FIGURE 3.

A. nidulans vps24 is a virtually essential gene. A, scheme of a conidiophore of an A. nidulans heterokaryotic strain carrying untransformed vps24⁺, pyrimidine-requiring haploid nuclei (blue) and transformed vps24Δ, pyrimidine-independent nuclei (red) (see also the color scheme in B). During conidiophore development, individual nuclei segregate into conidia, thereby resolving the heterokaryotic situation. Thus, heterokaryotic pyrG^- strains carrying nuclei where an essential gene has been replaced by a pyrG⁺ allele can be propagated from conidiospores in the presence but not in the absence of pyrimidines, as the auxotrophic conidia corresponding to the untransformed nuclei required uracil supplementation for growth. Transformants 1 through 5, 7, and 8 are heterokaryotic, whereas transformant 6 is a spontaneous heterozygous diploid or aneuploid for vps24. B, Southern blot analysis of the transformants with indication of diagnostic bands. Note that, in heterokaryotic transformants 1–5, the intensities of the vps24Δ bands relative to the vps24⁺ band are lower than in the diploid/aneuploid transformant 6, indicating that the heterokaryons contain fewer vps24Δ than vps24⁺ nuclei.