Fungal Genetics

​Virus genetics are probably the simplest of pathogens, some not even bothering with DNA but using mRNA to directly cause the host cell to provide the more complex process of DNA code as well as the ribosomes to produce essential proteins for the pathogen.  Fungal pathogens of corn have more complex genetic systems than viruses and corn.
 
​Corn reproduction is relatively easy to understand when compared to most corn pathogens.  Most corn cells have a single nucleus with 10 pairs of chromosomes with only the exceptions of the triploid cells of the endosperm and the haploid cells in the pollen and egg cells.  The hyphae of most corn leaf pathogens are filaments composed of multiple cells separated by cell walls called septa that have pores allowing exchange of cytoplasm and even nuclei. The nuclei are composed of a single set of chromosomes. Furthermore, a single cell of the fungus causing northern leaf blight may have up to 30 nuclei, all haploid.
 
Genetics of diploid cells are affected by both sets of the pair, generally with the dominant form of a gene on one set of the chromosomes resulting in the amino acid components of the protein and therefor the trait expressed.  Recessive genes get expressed only when both sets have the same recessive i.e., homozygous recessive for that gene.  Mutations resulting in new recessive genes are often ineffective because of the dominant form in the other member of the paired chromosome.
 
In a haploid nucleus, there is only one form of a gene, allowing a recessive gene to be fully expressed.  Resistance, especially those with single gene inheritance, in corn to a pathogen such as Exserohilum turcicum, cause of northern leaf blight, is initiated when the host cells recognize the presence a pathogens product (effector).  A mutation in the pathogen’s gene for this effector, negates the plants ability to recognize the pathogen and therefore this single gene for resistance is not initiated.  This is the weakness of the single gene system for resistance to any pathogen.
 
Fungal pathogens have the statistical advantage of producing new races because of the haploid nuclei in most hyphae.  For example, if a homozygous recessive form is crossed with a homozygous dominant, the first generation of a diploid organism would only produce the dominant gene product.  In the haploid hyphae, half of the resulting cells would produce the recessively inherited product.  This explains why some pathogens can successfully overcome the single gene type of resistance.  Being able to asexually reproduce with thousands of spores with haploid nuclei from the successful pathogen allows quick spread of a new genetic type.