The Drosophila proventriculus lacks stem cells but compensates for age-related cell loss via endoreplication-mediated cell growth

The Drosophila proventriculus is a bulb-shaped structure at the juncture of the foregut and the midgut, which plays important roles in ingestion, peritrophic membrane synthesis, and the immune response to oral pathogens. A previous study identified a population of cells in the proventriculus which incorporate bromodeoxyuridine (BrdU), a marker of DNA synthesis, and proposed that these cycling cells are multipotent stem cells that replace dying cells elsewhere in the tissue. Here, we re-investigate these cycling cells and find that they do not undergo mitosis, do not generate clonal lineages, and do not proliferate in response to tissue damage, and are therefore not stem cells. Instead, we find that these cells continually endoreplicate throughout the fly’s life, increasing their ploidy and cell volume, while at the same time nearby cells are lost into the gut lumen in an age-dependent process. These cells play a critical role in the synthesis of peritrophic membrane components, and we show that when endocycling in these cells is experimentally increased or decreased, nuclear and cellular volume as well as peritrophic membrane synthesis is concomitantly enhanced or reduced. Further, we show that blocking endocycling in these cells leads to functional defects in the peritrophic membrane, ultimately leading to increased susceptibility to orally infectious bacteria. Altogether, we show that continual endocycling of these cells is critical for maintaining tissue size and function over the course of the adult lifespan.