Band-aids for Buchnera and B vitamins for all

Evolution lacks foresight, and hence, key adaptations may produce major challenges over the long run. The natural world is rife with examples of long-term ‘side effects’ associated with quick-fix tinkering, including blind spots in vertebrate eyes. An important question is how nature compensates for imperfections once evolution has set a course. The symbioses associated with sap-feed-ing insects present a fascinating opportunity to address this issue. On one hand, the substantial diversity and biomass of sap-feeding insects are largely due to ancient acquisitions of nutrient-provisioning bacterial symbionts. Yet, the insularity and small population sizes enforced by intracellular life and strict maternal transfer inevitably result in the degradation of symbiont genomes and, often, the beneficial services that symbionts provide. Sta-bilization through lateral transfer of bacterial genes into the host nucleus (often from exogenous sources) or replacement of the long-standing symbiont with a new partner are potential solutions to this evolutionary dilemma (Bennett & Moran 2015). A third solution is adoption of a cosymbiont that compensates for specific losses in the original resident. Ancient ‘co-obligate’ symbiont pairs in mealybugs, leafhoppers, cicadas and spit-tlebugs show colocalization, codiversification, metabolite exchange and generally nonredundant nutrient biosyn-thesis (Bennett & Moran 2015). But in this issue, Mese-guer et al. (2017) report on a different flavour of cosymbiosis among conifer-feeding Cinara aphids.