7.8 Parasexual cycle

We have described a number of separate events occurring in mitosis during vegetative fungal growth that might be arranged into a sequence. Mitotic segregants from the diploid prove to be haploid (produced by a process of regular chromosome loss during successive aberrant mitoses called haploidisation), partial diploids (aneuploids stabilised during the chromosome loss sequence) or diploids showing segregation for a few linked genetic markers, and remaining heterozygous for the others. Haploidisation is caused by nondisjunction (improper transport of chromosomes to the poles of the division spindle during mitosis) resulting in random chromosome loss over several divisions, so the diploid is reduced to a haploid state through a series of aneuploid intermediates (Stukenbrock & Croll, 2014). Overall, the fusion of genetically different haploid nuclei in a heterokaryon followed by mitotic crossing‑over; then completed by haploidisation, is a sequence termed the parasexual cycle.

On the face of it, the parasexual cycle has much the same effect as the sexual cycle by reassorting and recombining genes, thereby increasing genetic variation within the species. A plausible argument can be made that the parasexual cycle could be an alternative to sex in fungi lacking a sexual cycle (so-called imperfect fungi), but there is not much clear evidence for this. Not a great deal of practical use has been made of the parasexual cycle in fungi, even though several commercial processes depend on imperfect fungi like Penicillium chrysogenum. Prior to the advent of molecular biology industrial fungal geneticists at Glaxo's Laboratories in Ulverston used the parasexual cycle to improve strains of P. chrysogenum for penicillin production. At the time this was the only way that advantageous mutations could be recombined.

Ironically, however, the technique found its most extensive application in human genetics. A very large proportion of the gene assignments to human chromosomes were made, before the genomics era, using this analogous cycle:

  • mouse + human cell forming a hybrid fusion cell, which is a heterokaryon that suffers successive loss of chromosomes during subsequent mitoses. Eventually, aneuploid cell lines, sufficiently stable for genetic and cytogenetic characterisation are formed and co-segregation of genes reveals linkage.

It's another example of a phenomenon discovered in fungi being exploited to enhance some aspect of animal cell biology. Indeed, study of heterokaryons involving human cells is an expanding area of interest in medical research. Contributing to production of monoclonal antibodies, the generation of cell hybrids for cancer immunotherapy and organ repair therapies for several genetic and degenerative diseases. Experimental cell fusion heterokaryons of this sort have informed our understanding of malignancy and tumour-suppressor activity, and they are now fundamental to the study of nuclear reprogramming of differentiated cells into the pluripotent (stem-cell) state (Serov et al., 2011; Narbonne et al., 2012; Jang et al., 2016).

 Sexual reproduction is ubiquitous among eukaryotes, and fully asexual lineages are extremely rare; probably the most prominent of ancient, presumed asexual, lineages are the arbuscular mycorrhizal (AM) fungi (Glomeromycotina, see Section 3.6). This group of highly successful plant mycorrhizal symbionts have a multinucleate cytoplasm but no known sexual stage and are thought to have been asexual for approximately 400 million years. It has been presumed that in these organism’s evolution might depend on genetic variation resulting from accumulation of mutations occurring in a population of genetically different nuclei within individual arbuscular mycorrhizal fungi. And while there is some support for this concept (Kuhn et al., 2001), there is also evidence that genetic diversity in isolates of Rhizophagus irregularis, a model AM fungus, varies between isolates in a way that is like a fungus with a homokaryon-dikaryon sexual life cycle. A multi-allelic mating-type locus, containing two genes with structural and evolutionary similarities to the mating-type locus of some Dikarya, has also been identified (Ropars et al., 2016).

Updated July, 2019