Mating changes the genital microbiome in both sexes of the common bedbug Cimex lectularius across populations.

Many bacteria live on host surfaces, in cells and in specific organ systems. In comparison with gut microbiomes, the bacterial communities of reproductive organs (genital microbiomes) have received little attention. During mating, male and female genitalia interact and copulatory wounds occur, providing an entrance for sexually transmitted microbes. Besides being potentially harmful to the host, invading microbes might interact with resident genital microbes and affect immunity. Apart from the investigation of sexually transmitted symbionts, few studies have addressed how mating changes genital microbiomes. We dissected reproductive organs from virgin and mated common bedbugs, Cimex lectularius L., and sequenced their microbiomes to investigate composition and mating-induced changes. We show that mating changes the genital microbiomes, suggesting bacteria are sexually transmitted. Also, genital microbiomes varied between populations and the sexes. This provides evidence for local and sex-specific adaptation of bacteria and hosts, suggesting bacteria might play an important role in shaping the evolution of reproductive traits. Coadaptation of genital microbiomes and reproductive traits might further lead to reproductive isolation between populations, giving reproductive ecology an important role in speciation. Future studies should investigate the transmission dynamics between the sexes and populations to uncover potential reproductive barriers.

Female immunity in response to sexually transmitted opportunistic bacteria in the common bedbug Cimex lectularius.

Besides typical sexually transmitted microbes, even environmental, opportunistic microbes have been found in copulatory organs of insects and even humans. To date, only one study has experimentally investigated the sexual transmission of opportunistic microbes from male to female insects, whereas nothing is known about the transmission from females to males. Even if opportunistic microbes do not cause infection upon transmission, they might eventually become harmful if they multiply inside the female. While the immune system of females is often assumed to target sexually transmitted microbes, most studies ignore the role of mating-associated opportunistic microbes. Variation in immunity between populations has been linked to parasite or bacteria prevalence but no study has ever addressed between-population differences in immune responses to sexually transmitted opportunistic microbes. We here show that bacteria applied to the copulatory organs of common bedbugs, Cimex lectularius, are sexually transmitted to the opposite sex at a high rate, including the transmission from female to male. Bacterial growth in the female sperm-receiving organ was inhibited over the first hours after introduction, but after this initial inhibition bacterial numbers increased, suggesting a shift of investment from immune defence towards reproduction. However, 24 h after the injection of bacteria, male components, or saline as a control, the sperm-receiving organ showed lysozyme-like activity and inhibited the growth of Gram-negative and Gram-positive bacteria in vitro, potentially to mop up the remaining bacteria. Contrasting our prediction, neither bacterial growth nor immune responses differed between populations. Future studies should link transmission dynamics, immune responses and fitness effects in both sexes. Experimental manipulation of environmental bacteria could be used to investigate how transmission frequency and toxicity of sexually transmitted opportunistic microbes shapes bacteria clearance and immune responses across populations.