differences in pigmentation or sporulation. But sectors
can also differ in pathogenicity or biochemical prop-
erties, and these often go undetected.The significance of heterokaryosisHeterokaryosis is a potentially powerful phenom-
enon, but caution is required because the extent of
heterokaryosis in natural environments is largely
unknown. Many laboratory studies have involved paired
auxotrophic mutants, and these can be regarded as
“forced heterokaryons” – there is a strong selection
pressure to maintain the heterokaryotic condition.
Also, there are significant barriers to the creation ofheterokaryons in nature, because many fungi have
nuclear-encoded “heterokaryon incompatibility” (het)
gene loci. For example, Neurospora crassahas at least
10 such loci, with two alleles at each locus, so there
are potentially 2^10 (1024) different “vegetative” com-
patibility groups(VCGs). Pairings of strains of dif-
ferent VCGs lead to hyphal fusion at the points of
contact, followed by different degrees of cytoplasmic
incompatibility, depending at least partly on the
number of hetloci that any two strains have in
common. Typically, the cytoplasm of the fused cells
dies (Fig. 9.6) resulting in a clear demarcation zone
between opposing colonies (Fig. 9.7). Further details can
be found in Glass & Kaneko (2003).166 CHAPTER 9
Fig. 9.5(a–d) Heterokaryosis and the reversion to homokaryons. See text for details.