CYTOPLASMIC GENES. “Species with two mating-types coordinate uniparental inheritance of cytoplasmic genes more efficiently than do those with three, then, assuming the costs to mate finding are not too high, evolution from three to two sexes is expected as a response to the invasion of a costly selfish cytoplasmic factor that disrupts the normal pattern of inheritance.”
http://rspb.royalsocietypublishing.org/content/263/1369/415.short