Beyond the fly: fusome conservation and its implications for germ cell biology

The survival of sexually reproducing species depends on the production of competent germ cells. Before meiotic entry, the expansion of specialized totipotent cells is essential for establishing a sufficient germ cell pool during gametogenesis. Growing evidence suggests that the mechanisms guiding the earliest stages of germ cell development are evolutionarily conserved across invertebrate and vertebrate species. A defining feature of germ cell maturation is the formation of germline cysts, interconnected groups of germ cells that undergo mitotic divisions to expand the gamete progenitor pool. A second characteristic now emerging is the formation of a specialized, yet still enigmatic organelle known as the fusome. First characterized in insects and more recently identified in vertebrates, the fusome represents an underexplored component of germline development. Here, we review recent advances in fusome biology with particular focus on insights from female Drosophila, where its structure and function have been most extensively characterized.