Symbiosis in bed bugs, Cimex sp. (Hemiptera: Cimicidae)

Abstract

Bed bugs (Hemiptera: Cimidicae) were mainly forgotten for over half a century but have recently become one of the most intensively studied insect pests in the world, prompted by the difficulty in preventing and effectively treating infestations largely as a consequence of insecticide resistance. The common bed bug Cimex lectularius harbours multiple symbiotic partners, at least one of which has been implicated in the supplementation of the vitamin-deficient blood diet. The focus of this thesis was the symbiosis between the bed bug and its primary and secondary endosymbiotic bacteria with an aim to better understand the intimate relationship between host and symbiont. In the first experimental chapter (Chapter 2), field samples of bed bugs were collected from residential buildings in Jeddah City, Saudi Arabia and examined for the presence of endosymbionts using diagnostic PCR. The bed bugs were identified as Cimex hemipterus and all samples contained the primary endosymbiont Wolbachia and the secondary endosymbiont BLS. A third endosymbiont torix-Rickettsia that has recently been described from some bed bug populations was not detected. Phylogenetic analysis revealed that the Wolbachia belonged to Wolbachia supergroup F in common with other Wolbachia strains detected in Cimex lectularius, but not in C. hemipterus. In the second experimental chapter (Chapter 3), the population dynamics of the cohabiting symbionts Wolbachia and BLS were explored in a laboratory colony of C. lectularius using quantitative PCR. Insects were reared on the same food supply throughout the study and all samples were generated from the same initial starting cohort of insects. The bacterial load of Wolbachia and BLS diverged over bed bug development stages, egg, nymphs and adults, indicating different mechanisms of insect control over the two symbionts, and the bacterial load of Wolbachia in particular was elevated in females allowed to mate, but conversely it decreased during periods of starvation. The various experimental manipulations had the greatest impact on the bacterial load of the primary symbiont Wolbachia, whereas the bacterial load of BLS was more consistent across treatments. Finally, in the third experimental chapter (Chapter 4) the salivary gland proteome of insects that had either been starved or had been fed antibiotics to eliminate Wolbachia was explored using label-free quantitative mass spectrometry. Significantly different abundant proteins were observed in both starved and symbiont-free insects that clearly separated them from the control groups, with proteins related to blood feeding such as nitrophorin and apyrase maintained in both starved and antibiotic-treated insects indicative of their importance in processing the blood meal. The Wolbachia-derived protein chaperone DnaK was differentially abundant in the salivary glands from control insects, whereas in starved insects the protein LD-carboxypeptidase, derived from the facultative endosymbiont BLS, was differentially abundant potentially indicating a novel function for this symbiont. The final chapter (Chapter 5) discusses these findings in relation to the limited knowledge regarding host-symbiont interactions in bed bugs, and the potential implications for exploiting the symbiosis as a novel method of insect control.