Suppressed recombination and a pairing anomaly on the mating-type chromosome of Neurospora tetrasperma

Neurospora crassa and related heterothallic ascomycetes produce eight homok aryotic self-sterile ascospores per ascus. In contrast, asci of N. tetraspe rma contain four sell-fertile ascospores each with nuclei of both mating ty pes (matA and mata). The self-fertile ascospores of N. tetrasperma result f rom first-division segregation of mating type and nuclear spindle overlap a t the second meiotic division and at a subsequent mitotic division. Recentl y, MERINO et al. presented population-genetic evidence that crossing over i s suppressed on the mating-type chromosome of N. tetrasperma, thereby preve nting second-division segregation of mating type and the formation of self- sterile ascospores. The present study experimentally confirmed suppressed c rossing over for a large segment of the mating-type chromosome by examining segregation of markers in crosses of wild strains. Surprisingly, our study also revealed a region on the far left arm where recombination is obligato ry. In cytological studies, we demonstrated that suppressed recombination c orrelates with an extensive unpaired region at pachytene. Taken together, t hese results suggest an unpaired region adjacent to one or more paired regi ons, analogous to the nonpairing and pseudoautosomal regions of animal sex chromosomes. The observed pairing and obligate crossover likely reflect mec hanisms to ensure chromosome disjunction.