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Nonsexual variation: parasexuality


Many common mitosporic fungi, such as Aspergillus,
Penicillium,Fusarium, andTrichoderma, seem largely to
have abandoned sexual reproduction, because their


sexual stages are not seen in laboratory conditions and
are found only infrequently, if at all, in nature. For
example, the common fungi, Aspergillus fumigatusand
A. nigerhave never been found to produce a sexual stage.
This leads us to ask whether they have alternative
mechanisms of genetic recombination, and the answer

FUNGAL GENETICS, MOLECULAR GENETICS, AND GENOMICS 167

Fig. 9.6Anastomosis reactions of Rhizoctonia solani. (a) No reaction, when strains of different anastomosis groups show
no hyphal attraction. (b) Compatible reaction, when strains of the same anastomosis group orientate towards one another
and fuse to form a continuous hyphal network. (c) Incompatible reaction, when strains of the same anastomosis group
but with different vegetative compatibility genes undergo hyphal fusion, followed by cell death of the fused hyphal
compartments. The fused hyphae between the three arrows are dead. (Courtesy of H.L. Robinson.)


Fig. 9.7(a) Part of Fig. 9.6c at higher magnification, showing regrowth of a hyphal tip into the dead hyphal com-
partment. (b) Vegetative incompatibility between two strains (T1 and T2) of Rhizoctonia solani(Basidiomycota) opposed
on an agar plate. The clear demarcation zone between the colonies resulted from post-fusion death of the hyphae
where the colony tips fused (Courtesy of P.M. McCabe.)


(a) (b)
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