Mechanical reproductive isolation via divergent genital morphology betweenCarabus insulicolaandC. esakiiwith implications in species coexistence

Abstract

AbstractThe mechanical isolation hypothesis predicts that physical incompatibility between divergent reproductive morphologies hinders hybridization between populations. However, evidence for this hypothesis remains scarce. We examined this hypothesis using two parapatric carabid beetles,Carabus insulicolaandC.esakii, which are of the subgenusOhomopterusand exhibit a species‐specific genital lock‐and‐key system. Our interspecific crossing experiment revealed that incompatibility of genital morphologies served as a strong postmating‐prezygotic isolation barrier. This isolation was asymmetric: a decrease in female fitness was more costly in the cross with greater genitalic incompatibility between aC.esakiifemale and aC.insulicolamale. These two species share a limited sympatric area, but the mechanism responsible for their coexistence is unclear given no evidence of premating isolation via male mate choice. A comparison of the present results with those of previous studies that quantified reproductive isolation betweenOhomopterusspecies suggest that strong mechanical isolation via genitalic incompatibility plays a major role in species isolation, but that it may be less important in species coexistence.