Biology and genetics of the chestnut blight fungus, Cryphonectria parasitica
1) Positional cloning of vegetative incompatibility genes
Vegetative incompatibility in C. parasitica is controlled by at least six unlinked vegetative incompatibility (vic) genes, each with two alleles. We have identified all six loci by classical genetics and are now trying to clone some of them. Using the same strategy used for cloning het genes N. crassa, we are trying to clone vic genes in C. parasitica by chromosome walking from linked markers.
This project is being conducted by Cristina McGuire, a postdoc in the lab, in collaboration with Myron Smith (Carleton University), Tom Kubisiak (U.S. Forest Service, Southern Institute for Forest Genetics), and Alice Churchill (Cornell University).
Relevant recent publications:
--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.
Relevant recent publications:
--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 June 2006, a proposal to sequence the genome of C. parasitica was approved by the DOE Joint Genome Institute's Community Sequencing Program. The proposal was submitted by Donald Nuss (University of Maryland), Alice Churchill (Cornell University) and Michael Milgroom (Cornell University). The objectives of this project are to: A) assemble an 8-10 X sequence coverage for the genome of C. parasitica strain EP155; B) perform an automated and directed annotation of the assembled genome sequence; and C) provide a web-accessible database of the C. parasitica genome sequence with necessary tools for mining and comparative genome studies by the research community.