Cellcell Recognition 821 Mating Types

The fungi grouped in Basidiomycotina are remarkable for having thousands of potential mating partners or mating types. It is of interest to learn how these fungi select a mating partner in the absence of any morphological differentiation. They are remarkable too for having a life cycle in which the nuclei of opposite mating types divide synchronously and remain in close proximity as an (n + n) pair by a special type of hyphal growth called a clamp connection, in which a short hyphal branch grows backwards as a hook that fuses with the penultimate cell with the nucleus passing through it such that each binucleate compartment in the hypha acquires nuclei of opposite mating type (see Figure 2.5). The nuclei of opposite mating type in each hyphal compartment remain associated for an extended period of time without fusing—a diploid (2n) phase is postponed in favor of an extended dikaryophase (n + n), the significance of which is a mystery.

Among the fungi in this group used as a model to investigate the mate recognition process are the mushrooms Coprinus cinereus and Schizophyllum commune and the corn smut fungus, scientifically known as Ustilago maydis (Casselton, 2002). Here, we shall highlight the efforts aimed at understanding sexual reproduction in U. maydis. This fungus is of interest for other reasons as well: In this fungus, sexual development and patho-genecity are interconnected (Brachmann et al., 2003). Moreover, the fungus exhibits the

Gametangia

Progametangium

Zygophore

Progametangium

Zygophore

Young zygospore

Gametangia

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