Sperm read each other’s DNA, so why not ova?
As a child I once amused myself with a harmless fantasy. I imagined finding a solid gold ball underwater. The ball was about 20 feet in diameter, so I would be unable to take it away; I could only go down and claw at it with my fingernails.
Sometimes I think this project is like that. It is too big even if it is valuable beyond belief. And I can’t really see it so clearly, much less get my fingernails into it. New information puts me in that frame of mind again.
As you know, it is rather easy to demonstrate that most animals including humans must restrict gene pool size or face eventual extinction. And I have a computer program that models this. It seems beyond doubt that modern infertility is caused by failing to limit gene pool size. Yet it has proven technically feasible to inject a sperm directly into an ovum and accomplish a pregnancy that the gametes could not make on their own. So it seems quite plausible that the infertility is at least in part mediated by some process that this injection bypasses. The egg may be frisking the sperm to make sure it will be compatible. But the DNA of the sperm is tightly packed in the head, and it seemed just a trifle problematic that the egg could perform such an inspection.
There is now an article (Competition Drives Cooperation among Closely Related Sperm of Deer Mice, Heidi S. Fisher and Hopi E. Hoekstra, NATURE vol. 463 no. 7282 February 11, 2010 page 801) that lends a little more plausibility to the process. As it turns out, some sperm are able to cooperate in getting to an ovum.
The illustrations sort of remind me of the old fashioned “flying wedge.” When my grandfather played football as center, they still used it. He would hike the ball back to the quarterback. Then the men on either side would grab his elbows and the flanking men would grab their elbows until the whole team except for the quarterback was a single awesome formation against which no defender could stand. The play was outlawed but not in time to save my grandfather’s nose.
Well sperm can do that, and it increases their swimming speed. The point of the article is that the sperm are competing to get to the egg in promiscuous animals where there might be sperm from any number of would-be fathers around at the same time. It does not happen in monogamous animals like humans. (Sorry guys.) What they discovered was that any one sperm is more likely to team of with sperm it closely resembles from a genetic standpoint than to team up with sperm that are more different.
This requires that the sperm can somehow read each other’s DNA. And if the sperm can do it, it seems quite likely the ovum can do it. Then of course there is the question of whether the ovum actually does it or whether the infertility is due to something else.
I shall be sending a copy of this to Heidi Fisher at the Department of Organic and Evolutionary Biology, Museum of Comparative Biology, Harvard University, Cambridge, Massachusetts 02138. Perhaps she will take an interest.
On a different note, there is a recent article (Still Looking for that Woodpecker, NATURE vol. 463 no. 7282 February 11, 2010 page 718) about the search for the ivory billed woodpecker. It is thought to have been extinct these 50 years, but there have been a few reported sightings so there is much excitement. It was called the “good God bird” because it would peck not with the characteristic rat-a-tat-tat of familiar woodpeckers but with two quick and mighty blows that would cause people to exclaim in surprise. It should be easier to hear than to see, and I hear few reports of that. Nonetheless somebody thinks he has a picture of one. When he went to biologists and asked them to come see the bird they refused. I think I could have told him that would happen. Somehow he managed to file a formal complaint, and now the photograph is being analyzed. Perhaps it is I who should be asking for advice from him.
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