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.”
Figure
Polar cells (red) pass the polarization signal from posterior (right) to anterior
(left).
St. Johnston/Elsevier
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. ▪
Reference:
Torres, I.L., et al. 2003. Dev. Cell. 5:547–558.14536057