Chapter 10: Fungal Genetics: Mendelian and Molecular

The haploid nature of most fungi—where nuclei contain only a single set of chromosomes—means that all genes are immediately expressed in the observable phenotype, eliminating the masking effects of dominance that complicate genetic analysis in diploid organisms. Fungi offer six critical advantages for genetic research: their abundant haploid spores enable efficient mutation screening, hyphal fusion creates heterokaryons suitable for complementation testing, heteroplasmons permit investigation of extranuclear inheritance patterns, the ordered arrangement of spores in ascomycete asci reveals crossing-over with remarkable clarity, parasexuality allows recombination through mitotic mechanisms without sexual reproduction, and their capacity for bacterial-like culturing combined with eukaryotic complexity makes them remarkably tractable. The chapter thoroughly explores crossing-over phenomena, including first and second-division segregation patterns, recombination frequency calculations, chromosomal interference, and gene conversion processes, using model organisms like Neurospora crassa and Sordaria to demonstrate these mechanisms. Fungal mating systems range from homothallism, where organisms are self-fertile, to heterothallism, enforced by mating-type alleles that function as reproductive barriers, with basidiomycetes displaying sophisticated bipolar and tetrapolar systems governed by hundreds of mating-type variants. The chapter then shifts to molecular applications, detailing recombinant DNA techniques, plasmid vector construction, protoplast transformation, and lithium acetate-mediated gene introduction, illustrating how fungi serve as production hosts for pharmaceutical proteins including interferon and tissue plasminogen activator. Modern molecular taxonomy relies on DNA sequencing, ribosomal DNA barcoding, polymerase chain reaction, and metagenomic approaches, fundamentally reshaping fungal classification. The chapter concludes by addressing contemporary advances including CRISPR-Cas9 genome editing capabilities, mycovirus discovery and applications, and the emerging fields of epigenetics and proteomics in fungal biology.