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 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 this comes down to the timing of different events, which is in turn “a consequence of the whole thing being a production line. Because the previous cyst has already signaled, the response is biased to the side that is naïve.”
This timing mechanism would overcome the possibly symmetrical nature of the Delta signal. Delta establishes the identity of both anterior and posterior polar cells. But by the time the Delta arrives, a polarization signal has already passed through the precursors of the posterior polar cells, perhaps altering them. And the posterior cells were also born earlier (at the same time as the anterior polar cells of the older cyst). One or both of those differences might ensure that the polarization signal always travels from posterior to anterior, and never in reverse. ▪