The study of gene structure and function in fungi. Genetic research has provided important knowledge about genes, heredity, genetic mechanisms, metabolism, physiology, and development in fungi, and in higher organisms in general, because in certain respects the fungal life cycle and cellular attributes are ideally suited to both mendelian and molecular genetic analysis.
Fungal nuclei are predominantly haploid; that is, they contain only one set of chromosomes. This characteristic is useful in the study of mutations, which are usually recessive and therefore masked in diploid organisms. Mutational dissection is an important technique for the study of biological processes, and the use of haploid organisms conveniently allows for the immediate expression of mutant genes. See Mutation
Reproduction in fungi can be asexual, sexual, or parasexual (see illustration). Asexual reproduction involves mitotic nuclear division during the growth of hyphae, cell division, or the production of asexual spores. Sexual reproduction is based on meiotic nuclear divisions fairly typical of eukaryotes in general. In ascomycetes and basidiomycetes, the spores, containing nuclei that are the four products of a single meiosis, remain together in a group called a tetrad. The isolation and testing of the phenotypes of cultures arising from the members of a tetrad (tetrad analysis) permit the study of the genetic events occurring in individual meioses; this possibility is offered by virtually no other eukaryotic group. In other groups, genetic analysis is limited to products recovered randomly from different meioses. Since a great deal of genetic analysis is based on meiosis, fungal tetrads have proved to be pivotal in shaping current ideas on this key process of eukaryotic biology. See Eukaryotae, Meiosis
Because their preparation in large numbers is simple, fungal cells are useful in the study of rare events (such as mutations and recombinations) with frequencies as little as one in a million or less. In such cases, selective procedures must be used to identify cells derived from the rare events. The concepts and techniques of fungal asexual and parasexual genetics have been applied to the genetic manipulation of cultured cells of higher eukaryotes such as humans and green plants. However, the techniques remain much easier to perform with fungi.
The fact that each enzyme is coded by its own specific gene was first recognized in fungi and was of paramount importance because it showed how the many chemical reactions that take place in a living cell could be controlled by the genetic apparatus. The discovery arose from a biochemical study of nutritional mutants in Neurospora. See Enzyme
In genetically transformed organisms, the genome has been modified by the addition of DNA, a key technique in genetic engineering. The cell wall is temporarily removed; exogenous DNA is then taken up by cells and the cell wall is restored. The incorporation of DNA must be detected by a suitable novel genetic marker included on the assimilated molecule in order to distinguish transformed from nontransformed cells. The fate of the DNA inside the cell depends largely on the nature of the vector or carrier. Some vectors can insert randomly throughout the genome. Others can be directed to specific sites, either inactivating a gene for some purpose or replacing a resident gene with an engineered version present on the vector. A third kind of vector remains uninserted as an autonomously replicating plasmid. The ability to transform fungal cells has permitted the engineering of fungi with modified metabolic properties for making products of utility in industry. See Genetic engineering
A surprising development in the molecular biology of eukaryotes was the discovery of transposons, pieces of DNA that can move to new locations in the chromosomes. Although transposons were once known only in bacteria, they are now recognized in many eukaryotes. The transposons found in fungi mobilize by either of two processes: one type via a ribonucleic acid (RNA) intermediate that is subsequently reverse-transcribed to DNA, and the other type via DNA directly. In either case, a DNA copy of the transposed segment is inserted into the new site and may contain, in addition to the transposon itself, segments of contagious DNA mobilized from the original chromosomal site. Because of the rearrangements which transposons may produce, they have been important in the evolution of the eukaryotic genome. See Fungi, Genetics, Transposons