For millions of couples suffering the heartache of infertility, every new shred of knowledge in the field serves to broaden the chances that they will yet hold a newborn in their arms. Three scientists have just added an amazing piece of information to what we know about human fertilization. The researchers, hailing from Leeds and Bradford universities have uncovered a kind of DNA signature unique to sperm which appears to act like a key in unlocking the fertility of the egg so as to create a living being.
Dr. David Miller, Dr. David Iles (University of Leeds), and Dr. Martin Brinkworth (University of Bradford) discovered that sperm write a DNA script that can only be read by an egg of the identical species. The egg must be able to read this "signature," in order to unlock its potential. In addition to enabling fertilization, the scientists believe this signature may also tell us how it is that a species creates its individual genetic identity.
According to Dr. Iles, "What we have discovered is a previously unrecognized DNA packaging 'signature' in mammalian sperm that may be essential for successful fertilization of the egg and development of the embryo. We think it may also be ancient in origin."
The scientists believe that this "lock and key" mechanism may also explain why some men who are in otherwise good health, produce sterile sperm. Dr. Miller comments, "Up until now, Doctors have struggled to understand idiopathic male infertility. Our latest research offers a plausible explanation for why some sperm malfunction or fail to function correctly."
In order to understand the implications of the DNA signature in male infertility, it helps to imagine the strand of DNA contained by the sperm cell as longer than a meter in length. In order to fit the entire DNA coding into the microscopic space that is the head of the sperm cell, the DNA must be tightly wound and packaged. However, the research produced at Leeds discovered that the DNA of humans and mice are not always packaged in the same manner. Some paternal DNA was discovered to be packaged in a much more compact manner. The scientists believe that when the DNA is packaged in a looser fashion it becomes more vulnerable to toxins that may affect its potential, for instance cigarette smoke or anti-cancer drugs. Dr. Brinkworth comments, "This might mean that anything capable of causing genetic damage to sperm could have particular significance for the development of the embryo."
But there are other implications. The researchers speculate that this same discovery may explain why breeding various species of the same type of animal often fail. If the "key" refuses to fit into the "lock" no offspring can result, no matter how similar a dachshund is to a fox terrier, for instance. Better yet, think of horses and donkeys. They can produce offspring, but the sperm and egg signatures don't fit well so that embryonic development cannot proceed in any normal manner. This is why their offspring are with rare exceptions, infertile.
In terms of evolution, the new theory can help to explain why Neanderthals and humans coexisted without producing a common species. At least, no DNA evidence exists to support any offspring that may have resulted from a coupling between the two species. If any offspring survived, they may not have lived long, or may have been infertile.