A nucleus-encoded dynamin-like protein controls endosymbiont division in the trypanosomatid .

is a trypanosomatid of the Strigomonadinae. All members of this subfamily contain a single β-proteobacterial endosymbiont. Intriguingly, cell cycles of host and endosymbiont are synchronized. The molecular mechanisms underlying this notable level of integration are unknown. Previously, we identified a nucleus-encoded dynamin-like protein, called ETP9, that localizes at the endosymbiont division site of . Here, we found by comparative genomics that endosymbionts throughout the Strigomonadinae lost the capacity to autonomously form a division septum. We describe the cell cycle-dependent subcellular localization of ETP9 that follows accumulation of the bacterium-encoded division protein FtsZ at the endosymbiont division site. Furthermore, we found that ETP9 is essential in symbiotic but dispensable in aposymbiotic that lost the endosymbiont. In the symbiotic strain, ETP9 knockdowns resulted in filamentous, division-impaired endosymbionts. Our work unveiled that in an endosymbiont division machinery of dual genetic origin evolved in which a neo-functionalized host protein compensates for losses of endosymbiont division genes.