Self/Nonself Recognition in Fungi: Old Mysteries and Simple Solutions

to generate new alleles by meiotic recombination have failed (Puhalla, 1970). Genetic studies had revealed that Institut fü r Genetik und Mikrobiologie der Universitä t Mü nchen Maria-Ward-Str. 1a the b locus is the master control locus for pathogenicity and that the combination of two different b alleles in the 80638 Mü nchen, Germany same cell is absolutely required for the parasitic stage (see Banuett, 1992). Molecular analysis revealed that In fungi, sexual reproduction is governed by incompatibility genes which determine the pattern of mating the b locus contains two divergently transcribed genes (bE and bW) that encode homeodomain proteins (Schulz among individual strains. This occurs in the absence of morphological differences between mating partners. Except for the homeodomain the bE Incompatibility loci are genetically complex and often exist in multiple alleles. Recognition mediated by these and bW polypeptides lack significant amino acid sequence similarity. The allelic variants of the bE gene loci can occur extracellularly (to allow or prevent cell fusion) or intracellularly (to allow or prevent sexual devel-products are highly similar in their C-terminal domains, while the N-terminal domains contain multiple amino opment). These mating systems restrict inbreeding among members of the same race and promote out-acid substitutions. The same organization into constant and variable domains is also seen in the bW proteins. breeding with members of a different race. The phenomenon of multiallelic recognition was discovered more Deletion analysis revealed that the b gene products play an active role in triggering development. This regulatory than 70 years ago (Kniep, 1920) and has been puzzling researchers ever since. Recent breakthroughs towards activity is seen only when a situation is generated in which at least one bE gene and one bW gene from a understanding the molecular basis of these processes came from a combination of genetical and biochemical different allele are combined (Gillissen et al., 1992). The bE1-bW2 gene pair, for example, is as active as the bE2-bW1 gene pair while neither the bE1-bW1 nor the bE2-bW2 gene pairs are active. Furthermore all combinations The fascinating and unifying scheme emerging from of different bE genes or those of different bW genes are inactive. What is the difference between active and these studies is that recognition always involves linked genes arranged in pairs. During sexual reproduction the inactive gene pairs? Recent experiments have demonstrated that the gene products from an active gene pair products from one gene pair have …