The adult Drosophila testis lacks a mechanism to replenish missing niche cells

The adult Drosophila testis contains a well-defined niche created by a cluster of hub cells which secrete signals that maintain adjacent germline stem cells (GSCs) and somatic cyst stem cells (CySCs). Hub cells are normally quiescent in adult flies but can exit quiescence, delaminate from the hub, and convert into CySCs after ablation of all CySCs. The opposite event, CySC conversion into hub cells, was proposed to occur under physiological conditions, but the frequency of this event is debated. Here, to further probe the question of whether or not hub cells can be regenerated, we developed methods to genetically ablate some or all hub cells. Surprisingly, when flies were allowed to recover from ablation, the missing hub cells were not replaced. Hub cells did not exit quiescence after partial ablation of hub cells, and labeled cells from outside the hub did not enter the hub during or after ablation. Despite its ability to exit quiescence in response to CySC ablation, we conclude that the hub in the adult Drosophila testis does not have a mechanism to replenish missing hub cells.