THE LOGIC. “There exists also a limited domain in parameter space (in which costs to mate finding are moderate), in which collapse from four mating-types to three is possible, but the collapse from three to two is not. If costs to not finding a mate are high then it is expected that gametic fusion should be abandoned for a system in which nuclei are exchanged but cytoplasm is not.”
http://rspb.royalsocietypublishing.org/content/263/1369/415.short