GENITAL LOCK-AND-KEY AS A SELECTIVE AGENT AGAINST HYBRIDIZATION

Area of Science:

• Evolutionary Biology
• Behavioral Ecology
• Speciation

Background:

• Closely related species often maintain reproductive isolation through prezygotic barriers.
• Genital morphology (the "lock-and-key" mechanism) is a proposed prezygotic isolation factor in many animal groups.
• Hybrid zones offer opportunities to study the strength and mechanisms of reproductive isolation.

Purpose of the Study:

• To experimentally investigate the direct costs of interspecific copulation in two closely related carabid beetle species.
• To assess the role of genital "lock-and-key" mechanisms in preventing hybridization.
• To quantify the fitness consequences of interspecific mating for both sexes.

Main Methods:

• Experimental interspecific and intraspecific mating trials were conducted with *Carabus (Ohomopterus) maiyasanus* and *C. (O.) iwawakianus*.
• Female mortality, fertilization rates, and male genital damage were recorded.
• Fitness costs were estimated based on the reduction in viable offspring.

Main Results:

• Males attempted copulation indiscriminately with conspecific and heterospecific females.
• Heterospecific mating caused female mortality (vaginal rupture) and significantly lower fertilization rates.
• Males of *C. maiyasanus* experienced genital damage after interspecific copulations, potentially impairing future mating success.

Conclusions:

• Genital incompatibility imposes substantial direct fitness costs, including mortality and reduced reproductive success, on interspecific matings.
• The "lock-and-key" mechanism acts as a strong selective force against hybridization in this carabid beetle hybrid zone.
• These findings highlight the role of direct mating costs in maintaining species boundaries.