What is the basis of pederin polymorphism in Paederus riparius rove beetles? The endosymbiotic hypothesis

Paederus riparius (Coleoptera:Staphylinidae) females are polymorphic with regard to their ability to synthesize the hemolymph toxin pederin, a character confined to that sex: (+)‐females endow their eggs with pederin whereas (−)‐females lay eggs devoid of the substance. When reared without access to conspecifics, (+)‐females can only be obtained from (+)‐mothers. Ingestion of (+)‐eggs during larval development, however, enables the offspring of (−)‐females to also become (+)‐females. This is only the case if untreated (+)‐eggs are eaten, while sterilized (+)‐eggs do not alter larval fate as (−)‐females. The character can thus be transferred to unrelated specimens. These experiments suggest that microorganisms might be involved in the biosynthesis of this unusual compound. Pederin appears to be the first defensive substance in insects traceable to endosymbionts.