ABSTRACT
Sexual development is the classical parameter for the taxonomical classification of animals, plants, and fungi. This is reflected in the nomenclature of numerous taxons of the fungal kingdom. The typical feature of many ascomycetes as a result of their sexual reproductive cycle is a saclike bag, which is called the ascus (Gr. askos=sac, goat skin). The ascus is filled with ascospores (Gr. askos; spora=seed, spore), which are formed in a process originally termed “free-cell formation” [1]. These ascospores are meiospores and are the final products of a complex series of events. The number of ascospores within an ascus varies between one and >1000 depending on the species. In many ascomycetes, ascosporogenesis results in either four or eight ascospores. Many ascomycetes do not seem to reproduce sexually, which is a principal problem for taxonomists. Since ascomycetes-and many other fungi-were originally classified primarily on the basis of their sexual reproduction, an artificial classification group was created for organisms lacking a sexual life phase [2]. This classification as Deuteromycota or Fungi imperfecti denotes their status as second (deutero)-class members in comparison to sexually reproducing organisms which were considered as “perfect.” For some fungi, this taxon was only a transient classification until proper conditions for sexual reproduction were discovered. When morphological, biochemical, and genetic criteria are applied, many members of this group are found to be closely related to ascomycetes. Thus, species without recorded sexual cycle are now usually grouped in their respective related “teleomorphic” families and designated as “anamorphic” species. For more than a century it was assumed that many deuteromycetes, including many Aspergillus spp., have completely lost their sexual cycle [2,3]. For some imperfect fungi this view might still hold true; for others, e.g., the well-studied human pathogen Candida albicans, however, recent genomics studies have challenged this view owing to the discovery of mating typespecific genes. It was also shown that C. albicans could be forced to mate, suggesting that necessary elements of a sexual cycle are retained in its genome. Since the genome sequencing project of C. albicans also uncovered homologs of genes required for meiosis, a full sexual cycle could-if only rarely-happen in nature [4,5]. Presumably,
the analyses of additional fungal genomes will soon shed more light on the presence or absence of sexual life phases of numerous anamorphic fungi.
