Practical sperm-wrangling

February 24, 2008 7:39 PM

Speak to me of blocks to polyspermy.

If you care about the molecular biology of proteostome and deuterostome fertilization, (and lord, who doesn't?) you know that polyspermy (entry into the egg by multiple sperm) can be a huge problem. Sperm carry both a load of genetic matrial, and the machinery with which to divide it. Too much division machinery can cause the egg to divide improperly, and too much genetic material can cause just about every problem you can think of.

To prevent polyspermy, animals have evolved a number of sperm-management techniques. These include:

(1) The fast block to polyspermy-- changing the membrane potential of the egg cell. (Sperm can only fuse with eggs whose resting membrane potential is negative. In sea urchins, the magic number is about -70mV. Once sperm enters, the egg takes up sodium cations, and its membrane potential becomes positive).

(2) The slow block to polyspermy-- where cortical granules release their contents into the space between the egg's cell membrane and its outer coating, creating a hard envelope that sperm can't get through.

(3) The micropyle-- In Drosophila (fruit fly) eggs, the sperm can only enter through one tiny opening, called the micropyle. After a sperm enters, its extremely lengthy tail stays in the micropyle, blocking it so that no other sperm can follow.

I just learned about the micropyle mechanism, and I'm absolutely wonderstruck by it. My question is, what other interesting or unusual blocks to polyspermy exist? I'm looking specifically for things that happen on the gamete level.

Preliminary (and admittedly cursory) research on les internets hasn't turned up anything other than these three mechanisms.

posted by palmcorder_yajna to science & nature (3 comments total)

My botanist sweetie says that plants have a micropyle mechanism also (but no sperm tails in plants of course). She can't think of any other blocks offhand though, besides the ones you mention.
posted by hattifattener at 8:52 PM on February 24, 2008

A quick pubmed search shows you probably have it down. Of course, you know a pubmed search limited to reviews will often give you a good background in such matters.

If you want to dig further, bivalves often have cool stuff going on here as they have all sorts of crazy mating systems - eg "explosive egg reactions"
posted by scodger at 2:42 AM on February 25, 2008

Check out "Promiscuity: an evolutionary history of sperm compitition" by Tim Birkhead.

I would guess that it has some information about polyspermy but it also has a lot of cool stuff about sperm competition in general.
posted by hydrobatidae at 12:43 PM on February 25, 2008