in gene pan-2. This gene is necessary for the synthesis of pantothenic acid (vitamin B5), and mutants defective in this gene can be experimentally identified by their requirement for pantothenic acid in their growth medium. The two pan-2 mutations B5 and B3 are located at different sites in the pan-2 gene, so that a cross of B5 ´ B3 yields wild-type recombinants at low frequency. An analysis of 939 asci in which the genotypes of all meiotic products (ascospores) could be determined found 11 asci with an exceptional segregation pattern. These included six asci in which there was one wild-type meiotic product but no expected reciprocal double-mutant (B5B3) product. Furthermore, in three asci the ratio of meiotic products was 1B5:3B3, rather than in the expected 2:2 ratio. This study, as well as numerous additional studies in N. crassa and other fungi (reviewed by Whitehouse), led to an extensive characterization of gene conversion. It became clear from this work that gene conversion events arise when a molecular recombination event happens to occur near the genetic markers under study (e.g. pan-2 mutations in the above example). Thus studies of gene conversion allowed insight into the details of the molecular mechanism of recombination. Over the decades since the original observations of Mary Mitchell in 1955, a sequence of molecular models of recombination have been proposed based on both emerging genetic data from gene conversion studies and studies of the reaction capabilities of DNA. Current understanding of the molecular mechanism of recombination is discussed in the Wikipedia articles Gene conversion and Genetic recombination. An understanding of recombination is relevant to several fundamental biologic problems, such the role of recombination and recombinational repair in cancer (see BRCA1) and the adaptive function of meiosis (see Meiosis). Adaptive function of mating type That mating in N. crassa can only occur between strains of different mating type suggests that some degree of outcrossing is favored by natural selection. In haploid multicellular fungi, such as N. crassa, meiosis occurring in the brief diploid stage is one of their most complex processes. The haploid multicellular vegetative stage, although physically much larger than the diploid stage, characteristically has a simple modular construction with little differentiation. In N. crassa, recessive mutations affecting the diploid stage of the life cycle are quite frequent in natural populations. These mutations, when homozygous in the diploid stage, often cause spores to have maturation defects or to