Among the 250 known species of aspergilli, about 33% have an identified sexual state. Among those "Aspergillus" species that exhibit a sexual cycle the overwhelming majority in nature are homothallic (self-fertilizing). Selfing in the homothallic fungus "Aspergillus nidulans" involves activation of the same mating pathways characteristic of sex in outcrossing species, i.e. self-fertilization does not bypass required pathways for outcrossing sex but instead requires activation of these pathways within a single individual. Fusion of haploid nuclei occurs within reproductive structures termed "cleistothecia", in which the diploid zygote undergoes meiotic divisions to yield haploid ascospores.