The testis of a male fruit fly, just like that of male humans, is constantly making sperm throughout a fly’s lifetime. Testes can do this because they have a group of germline stem cells (or GSCs) that are constantly dividing asymmetrically to produce new copies of themselves (maintaining the GSC pool) and a progenitor gonioblast that will undergo a series of amplifying mitotic divisions before undergoing meiosis and differentiating into sperm, in a process called spermatogenesis. The GSCs in the fly testis (as well as the GSCs in the fly ovaries) have for long been a prime experimental model for studies of stem cells in vivo. They were first described and characterized many decades ago, and have been intensely studied throughout the years.
The fly testis contains another population of stem cells, called cyst stem cells (or CySCs), which divide asymmetrically to self-renew and generate progeny that will differentiate into cyst cells. When GSCs divide and give rise to a gonioblast, the GB becomes encapsulated by two cyst cells, which have to remain in contact with the GB and its progeny for spermatogenesis to proceed normally. Therefore, for every new GB that will commit to spermatogenesis, the testis needs to produce two cyst cells provided by CySCs. What makes the fly testis (and ovaries) a particularly interesting experimental model is that the divisions of GSCs and CySCs needs to be extremely tightly coordinated. Therefore, the fly testis not only provides two independent models to study the genetic regulation of stem cells in vivo (GSCs and CySCs), but also a unique model to investigate the coordination, communication and cross-regulation between two stem cell populations within a tissue.
Moreover, understanding the communication and inter-regulatory mechanisms between stem cells in vivo, may prove to be critical to understand the behavior of cancer cells within a tumor. It is thought that cancer cells share many molecular and regulatory mechanisms with normal stem cells, and a better understanding of how stem cells communicate with one another within organs under normal conditions may shed light on how cancer cells interact within a tumor.