A sperm fast and lucky enough to fertilize an egg gets to pass on its genes. But now researchers have shown, for flies at least, that the champion meets an ignoble end: After hanging out in the embryo during development, the sperm's remnants are excreted when the larvae hatches. This finding, reported in the May Proceedings of the Royal Society London B, is the first for any species to track a successful sperm's ultimate fate.
A fly sperm consists of a tiny head containing its nucleus and a whiplike tail up to 20 times longer than the egg. During sperm development, energy-producing mitochondria clump and elongate down the tail and fuel the frantic race to the egg. Researchers have long known that the sperm delivers its genetic cargo and centriole, a structure that coordinates chromosome separation during the fertilized egg's division. But no one had bothered to follow what happened to the rest of the spent sperm. Tim Karr, a developmental biologist at the University of Chicago, and colleague Scott Pitnick, an evolutionary biologist at Syracuse University, decided to look at several species of Drosophila to find out.
To follow the sperm's fate, Karr and Pitnick hooked a fluorescent dye to a monoclonal antibody that latches onto proteins coating the sperm membrane. After fertilization between an egg and a sperm in a petri dish, the researchers spotted glowing patches in the egg's yolk. Hours later the glow showed up in the developing midgut. The sperm membrane and tail then disintegrated in the midgut, leaving behind mitochondria that the larvae excreted after hatching. "This is an orchestrated and highly regulated phenomenon that we know almost nothing about," Karr says.
Experts are impressed with the spermatic sleuthing. "The beauty of their work is that they are able to trace the fate of [the sperm's tail]," says Gerald Schatten, a zoologist at the University of Oregon. The finding could be an intriguing new avenue for developmental biologists, says Pitnick. He thinks fly embryos might sequester mitochondria to keep them from entering cells, since most plants and animals get their mitochondria only from the mother.