Extracellular Recognition 831 The a Locus

The fusion of sporidia is controlled by two unlinked mating type loci, designated as a and b. Compatible mates have different alleles of genes at both loci. The a mating type locus has two "alleles," a1 and a2, that are structurally quite dissimilar even though they are at the same position in the chromosome. This unusual situation of dissimilar mating type genes being present at the same locus in the chromosome is distinguished from the term "allele" by the term idiomorph. The idiomorphs are responsible for extracellular cell-cell recognition and the fusion of sporidia (Figure 8.4).

To understand the mating process at molecular level, Regine Kahmann's group cloned and determined the DNA sequence of a and b mating type loci and performed functional assays based on transformation.

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