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. 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 (called imperfect fungi), but there is not much clear evidence for this. Indeed, not a great deal of practical use has been made of the parasexual cycle in the laboratory, 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 they could recombine advantageous mutations. 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 the analogous cycle: mouse + human cell forming a hybrid fusion cell which 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.

Updated December 17, 2016