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

## TL;DR

The Drosophila proventriculus maintains tissue function without stem cells by using endocycling to grow cells as others are lost.

## Contribution

This study reveals that endocycling, not stem cells, compensates for cell loss in the Drosophila proventriculus.

## Key findings

- Cycling cells in the proventriculus do not act as stem cells but undergo endocycling.
- Endocycling increases cell ploidy and size, compensating for age-related cell loss.
- Endocycling supports peritrophic membrane synthesis and protects against oral pathogens.

## Abstract

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 endocycle throughout the fly’s life, increasing their ploidy and size, while at the same time cells in this tissue are lost into the gut lumen as the fly ages. Functionally, these cells play a critical role in the synthesis of peritrophic membrane components, and we show that when their endocycling is experimentally increased or decreased, there is a concomitant change in ploidy, tissue size, and peritrophic membrane synthesis. Further, we show that inhibition of endocycling makes flies more susceptible to orally infectious bacteria. Altogether, we show that continual endocycling of these cells is critical for maintaining tissue size and function in the face of cell loss due to aging or tissue damage.

This study demonstrates that the Drosophila proventriculus, the structure at the anterior of gut that synthesizes the peritrophic membrane, lacks stem cells but undergoes continual endocycling to compensate for cells loss during aging or damage.