Nitric oxide radicals are emitted by wasp eggs to kill mold fungi

Detrimental microbes caused the evolution of a great diversity of antimicrobial defenses in plants and animals. Here we show that the eggs of a solitary digger wasp, the European beewolf Philanthus triangulum, emit large amounts of the gaseous free radical nitric oxide (NO{dot}) to protect themselves and their provisions, paralyzed honeybees, against mold fungi. Despite the extraordinary concentrations of nitrogen radicals (NO{dot} and its oxidation product NO2{dot}) in the brood cells (~1500ppm), NO{dot} is synthesized from L-arginine by an NO-synthase (NOS) as in other animals. The beewolf NOS gene revealed no conspicuous differences to related species. However, due to alternative splicing, the NOS-mRNA in beewolf eggs lacks a 144bp exon near the regulatory domain. This preventive external application of high doses of NO{dot} by seemingly defenseless wasp eggs represents an evolutionary key innovation that adds a remarkable novel facet to the array of functions of the important biological effector NO{dot}.