Introduction

One of the most characteristic features of the fungal mycelium is its highly polarized mode of growth. Mycologists have devoted considerable effort towards understanding the basic mechanisms underlying hyphal elongation and branching, and these subjects have been reviewed extensively (Trinci, 1979; Prosser, 1983; Trinci, Wiebe & Robson, 1994; Gow, 1994; Trinci et al., Chapter 5, this volume). One approach which has been employed to investigate these mechanisms is the identification and characterization of mutants defective in hyphal morphogenesis. Such mutants are relatively easy to detect since they typically cause severe alterations in colony morphology. Indeed, a useful benefit of early experiments in the biochemical genetics of Neurospora crassa was the generation and description of a large collection of colonial mutants (Murray dr. Srb, 1962). Although these mutants have been characterized to a limited extent, the nature of the affected genes is in most cases unknown. Recently, with rapid progress being made in understanding cellular morphogenesis in the yeasts Saccharomyces cerevisiae andSchizosoccharomyces pombe (Snell & Nurse, 1994; Simanis, 1995;Roemer, Vellier&Snyder, 1996), there has been renewed interest in understanding the genetic basis of filamentous growth in fungi. Sincemolecular genetic analyses in the model filamentous fungi Aspergillusnidulans and N. crassa have yielded considerable insight into metabolic control, development, and mitosis (Bennett&Lasure, 1991; Martinelli & Kinghorn, 1994), it is reasonable to presume that this approach will alsocontribute towards achieving a thorough understanding of hyphal morphogenesis.