Every embryo of an organism that reproduces sexually faces an early and consequential decision: which of its cell lineages will make up the mortal body, and which will become the germline, the lineage that eventually makes eggs or sperm and carries genetic information to the next generation. Get this wrong, and an organism can live but cannot reproduce and, evolutionarily, ceases to exist.
Like most embryos, fruit fly embryos begin as one mega-cell, the fertilized egg cell, full of the ingredients for future development. What's different in fly embryos is that not all cells form at the same time. Before most of the embryo has been subdivided into individual cells, a small group of cells that will eventually produce eggs or sperm has already begun to form. These primordial germ cells emerge at the back end of the embryo, where small buds rise from its surface and pinch free from the shared interior.
A new study from the lab of Whitehead Institute Director Ruth Lehmann reveals that this process depends on preparations made before the buds appear. The researchers found that a protein, aptly named Germ Cell-less (GCL), produced at the posterior, organizes a distinct region of the embryo's membrane so that the molecular machinery needed to separate the future germ cells can assemble.
The findings, published on July 22 in the Journal of Cell Biology , show how the local organization of lipids, the molecules that form cell membranes, helps to establish a boundary between the germline and the soma or body cells. This separation is essential for the animal's ability to produce offspring.
How that separation begins depends on location. As the embryo's nuclei travel outward to its surface, only those that reach the posterior go on to become germ cells, each sealing into a membrane and pinching away from the rest of the embryo as one of its first true cells. The germplasm, a specialized material the mother deposits at the posterior end, carries the factors that set these cells apart.
Previous work from the Lehmann lab showed that GCL is essential for this pinching-off. GCL directs the cell's protein-disposal machinery to recognize and destroy Torso, a receptor that transmits signals from the embryo's surface. Torso is best known for its role in the soma, where it triggers a signaling cascade switching on genes needed for head and tail development. Without GCL, Torso remains active at the posterior end, and germ cells usually fail to form. What Torso was doing to interfere with the process, however, remained unclear, since none of the hallmarks of its gene-activating pathways seemed involved.
To trace the signal downstream of Torso, the team, including lead author Mariyah Saiduddin, a graduate student in the Lehmann lab, used genetic experiments, live imaging, and optogenetics (a method that uses light to switch protein activity on or off with precise timing and spatial control). The results revealed a previously unknown and surprising role for Torso in early development: activating an enzyme called PI3K. That enzyme produces PIP3, a signaling lipid embedded in the embryo's membrane.
Collaborating with Whitehead's W.M. Keck Microscopy Innovation Center , the researchers developed image-analysis approaches to measure subtle changes in PIP3 signals in living embryos in real time.
In a normal embryo, PIP3 was abundant near the posterior end but excluded from the region where germ cells form. Without GCL, PIP3 spread into that region. Experimentally boosting PI3K activity sharply reduced germ cell formation, while dialing it down allowed extra cells to form. The lipid acted like a chemical switch: with PIP3 levels reduced by GCL's action, Myosin II, a motor protein that generates contractile force, assembled into a ring-like structure to pinch each bud coming off of the embryo membrane at its base, separating each developing germ cell from the rest of the embryo. With too much PIP3 on the membrane, the Myosin structure was unable to assemble, the buds flattened, and germ cells failed to form.
"What was surprising was that the membrane was already being prepared before the nuclei reached it," says Saiduddin. "We had thought of germ cell formation as beginning when the nuclei arrived at the surface, but GCL is acting earlier, preventing PIP3 from building up in advance."
This study shows that the boundary between soma and germline isn't drawn by turning genes on or off in the early embryo, but depends in part on locally shaping the chemistry of the cell membranes. In Drosophila, the antagonistic relationship between Torso and its destroyer, GCL, determines where in the membrane signaling molecules are positioned before the embryo even starts developing. This in turn provides constraints on where to exert mechanical force to round up and bud into a new cell, thereby helping to specify germ cell fate.
"By regulating lipid composition as soon as an egg is laid, boundaries are established that control cell fate," says Lehmann, who is also a professor of biology at MIT.
Because the signaling pathways involved here are conserved or present across the animal kingdom, the work offers broader clues about how local signaling cascades can organize membrane lipids and shape cell behavior and fate, beyond fruit flies.
The work also raises new questions about why the embryo uses more than one mechanism to protect germ cells from signals that would otherwise push them toward a somatic fate.
"This is a beautiful illustration of discovery science," Lehmann says. "We solve one puzzle only to realize that there is so much more we do not understand."
