles (these are called heterokaryon incompatibility or het genes in some fungi). We have identified all six loci by classical genetics and are now trying to clone some of them. Current efforts to clone vic genes in C. parasitica are being undertaken in collaboration with Don Nuss's laboratory at the University of Maryland. Using the genome sequence of C. parasitica and resequencing of a vegetatively incompatible, in conjunction with linkage mapping, we are in the process of identifying several vic genes.
--Liu, Y.-C., and Milgroom, M. G. 2007. High diversity of vegetative compatibility types in Cryphonectria parasitica in Japan and China. Mycologia 99: 279-284. [Abstract] [pdf].
--Kubisiak, T. L., and Milgroom, M. G. 2006. Markers linked to vegetative incompatibility (vic) loci and a region of reduced recombination near the mating type locus (MAT) in Cryphonectria parasitica. Fungal Genet. Biol. 43: 453-463. [PubMed]
--Smith, M. L., Gibbs, C. C., and Milgroom, M. G. 2006. Heterokaryon incompatibility function of vegetative incompatibility genes (vic) in Cryphonectria parasitica. Mycologia 98: 43-50. [Abstract] [pdf]
2) The formation of mating-type heterokaryons in clonal populations of C. parasitica
Cryphonectria parasitica, like many ascomycete fungi, has two mating types, such that mating only occurs between individuals with opposite mating type (a bipolar, heterothallic system). Studies in natural populations, however, have shown that C. parasitica is highly inbred, sometimes lacking segregation of any markers except for mating type. We have shown that C. parasitica often exists as heterokaryons, i.e., two unlike nuclear types in the same thallus. The two nuclear types differ in mating type, thereby allowing an individual to self-fertilize. A study of a clonal population of C. parasitica in Wisconsin showed that the opposite mating types most likely were derived from different vegetative compatibility types. However, stable heterokaryons do not form between vegetatively incompatible individuals. Understanding the mechanism for heterokaryon formation in nature is the main goal for this current research.
This project is being conducted in collaboration with Kiril Sotirovski (University Sts. Kiril & Metodij, Macedonia) and Paolo Cortesi (University of Milan, Italy). The populations of C. parasitica in southeastern Europe are of interest because some of them are clonal; thus, recombination will not occur between vegetative compatibility types, allowing us to test whether heterokaryons form between types.
--Milgroom MG, Sotirovski K, Risteski M, Brewer MT (2009) Heterokaryons and recombinants of Cryphonectria parasitica in two clonal populations in southeastern Europe. Fungal Genet. Biol.46: 849-854 [abstract][full text].
--Milgroom MG, Sotirovski K, Spica D, Davis JE, Brewer MT, Milev M, Cortesi P, 2008. Clonal population structure of the chestnut blight fungus in expanding ranges in southeastern Europe. Molecular Ecology 17: 4446-445 [abstract][pdf]
--McGuire, I. C., Davis, J. E., Double, M. L., MacDonald, W. L., Rauscher, J. T., McCawley, S., and Milgroom, M. G. 2005. Heterokaryon formation and parasexuality between vegetatively incompatible lineages in a population of the chestnut blight fungus, Cryphonectria parasitica. Mol. Ecol. 14: 3657-3669. [PubMed]
--Marra, R. E., Cortesi, P., Bissegger, M., and Milgroom, M. G. 2004. Mixed mating in natural populations of the chestnut blight fungus, Cryphonectria parasitica. Heredity 93:189-195. [PubMed]
--McGuire, I. C., Marra, R. E., and Milgroom, M. G. 2004. Mating-type heterokaryosis and selfing in Cryphonectria parasitica. Fung. Genet. Biol. 41:521-533. [PubMed]
3) Sequencing the genome of C. parasitica
In April 2008, the sequence the genome of C. parasitica was made public by the DOE Joint Genome Institute's Community Sequencing Program. The project was undertaken by JGI based on the proposal submitted by Donald Nuss (University of Maryland), Alice Churchill (Cornell University) and Michael Milgroom (Cornell University).
A complete description of the genome of C. parasitica is available at the JGI website. Click here to link to JGI.