Steady and escort cyst cells regulate Drosophila germline stem cell differentiation and death.

Somatic cells in both mammalian and Drosophila testes perform diverse roles in regulating germline stem cell differentiation into sperm. Beyond their supportive functions, such as encapsulation and signaling, somatic cells also act as tissue-resident, non-professional phagocytes. In Drosophila testis, somatic cyst cells eliminate approximately a quarter of newly emerged spermatogonial progenitors, a role seemingly contradictory to their supportive function. Here, we examined individual events in which cyst cells alternated between supporting germ cells and promoting their death, revealing distinct morphological features. Our data indicate that, in addition to well-defined cyst cells derived from stem cell divisions and escorting differentiating spermatogonia, a distinct population of long-lived steady cyst cells arises during larval development. These steady cyst cells persist at the apical tip of the testis for extended periods and engage in phagoptosis. This distinction separates cyst cells into two subpopulations based on function and morphology, highlighting how genetically similar cells adopt specialized roles depending on their developmental origin and timing.