Ascus and ascospore morphogenesis
The diagnostic feature for members of the subdivision Ascomycotina is that sexual reproduction results in the production of endogenous ascospores within asci. A range of ascus and ascospore types is found within 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 ascospore walls, ascus apical structures, septal pore structures, ascospore size and shape, ascospore pigmentation, the number of cell compartments within each ascospore, the number of ascospores per ascus and the arrangement of ascospores within an ascus. In filamentous ascomycetes, asci arise from ascogenous hyphae and this usually takes place asynchronously within an ascoma. Crozier formation is involved in ascus initiation in many, but not all, filamentous species. Asci exhibit determinate growth and usually represent highly specialized hyphal elements. Following ascus extension a simple to complex apical structure is commonly formed in those ascomycetes which discharge their ascospores actively. Ascospore formation involves cytoplasmic compartmentalization by double delimiting membranes between which the ascospore wall develops. Confusion in the literature about the origin of delimiting membranes is resolved by using the endomembrane concept which comprises one membrane-generating system with different levels of membrane transformation between the nuclear envelope and plasma membrane. Spindle pole bodies and microtubules play important roles in the organization of nuclei and the assembly of the delimiting membranes. Major problems with ascospore wall terminology are overcome by using a simple scheme in which wall material is either primary or secondary. Ascospore liberation from mature asci may be either an active or passive process. In actively discharging species, the motive force for discharge commonly arises from an increase in internal hydrostatic pressure arising from water uptake. To date most studies on ascus and ascospore morphogenesis have been descriptive involving light microscopy and/or electron microscopy. However, much useful information has also been obtained from mutant analysis and significant progress has recently been made in understanding the role of the mat locus in controlling dikaryon formation. Of significance in future studies will be the increased integration of traditional approaches with newer cell biological techniques to study ascus and ascospore morphogenesis.