Too much of a good thing? Hybrid necrosis as a by-product of plant immune system diversification

Abstract

Plants defend themselves against their enemies with an impressive arsenal of physical barriers, surveillance and defense proteins, enzymes, and toxic chemicals. Many different molecules are involved in the detection of invaders, suggesting that pathogen pressure selects for a broad array of defense strategies and a high diversity of recognition specificities in host species. Recent results in plants, however, show that immune system diversification can also have negative consequences; epistatic interactions among divergent immune system components can cause hybrid necrosis, a form of genetic incompatibility. This type of hybrid failure is frequently lethal, and characterized by the widespread induction of programmed cell death leading to tissue necrosis. In characterized examples, this is caused by hyperactivation of defense responses. Both the prevalence of hybrid necrosis in diverse plant taxa, and the growing indication that it may arise as a by-product of adaptation to the biotic environment, emphasize that it is likely a general factor in plant evolution. Since hybrid necrosis negatively impacts the progeny of certain crosses, divergence of the plant immune system may indirectly affect gene flow among populations, and perhaps contribute to the establishment or maintenance of species barriers.