A fascinating eukaryotic exception is the white-throated sparrow, which functionally has four genders in an equilibrium where the tan-striped males mostly mate with white-striped females and vice versa. (I first read about it in Joan Strassmann’s book Slow Birding; the Wikipedia page for White-Throated Sparrow also has some introductory info. It seems to involve a chromosomal inversion.)
A fascinating eukaryotic exception is the white-throated sparrow, which functionally has four genders in an equilibrium where the tan-striped males mostly mate with white-striped females and vice versa. (I first read about it in Joan Strassmann’s book Slow Birding; the Wikipedia page for White-Throated Sparrow also has some introductory info. It seems to involve a chromosomal inversion.)