Portions of the tail of gigantic fruit fly sperm, which enters the egg entirely during fertilization, persist in the developing midgut of the embryo and are excreted soon after the larva hatches, reports Tim Karr, a developmental biologist at the University of Chicago, and Scott Pitnick, an evolutionary biologist at the University of Syracuse in the May 7 issue of Proceedings of the Royal Society London B.
Karr and Pitnick, who have been studying the often bizarre sperm-egg interactions of Drosophila for several years, previously noticed that the long tails of fruit fly sperm entered the egg during fertilization, undermining the idea that the only role of the sperm tail is to propel its genetic cargo to the egg.
"It was remarkable that the egg accepted so much spermic material, especially since the tails of some sperm are over twenty times the length of the egg!" says Karr. "It became clear that the tail might be involved in the embryo's early development."
Now, using fluorescence staining, Karr and Pitnick have tracked the fate of the giant sperm tails of two species of fruit fly, D. melanogaster and D. pachea, throughout embryogenesis. They noticed that after the sperm entered the egg, its tail was partially degraded by the egg's enzymes, but otherwise remained intact and was sequestered in the developing embryo's midgut. One section of the tail was observed to remain untouched by the egg's enzymes. This particular segment could be seen in the guts of recently hatched larvae, where it appeared as a tight coil in photographs where the sperm had been stained with a fluorescent dye.
But when Karr and Pitnick looked for these pieces after the larvae were allowed to feed for a few hours, they were no longer detectable. Apparently, the larvae excrete the piece with their waste, says Karr, "We were pretty surprised that this
Contact: Sharon Parmet
University of Chicago Medical Center