Supergenes can also complicate the process of mating. In some species, supergenes create a breeding system that in effect has four sexes. Because of a supergene in the North American birds called white-throated sparrows, for example, there are two “morphs” with dissimilar coloration and behaviors. Not only do males have to find females, but they must find a partner from the opposing morph. Otherwise, offspring will die either from inheriting supergenes from both parents or from inheriting none. Only chicks that receive a “balanced lethal” inheritance of one supergene and one ordinary segment of chromosome survive.
White Throats were everywhere when I was young. Now it is much rarer to hear their sweet little song.
My wife and I were just recently discussing the phenomenon of black-and-white individuals always pairing with brown-and-tan in WTSP (well, preferentially, with a strong bias), but I couldn’t explain the mechanism for this segregation. Now I know…