Karyotype polymorphism correlates with intraspecific infertility in the homothallic ascomycete Sordaria macrospora

The homothallic fungus Sordaria macrospora produces perithecia with meiotic ally derived ascospores. In most cases, intraspecies crosses between strain s from different culture collections generate fertile hybrid perithecia in the contact zone of two mycelia. However, in some of these crosses we obser ved a significant decrease in the fertility of the hybrid perithecia when s trains of different origin were used for mating. Since we assumed that chro mosome variability between the culture collection strains might contribute to this reduction in fertility, we performed pulsed-field gel electrophores is. In the course of our study, we were able to identify two major groups o f electrophoretic karyotypes in S. macrospora culture collection strains. A quantitative analysis revealed that polymorphic karyotypes contribute to a reduction of fertility in forced crosses between strains carrying differen tly sized chromosomes. The observed intraspecific chromosome length polymor phism might have consequences on the speciation process of a homothallic fu ngus capable of sexual but not of asexual spore formation.