PSEUDOHOMOTHALLISM AND EVOLUTION OF THE MATING-TYPE CHROMOSOME IN NEUROSPORA-TETRASPERMA

Ascospores of Neurospora tetrasperma normally contain nuclei of bath m ating-type idiomorphs (a and A), resulting in self-fertile heterokaryo ns (a type of sexual reproduction termed pseudohomothallism). Occasion al homokaryotic self-sterile strains (either a or A) behave as heterot hallics and, in principle, provide N. tetrasperma with a means for fac ultative outcrossing. This study was conceived as an investigation of the population biology of N. tetrasperma to assess levels of intrastra in heterokaryosis (heterozygosity). The unexpected result was that the mating-type chromosome and autosomes exhibited very different pattern s of evolution, apparently because of suppressed recombination between mating-type chromosomes. Analysis of sequences on the mating-type chr omosomes of wild-collected self-fertile strains revealed high levels o f genetic variability between sibling A and a nuclei. In contrast, seq uences on autosomes of sibling A and a nuclei exhibited nearly complet e homogeneity. Conservation of distinct haplotype combinations on A an d a mating-type chromosomes in strains from diverse locations further suggested an absence of recombination over substantial periods of evol utionary time. The suppression of recombination on the N. tetrasperma mating-type chromosome, expected to ensure a high frequency of self fe rtility, presents an interesting parallel with, and possible model for studying aspects of, the evolution of mammalian sex chromosomes.