Isolation of two apsA suppressor strains in Aspergillus nidulans

Aspergillus nidulans reproduces asexually with single nucleated conidia. In apsA (anucleate primary sterigmata) strains, nuclear positioning is affected and conidiation is greatly reduced. To get further insights into the cellular functions of apsA, aconidial apsA strains were mutagenized and conidiating suppressor strains were isolated. The suppressors fell into two complementation groups, samA and samB (suppressor of anucleate metulae), samA mapped on linkage group 1 close to pyrG. The mutant allele was dominant in diploids homozygous for apsA. Viability of conidia of samA suppressor strains (samA-; apsA-) was reduced to 50% in comparison to wild-type conidia. Eighty percent of viable spores produced small size colonies that were temperature and benomyl-sensitive. samB mapped to chromosome VIII and was recessive. Viability of conidia from samB suppressor strains (apsA-; samB) was also affected but no small size colonies were observed. Both suppressors produced partial defects in sexual reproduction and both suppressed an apsA deletion mutation. In wild-type background the mutant loci affected hyphal growth rate (samA) or changed the colony morphology (samB) and inhibited sexual spore formation (samA and samB). Only subtle effects on conidiation were found. We conclude that both suppressor genes bypass the apsA function and are involved in microtubule-dependent processes.