Natvig 96-03902 Unlike N. crassa, which is heterothallic and produces haploid ascospores of two separate mating types, N. tetrasperma is self fertile as a consequence of each ascopore receiving one nucleus of each mating type. This reproductive system has been termed pseudohomothallism, because occasional single mating-type spores are self sterile and heterothallic, suggesting the potential for outcrossing in nature. At one level, the mechanism of pseudohomothallism in N. tetrasperma appears quite simple. An overlap of spindles at the second meiotic division, together with subsequent programmed nuclear movements, leads to four heterokaryotic ascospores per ascus, instead of the eight typical of N. crassa and other truly heterothallic species. Despite this deceptively simple mechanism for creating self-fertile ascospores, it is now clear that pseudohomothallism in N. tetrasperma has important and complex implications. One recent study revealed that laboratory outcrossing with geographically diverse strains of N. tetrasperma results in high levels of sexual dysfunction. Another demonstrated that recombination between N. tetrasperma mating-type chromosomes is suppressed relative to that observed for truly heterothallic species, such as N. crassa; and it demonstrated that the mating-type chromosomes and autosomes of N. tetrasperma reflect very different evolutionary histories. Paradoxically, suppressed recombination on the mating-type chromosome, coupled with the unique sexual developmental pathway of N. tetrasperma, has produced wild-type strains with high levels of heteroallelism on the mating-type chromoqome, but with virtually complete homoallelism for sequences on autosomes. The results of these two studies appear to be very much interrelated. For example, one aspect of sexual dysfunction seems to involve protoplasmic incompatibility brought about by heteroallelism at certain loci. The high levels of homoallelism observed for autosomal sequences in wild type strains may reflect an avoidance of this incompatibility. This research will investigate (1) the genetic bases of sexual dysfunction and (2) the mechanism and biological significance of suppressed recombination on the mating-type chromosome. Our study focuses on several fundamental aspects of developmental and life-cycle biology, and it therefore has broad implications for reproductive genetics in higher organisms. This study will go far toward developing N. tetrasperma as an experimental organism for future genetic studies.
