ASCUS AND ASCOSPORE MORPHOGENESIS

The diagnostic feature for members of the subdivision Ascomycotina is that sexual reproduction results in the production of endogenous ascos pores within asci. A range of ascus and ascospore types is found withi n the group. Traditionally, asci have been classified as unitunicate, bitunicate or prototunicate, although a spectrum of intermediate types of asci also occurs. Variations occur in the structure of ascus and a scospore walls, ascus apical structures, septal pore structures, ascos pore size and shape, ascospore pigmentation, the number of cell compar tments within each ascospore, the number of ascospores per ascus and t he arrangement of ascospores within an ascus. In filamentous ascomycet es, asci arise from ascogenous hyphae and this usually takes place asy nchronously within an ascoma. Crozier formation is involved in ascus i nitiation in many, but not all, filamentous species. Asci exhibit dete rminate growth and usually represent highly specialized hyphal element s. Following ascus extension a simple to complex apical structure is c ommonly formed in those ascomycetes which discharge their ascospores a ctively. Ascospore formation involves cytoplasmic compartmentalization by double delimiting membranes between which the ascospore wall devel ops. Confusion in the literature about the origin of delimiting membra nes is resolved by using the endomembrane concept which comprises one membrane-generating system with different levels of membrane transform ation between the nuclear envelope and plasma membrane. Spindle pole b odies and microtubules play important roles in the organization of nuc lei and the assembly of the delimiting membranes. Major problems with ascospore wall terminology are overcome by using a simple scheme in wh ich wall material is either primary or secondary. Ascospore liberation from mature asci may be either an active or passive process. In activ ely discharging species, the motive force for discharge commonly arise s from an increase in internal hydrostatic pressure arising from water uptake. To date, most studies on ascus and ascospore morphogenesis ha ve been descriptive involving light microscopy and/or electron microsc opy. However, much useful information has also been obtained from muta nt analysis and significant progress has recently been made in underst anding the role of the mat locus in controlling dikaryon formation. Of significance in future studies will be the increased integration of t raditional approaches with newer cell biological techniques to study a scus and ascospore morphogenesis.