Two genomes are better than one: history, genetics, and biotechnological applications of fungal heterokaryons

Abstract

Heterokaryosis is an integral part of the parasexual cycle used by predominantly asexual fungi to introduce and maintain genetic variation in populations. Research into fungal heterokaryons began in 1912 and continues to the present day. Heterokaryosis may play a role in the ability of fungi to respond to their environment, including the adaptation of arbuscular mycorrhizal fungi to different plant hosts. The parasexual cycle has enabled advances in fungal genetics, including gene mapping and tests of complementation, dominance, and vegetative compatibility in predominantly asexual fungi. Knowledge of vegetative compatibility groups has facilitated population genetic studies and enabled the design of innovative methods of biocontrol. The vegetative incompatibility response has the potential to be used as a model system to study biological aspects of some human diseases, including neurodegenerative diseases and cancer. By combining distinct traits through the formation of artificial heterokaryons, fungal strains with superior properties for antibiotic and enzyme production, fermentation, biocontrol, and bioremediation have been produced. Future biotechnological applications may include site-specific biocontrol or bioremediation and the production of novel pharmaceuticals.

Keywords: Biotechnology; Complementation; Heterokaryon; Heterosis; Parasexual cycle; Protoplast fusion; Vegetative compatibility groups.

Fig. 1

The parasexual cycle. The parasexual cycle parallels events in the sexual cycle, resulting in genetically unique haploid offspring but without a meiotic reduction. a Hyphae of genetically unique homokaryotic parents grow towards each other by chemotaxis and fuse. b Nuclei from each unique strain migrate within the fused hypha, which is now considered a heterokaryon. c Haploid nuclei in the heterokaryon undergo karyogamy to create a heterozygous diploid nucleus. d The diploid nucleus undergoes mitotic recombination to produce a recombined genotype. e In growing hyphae, gradual loss of chromosomes due to repeated rounds of mitotic non-disjunction results in haploidization and unique genotypes in various sectors of mycelium

Fig. 2

Protoplast fusion. a Fungal cell walls are digested with cell wall degrading enzymes, exposing protoplasts (b). c Polyethylene glycol (PEG) facilitates fusion of protoplasts, which may belong to different strains or species, as represented by red or blue chromosomes. d Fused protoplasts form heterokaryons and may progress partway or completely through the parasexual cycle, resulting in diploid or recombined haploid hybrids. e Applications of fungal heterokaryons or hybrids resulting from protoplast fusion include genetic analyses, fermentation, pharmaceutical production, bioremediation, and biocontrol

Fig. 3

Vegetative incompatibility. a Hyphae of vegetatively incompatible filamentous fungi grow towards each other by chemotaxis. b Hyphal fusion occurs, resulting in a heterokaryon. c Expression of incompatible het genes in the heterokaryon leads to sealing of septal pores, autophagy, and programmed cell death. Vacuolization and the formation of additional septa (not pictured) also occur. Autophagy (represented by small circles in the cytoplasm of the heterokaryon) may prevent the spread of pro-death signals and is not thought to contribute to the cell death process. Thinning of the heterokaryotic filament accompanies cell death