Evolution of sex chromosome dimorphism in haploids -- While several very cogent mechanisms have been proposed for the evolution of sex chromosome dimorphism in diploids, there is little a priori reason to expect such dimorphism when sex determination is in the haploid stage of the life-cycle. However, the pair of chromosomes that determine haploid mating compatibility in Microbotryum are highly dimorphic for size (as is found in some mosses that also determing mating compatibility as haploids). Furthermore, we have shown that this dimorphic chromosome pair has much higher density of transposable elements than do the autosomes, and the two fungal sex chromosomes are highly different from each other in their densities of functional genes. All these traits are similar to sex chromosomes found in diploid mating animals and plants. Current research focuses on which evolutionary forces all of these organisms have in common that could account for their shared sex chromosomes characteristics.