A germline-specific reporter reveals conserved primordial germ cell developmental dynamics between jawless and jawed fish species

Every living animal carries a secret lineage within its body, a tiny group of cells set aside at the very beginning of life to become the next generation. These are the primordial germ cells, the biological ancestors of sperm and eggs. While the rest of an animal's body is built from cells that divide and specialize to form skin, muscle, or bone, these germ cells remain distinct, tasked with the singular job of passing genetic information forward. For decades, scientists have studied how these cells form and move in animals like mice and zebrafish, uncovering the rules that govern reproduction in jawed vertebrates. However, a massive gap in our knowledge remained: we knew very little about how these cells work in the most ancient branch of the vertebrate family tree, the jawless fish. Without the ability to see these cells clearly in these ancient creatures, researchers could not determine if the rules of reproduction were invented once, long ago, or if they evolved separately in different groups of animals.

To solve this puzzle, a team of researchers at Michigan State University turned their attention to the sea lamprey, a jawless fish that has existed for hundreds of millions of years. The challenge was that scientists had no way to track the germ cells inside a lamprey embryo; they were invisible to the naked eye and difficult to distinguish from other cells. The researchers needed a tool to light up these specific cells so they could watch them grow and move in real time. They designed a genetic switch based on a gene called piwil1, which is known to be active only in germ cells across many different species. By attaching a gene for a green fluorescent protein to this switch, they created a biological beacon. When this beacon was turned on, it would glow green, but only in the cells destined to become sperm or eggs.

The team first tested this new tool in zebrafish, a well-studied animal, to ensure it worked correctly. They injected the genetic construct into fertilized zebrafish eggs and watched the embryos develop. The results were immediate and clear: the green light appeared exactly where it was supposed to, marking the primordial germ cells as they formed, migrated through the body, and settled into the developing gonads. The cells glowed green from the earliest stages of life all the way into adulthood, proving the tool was stable and reliable. Crucially, the injection did not harm the embryos; they survived and developed just as well as those that had not been injected. This success gave the researchers the confidence to use the same tool in the sea lamprey, an organism that had never before been equipped with such a specific genetic marker.

When the researchers injected the glowing construct into sea lamprey embryos, the result was a breakthrough in understanding ancient biology. The green light appeared in the lamprey embryos, marking the primordial germ cells just as it had in the zebrafish. The cells followed a similar path, moving through the body and eventually gathering in the region where the gonads would form. This observation provided strong evidence that the genetic instructions for identifying and maintaining these reproductive cells are deeply conserved. Despite the sea lamprey and the zebrafish having diverged from a common ancestor more than 500 million years ago, they both use the same molecular signals to find and protect their germ cells. The study suggests that the fundamental machinery for building the next generation is an ancient inheritance shared by all vertebrates, rather than a trait that evolved independently in jawed fish.

Beyond simply tracking the cells, the researchers used this new glowing tool to investigate a question about how sex is determined in fish. In zebrafish, it is known that the number of germ cells an embryo has early in life can influence whether it becomes male or female. To see if this rule applied to the lamprey, the team counted the glowing cells in zebrafish embryos and followed them to adulthood. They found that embryos with a higher number of germ cells tended to develop into females, while those with fewer cells tended to become males. This confirmed that the tool could accurately measure cell abundance and link it to sexual outcome. While the study focused primarily on validating the tool in lampreys, the ability to count these cells in real time opens the door for future research to see if this same relationship between cell number and sex exists in the jawless fish.

The development of this glowing reporter system is more than just a technical achievement; it provides a window into the evolutionary history of reproduction. Before this work, scientists could only guess how germ cells behaved in jawless fish because they lacked the tools to see them directly. Now, with a method that allows them to visualize these cells throughout the entire life of the animal, researchers can finally compare the developmental journeys of ancient and modern fish side by side. The study confirms that the regulatory mechanisms controlling the germline are remarkably stable over deep time, surviving hundreds of millions of years of evolution. This new capability not only deepens our understanding of how vertebrates reproduce but also offers a potential path for managing invasive lamprey populations in the Great Lakes, where scientists hope to use similar genetic tools to control their numbers. The work stands as a testament to how a simple, glowing marker can illuminate the hidden history of life on Earth.