What is Ashbya?
Ashbya is a filamentous fungus that has a dual role as a model multinucleated cell in the lab and as a cotton and citrus pathogen in the natural world. The whole genome sequence of Ashbya (carried out in the laboratory of Peter Philippsen, the pioneer of Ashbya cell biology, in collaboration with Syngenta) demonstrated that it diverged from a common ancestor with the budding yeast about 100 million years ago. Unlike budding yeast, the Ashbya genome did not undergo detectable duplication events giving it a smaller (4.2Mb and 4700 ORFs) and more compact genome that lacks much of the redundancy that remains in the yeast genome. Synteny or gene order is conserved for over 90% of the Ashbya genome making it a powerful tool for studying how whole systems, such as the cell cycle machinery, evolved to function within a long multinucleated, hyphae as opposed to uninucleated, spherical yeast cells.
Ashbya cells are constitutively filamentous and multinucleated. Each nucleus contains one complete haploid genome and nuclei of different genotypes can reside within one single cell. Cells with genetically different nuclei are called heterokaryons. These may arise in the wild due to DNA damage that is limited to a subset of nuclei and in the lab are formed when Ashbya cells are transformed with either plasmids or linear pieces of DNA for gene targeting. Gene deletions or gene fusions are readily made with PCR fragments containing 45-50bp homology to the loci of interest and a dominant selectable marker encoding genes that supply resistance to compounds such as G418. Heterokaryons can be sporulated to generate asexual spores with a single nucleus and thus a single genotype which are called homokaryons. The phenotypes of mutants are studied in this homokaryon stage.
Ashbya has been exploited as a model organism for understanding novel mechanisms of cell cycle control and polarized growth. Here is a movie showing Ashbya polar growth and nuclear division.