Karyotype plasticity -- The genome of sexual organisms is thought to be constrained by the complicated orchestration of chromosome pairing and segregation during meiosis. However, many sexual fungi exhibit tremendous karyotypic variation within species, and often even within populations. As an extreme example, we have found in some lineages of Microbotryum that the autosome pairs are frequently of unequal size, yet this size variation is inherited in a normal Mendelian fashion. (Such size dimorphism for autosomes is sometime used as an indicator that the lineage is ameiotic, or apomictic, but Microbotryum is obligately meiotic each generation!) Current research is directed toward understanding the mechanisms that give rise to the chromosome size dimorphisms and how they are maintained in populations by natural selection.