| Summary: | The mating-type genes in Neurospora crassa, called "idiomorphs" and designated A and "a", control entry into the sexual cycle and aspects of meiosis. In heterothallic species, nuclei with unlike mating-types must come together before karyogamy and meiosis can occur. Homothallic species, however, can enter the sexual cycle spontaneously and without a mating event per se. Two homothallic classes exist in Neurospora, those with only the A-specific sequence (the majority) and those with both the A-specific and "a"-specific sequence (Neurospora terricola). Neurospora terricola and a number of homothallic Gelasinospora and Anixiella species are self-fertile but harbour both A and "a" idiomorphs in a single nucleus. Of what use are mating-types to organisms that do not mate? A molecular analysis of the mating-type loci of selected Sordariaceae homothallics has investigated the degree to which the homothallic A and "a" genes are similar to those of heterothallic and A-type homothallic species of Neurospora. Mapping of the N.terricola, Gelasinospora, and Anixiella mating-type loci has demonstrated that the majority of the N.crassa A and "a" sequences are conserved in these homothallics with the exception of approximately 1.1 kb of A absent in N.terricola and approximately 600bp of "a" absent in all homothallics examined. Further analysis was made of the sequences that flank the mating-type genes themselves to further define the locus. The sequence that in N.crassa flanks both A and "a" idiomorphs to the left is present in the genomes of all homothallic species examined. In most of the homothallics examined, this flank sequence is contiguous with the "a" idiomorph. A similar right flank sequence is absent in Sordariaceae homothallics. The ORF portions of the N.terricola A and "a" genes were amplified by PCR and sequenced. A sequence comparison of N.crassa and N.terricola A ORFs has demonstrated an average of 89% DNA identity. Functional analysis has shown that the N.terricola ORF A confers function (ability to mate with strains of opposite mating-type and ability to form perithecia) when transformed into N.crassa sterile mutant spheroplasts.
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