There are some 17,000 described species of imperfect
fungi (figure 36.12). Even though sexual reproduction is
absent among imperfect fungi, a certain amount of genetic recombination occurs. This becomes possible when
hyphae of different genetic types fuse, as sometimes happens spontaneously. Within the heterokaryotic hyphae
that arise from such fusion, a special kind of genetic recombination called parasexuality may occur. In parasexuality, genetically distinct nuclei within a common hypha
exchange portions of chromosomes. Recombination of
this sort also occurs in other groups of fungi and seems to
be responsible for some of the new pathogenic strains of
wheat rust.