Social behavior, ovarian status and juvenile hormone titer in a nest-sharing casteless wasp, Zethus miniatus (Vespidae, Eumeninae)
Klaus Hartfelder, Hans C. Kelstrup , Mary Jane West-Eberhard , Klaus Hartfelder , Fábio Santos do Nascimento , Lynn M. Riddiford
Faculdade de Medicina de Ribeirão Preto, Universidade de São Paulo, Ribeirão Preto, SP, Brazil ; Janelia Research Campus, Howard Hughes Medical Institute, Ashburn, VA, USA ; Smithsonian Tropical Research Institute and Louisiana State University, c/o Escuela de Biología, Universidad de Costa Rica, Costa Rica ; Faculdade de Medicina de Ribeirão Preto, Universidade de São Paulo, Ribeirão Preto, SP, Brazil ; Departamento de Biologia da Faculdade de Filosofia, Ciȇncias e Letras de Ribeirão Preto, Universidade de São Paulo, Ribeirão Preto, SP, Brazil ; Janelia Research Campus, Howard Hughes Medical Institute, Ashburn, VA, USA
The nest-sharing progressively provisioning wasp Zethus miniatus, a member of the primarily solitary subfamily Eumeninae, is considered a key species in the efforts to understand how a reproductive division of labor emerged from solitary antecedents. This is the first study of any eumenine wasp to combine behavioral observations with studies of ovarian status and juvenile hormone (JH) titer. Consistent with the prevailing hypothesis on caste origins, aggressive reproductives seeking to reuse or usurp brood cells (a hypothesized queen-like phase) possessed longer oocytes than provisioners of young larva (a hypothesized worker-like phase). Brood killings by reproductive cell-seekers were more common when the female/open-brood-cell ratio of the nest was high – an indicator of strong competition – which positively correlated with JH titer. Females lacking brood (e.g., victims of cell usurpation attempts) were observed to perform tasks relating to nest defense, nest maintenance and/or adoption. Compared to other wasps, the ovarian growth of Z. miniatus is intermediate between that of a progressive provisioning solitary female able to rear one larva at a time, and that of a female with even more increased oviposition opportunities as in eusocial queens. We hypothesize that competition for brood cells may have shaped the reproductive physiology of this group-living species, with JH playing a context-dependent role in reproduction and behavior. These and other data on developmental mechanisms support a competition-mediated, but not a diapause-mediated origin for workers in social wasps.