Common alleles of an immunity gene reduce plant compatibility.
Crosses between different plants can sometimes result in progeny that is compromised in growth and fertility, but at the same time resists pathogens more vigorously. Both are consequences of an autonomous activation of the immune system, associated with the hybrid necrosis syndrome, in which endogenous factors are accidentally recognized as non-self. Interactions between alleles of a single defense-related gene were found to be responsible for several hybrid necrosis cases in Arabidopsis thaliana. These interacting alleles co-exist and form hybrids in natural populations. See Todesco et al.
Image Credit: Marco Todesco and Detlef Weigel, Max Planck Institute for Developmental Biology