Passing along polarity

Fly ovarioles spit out oocyte-containing cysts “like a sausage machine,” says Daniel St. Johnston. In the process, the cysts are endowed with a clearly defined anterior-posterior polarity axis based on the posterior positioning of the oocyte. “The [axis] is hardwired into the geometry of the ovary,” says St. Johnston. “What we're describing is the mechanism that transmits this.”

That mechanism is based on a relay of axis information from older to newer cysts, according to Isabel Torres, Hernán López-Schier and St. Johnston (University of Cambridge, Cambridge, UK). They started with the observation that Delta in the germline cyst cells signals to Notch in the surrounding follicle cells, thus inducing the formation of specialized polar cells at each end of the cyst. Earlier work had shown that the oocyte sticks to...