Introduction

Asexual sporulation is generally the most prolific reproductive mode for fungi. Asexual spores of higher fungi are called conidia, which are non-motile asexual propagules made from the side or tip of specialized sporogenous cells and do not form through progressive cleavage of the cytoplasm. The process of conidiation is complex and involves temporal and spatial regulation of gene expression, cell specialization and intercellular communication. However, the genetic mechanisms controlling fungal sporulation have only been addressed in detail in two wellstudied ascomycetes, Aspergillus nidulans and Neurospora crassa. In this chapter we will describe the genetic regulation of development in A. nidulans. It is presumed that variations on this theme will apply in many cases to understanding conidiogenesis in other fungi as well.

The A. nidulans asexual reproductive cycle can be divided into three conceptual stages: (1) a growth phase that is required for cells to acquire competence to respond to induction signals; (2) initiation of the developmental pathway; (3) the events leading to sporulation.

Colony formation

Vegetative growth in A. nidulans begins with the germination of a spore. Spore germination leads to the formation of tubular structures, termed hyphae, that grow in a polar fashion by apical extension to form a network of interconnected hyphae known as a mycelium. The mycelium forms a radially symmetrical colony that expands indefinitely at a constant rate of about 0.5 mm h-1 (at 37 °C).