PLANTS. “Most mating-type loci have no such asymmetry, all alleles generally being heterozygous in the diploid parts of the life cycle; in plants with the distylous form of heterostyly, however, one incompatibility type is heterozygous while the other is homozygous, similarly to sex chromosomes.”
http://www.nature.com/hdy/journal/v95/n2/full/6800697a.html