The First Rejection: Eggs Sequester Bits Of Sperm For Later E...( segment persisted throughout the develo...)

Karr and Pitnick decided to look more closely at this stubborn section of sperm tail. Using electron microscopy, they determined that it was composed of what is called the mitochondrial derivative a crystallized form of the sperm's mitochondria. Mitochondria are normally inherited only from the mother, and provide energy for the cells. Because mitochondria have their own DNA, and replicate separately from the rest of the cell, they can pick up hazardous mutations just as the DNA of a cell's nucleus can. If mitochondria were inherited both maternally and paternally, the cell would have two genetically different sets of mitochondria, each of which could pick up mutations. By having only one genetic type of mitochondria inherited from the mother, the cell avoids the possibility of accumulating too many mutations in the mitochondria.

The fact that the entire sperm, including the mitochondrial derivative (which makes up about 50 percent of the tail) enters the egg means that a substantial amount of the father's mitochondrial DNA is present in the developing zygote. The researchers suspect that the egg sequesters the mitochondrial derivatives to the midgut as an active mechanism to avoid the inheritance of the father's mitochondrial DNA.

"Breaking down the derivative would require considerable amounts of energy," says Karr, "Sequestering it to the midgut where it will be eliminated is much easier." This mechanism works no matter how long the sperm's tail (and therefore the amount of mitochondrial derivative that can be packed into it) gets. The egg still rejects the paternal mitochondrial DNA by sequestering it to the developing fly's gut for future elimination.

This work indicates that "previously unrecognized sperm-egg interactions are important to the success of a developing zygote," says Karr. One intriguing possibility, he suggests, is that these interactions, which have been obs
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Contact: Sharon Parmet
sparmet@mcis.bsd.uchicago.edu
773-702-6241
University of Chicago Medical Center
4-May-1998