Passing along polarity

ly 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 LópezSchier 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 the posterior follicle cells because both cell types express high levels of the same cadherin. But, says St. Johnston, “there was one thing that was missing. How do the posterior follicle cells know that they are posterior?” This is where the relay comes in. The Cambridge team now reports that when only a single cyst lacks Delta, it is always fused to the more anterior (younger) cyst. And it is the younger cyst that fails to position its oocyte correctly. Thus the older cyst induces its anterior polar cells to induce the formation of stalk cells between two cysts, and then the stalk cells induce the cadherin expression that defines the posterior of the younger cyst. Somehow the relay is made unidirectional. St. Johnston believes F