Together forever: patterns and strategies of vertical symbiont transmission in planthoppers

Insects host a wide variety of microbial symbionts, including bacteria and fungi, which contribute to their survival by supplementing essential nutrients and aiding in adaptation to diverse ecological niches. The mode of symbiont transmission between generations, particularly vertical transmission, plays a central role in maintaining these mutualistic relationships. In this review, we focus on the transmission mechanisms of symbionts in planthoppers (Fulgoromorpha), a group of sap-feeding insects that rely heavily on obligate, nutritional symbionts for amino acid biosynthesis. These symbionts are transmitted through the ovary, with ancestral and newly acquired symbionts utilizing distinct strategies for vertical transmission. Here, we describe the conservative transmission mode for the ancient Sulcia and Vidania symbionts and show how more recent associates, including Sodalis and Acetobacteraceae, may adopt varied transmission routes. Moreover, we discuss the transmission strategies of facultative symbionts like Wolbachia and Rickettsia, illustrating the diversity of transmission pathways across different insect species. Lastly, we discuss the evolutionary consequences of long-term, strictly vertical symbiont transmission. This review synthesizes current knowledge on symbiont inheritance in planthoppers and identifies key areas for future research on insect-microbe symbioses.