Evidence of Environmental and Vertical Transmission of Burkholderia Symbionts in the Oriental Chinch Bug, Cavelerius saccharivorus (Heteroptera: Blissidae)

ABSTRACTThe vertical transmission of symbiotic microorganisms is omnipresent in insects, while the evolutionary process remains totally unclear. The oriental chinch bug,Cavelerius saccharivorus(Heteroptera: Blissidae), is a serious sugarcane pest, in which symbiotic bacteria densely populate the lumen of the numerous tubule-like midgut crypts that the chinch bug develops. Cloning and sequence analyses of the 16S rRNA genes revealed that the crypts were dominated by a specific group of bacteria belonging to the genusBurkholderiaof theBetaproteobacteria. TheBurkholderiasequences were distributed into three distinct clades: theBurkholderia cepaciacomplex (BCC), the plant-associated beneficial and environmental (PBE) group, and the stinkbug-associated beneficial and environmental group (SBE). Diagnostic PCR revealed that only one of the three groups ofBurkholderiawas present in ∼89% of the chinch bug field populations tested, while infections with multipleBurkholderiagroups within one insect were observed in only ∼10%. Deep sequencing of the 16S rRNA gene confirmed that theBurkholderiabacteria specifically colonized the crypts and were dominated by one of threeBurkholderiagroups. The lack of phylogenetic congruence between the symbiont and the host population strongly suggested host-symbiont promiscuity, which is probably caused by environmental acquisition of the symbionts by some hosts. Meanwhile, inspections of eggs and hatchlings by diagnostic PCR and egg surface sterilization demonstrated that almost 30% of the hatchlings vertically acquire symbioticBurkholderiavia symbiont-contaminated egg surfaces. The mixed strategy of symbiont transmission found in the oriental chinch bug might be an intermediate stage in evolution from environmental acquisition to strict vertical transmission in insects.