Introduction

Fungi form cells by inserting cross walls called septa. Even the so-called cenocytic fungi (Chytrids and Zygomyoetes) delimit sporangia, zoospores and other structures with septa. ln the filamentous Basidiomycetes and Ascomycetes, septa are formed along the mycelial thallus denning cell compartments of a uniform length and nuclear content. In all true fungi, septa are formed by a similar process. A site is chosen within the cell for the assembly of the septum. Actin is recruited to this site and can be seen as a complex of dots (in the case of unicellular yeasts) (Adams & Pringle, 1984; Marks & Hyanns, 1985) or as a micro filamentous belt in the case of higher fungi (Girbardt. 1979). The actin cytoskeleton facilitates the highly localized, circumferential deposition of cell wall material outside the plasma membrane. In unicellular yeast such as Saccharomyces cerevisiae or Schizasaccharamyces pombe a primary cell wall layer is depositedfollowed by the symthesis of a secondary septal wall on either side of the primary septum. The primary wall is enzymatically removed to allow cell separation. In the mycelia of higher fungi, septation is incomplete, leaving a complex structure called the septal pore. There is no obvious stage of mondary septal wall synthesis and no cell separation. Cell separation does occur during the septation processes that produce aerial spores.

This chapter focuses on the mechanisms controlling the pattern of septation in the filamentous fungus, Aspergillus nidulans.