Exploring sex differences in bacterial composition in cantharidin-producing blister beetles (Coleoptera: Meloidae)

Sex is an important factor affecting bacterial diversity in insects (Xu et al. 2016, J. Insect Phy, 95: 110–117). Divergence in some life-history traits between males and females could provide differential opportunities for bacterial colonization (Tang et al. 2012, FEMS Microb Ecol 80: 659–670). Blister beetles include almost 3000 species (Riccieri et al. 2022, Syst Ent 1-12) and are characterized by sexual dimorphism in body content of the toxic terpene cantharidin (CA). CA is produced and exuded by blister beetles in yellowish oily hemolymph droplets from leg and antennal joints as a defensive strategy (Gisondi et al. 2019, Entomol Sci 22: 258-263). Although the latest steps of CA production are still elusive (Fratini et al. 2021, BMC Genomics 22: 808), males are known to produce more CA than females and accumulate large amounts of this terpene in their reproductive organs (Carrel et al. 1993, Experientia 49: 171–174). Females are especially attracted by males with high amount of CA, which is transferred to females upon mating for protecting eggs from predators or parasites (Sierra et al. 1976, Experientia 32: 142-144). Through 16S metabarcoding we compared the bacterial composition between males and females in five species of blister beetles (Hycleus polymorphus, Mylabris variabilis, Lydus trimaculatus, Zonitis flava, Meloe proscarabeus). Gammaproteobacteria, Bacilli, Actinobacteria and Alphaproteobacteria were the most prevalent classes of bacteria in all species. Overall, we observed a sexual dimorphism in the microbial component in all examined species, with males harboring lower values of microbial alpha-diversity, as in other insects (Wan et al. 2020, Animals 10: 2352). Plausible roles of sex-specific microbes are hypothesized and, as in other terpene-producing beetles (i.e., Paederus fuscipennis, Song et al. 2022, Insect Mol Biol 1-14), the potential involvement of bacteria in the de novo biogenesis of CA in blister beetles is discussed