Phenotypic variation in a carpenter bee which uses alternative nesting host-plants

The implications of host-mediated differentiation for insect diversification, under an ecological scenario, have broadly been documented in insects. The study of this phenomenon requires the identification of the traits which enable them to become specialized in the use of a given resource. Numerous reports have addressed these issues in phytophagous and parasitic insects which are characterized by host specialization. Contrastingly, research on host-mediated differentiation is scarce on wood-boring insects, despite the fact that these animals also depend in an extensive way on their nesting hosts. Carpenter bees are a group of wood-boring insects that possess traits that may be related to the use of diverse nesting resources. Among these traits, mandibles may play a critical role during nest construction because they are extensively used to excavate wood to construct galleries in which oviposition takes place, and cuticular chemicals may be influenced by larval rearing environment, thus highlighting the importance of nesting plants in host-mediated differentiation. Manuelia (Apidae, Xylocopinae), is a genus of carpenter bees which has been proposed as a relict taxon occurring predominately in Chile. Nest-mate recognition capacity mediated by cuticular compounds has been demonstrated in M. postica, a species which nests in three plant species, the alloctonous shrub Rubus ulmifolius (Rosaceae), the native bamboo Chusquea quila (Poaceae) and the native tree Aristotelia chilensis (Elaeocarpaceae). We have determined the morphological variation of mandibles by means of geometric morphometric analysis, and the variation of cuticular chemicals through analyses of gas chromatographic profiles, in M. postica females that construct their nest in the three mentioned hosts. Our main results show that females nesting in C. quila exhibit less variance in mandibular shape and size and in cuticular compounds than those that use R. ulmifolius. Moreover, a positive and significant linear selection on mandibular area in bees from C. quila was observed which was not detected in bees from R. ulmifolius. Considering that C. quila represents a selective force on mandible size, we hypothesized a high adaptive value of this trait, taking into account that M. postica also exhibits higher fitness values when nesting on this host with whom it shares a long-term evolutionary history.

This work was financially supported by CONICYT (Comisión Nacional de Ciencia y Tecnología, Chile) FONDECYT grant 11110075