The vitellogenesis pathway of the tick host Ixodes scapularis regulates Rickettsia buchneri transcriptional state and persistence

Abstract

Obligate endosymbionts relying on transovarial transmission must coordinate with host reproduction, yet the regulatory mechanisms remain poorly understood. Here we show that the vitellogenesis pathway of the tick Ixodes scapularis regulates the transcriptional state of its endosymbiont Rickettsia buchneri (Rb), separately from its abundance. In males, Rb DNA remained detectable, but bacterial transcription was strongly reduced across all examined genes, with markedly lower RNA/DNA ratios. In females, Rb was restricted to ovarian tissues (developing oocytes and interstitial cells), with no detection in salivary glands or midgut by TEM, FISH, or PCR. After blood feeding, Rb density within size-matched early-stage oocytes was significantly reduced. We further characterized two gene families mediating vitellogenesis: vitellogenin synthesis genes (Vgs, n = 20) and vitellogenin receptor genes (Vgr, n = 15). Vgs proteins showed conserved domain organization, whereas Vgr paralogs showed greater structural diversification. RNAi silencing of Vgs20 or Vgr12 altered Rb transcriptional profiles in both tick cells and ticks, although changes in bacterial load were not consistent between the two systems. Antibiotic depletion of Rb increased expression of both host genes. Together, these findings show that tick vitellogenesis pathways contribute to Rb regulation during reproduction and identify the I. scapularis-Rb system as a useful model for studying host control of obligate endosymbionts. IMPORTANCE Maternally inherited bacterial symbionts are commonly characterized by bacterial abundance. This study shows that bacterial abundance alone does not necessarily reflect symbiont functional state. In the blacklegged tick, the inherited symbiont Rickettsia buchneri persists in males but exhibits little transcriptional activity. In females, the host reproductive pathway determines whether the symbiont is active: silencing one component of this pathway changed bacterial gene expression without changing bacterial abundance. Hosts therefore regulate not only the abundance of inherited symbionts, but also their functional state. The same bacterial abundance can correspond to very different functional states. Understanding inherited symbioses therefore requires considering bacterial function alongside bacterial abundance.