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In this issue |
Size Counts
kathryn{at}biologists.com
Creatures that spread their gametes in the sea are gambling with high stakes: get it wrong and your genes are heading towards a dead-end in the evolutionary tree. This is a strong incentive to develop eggs with a high chance of getting fertilized, but only if the advantages come at a realistic cost. One frequently used low cost solution to improving an egg's fertilization odds is to surround the tiny gamete with a gel coat that interacts with sperm in a variety of chemical and physical ways. Up until now, most people had analysed the gel from a chemical perspective, but Robert Podolsky wanted to know how important the gel's size was to an egg's chances of survival. Weighing up the gel's advantages to sand dollar eggs from this alternative perspective, he discovered that 75% of that advantage comes simply from the egg's increased size and buoyancy (p. 1657).
Sand dollars have opted for small eggs so that they can produce enormous numbers of the gametes at little cost to the mother. But each egg's chance of bumping into a sperm drops enormously as its girth decreases, so the sand dollar developed a gel outer layer to compensate for the egg's drop in size.
The egg benefits from the gel in a variety of ways. The gel produces chemicals that encourage sperm to swim faster as well as protecting the egg from multiple fertilizations by blocking late arrivals. But is that the major driving force behind the gel's evolution? The sand dollar egg is more than ten times its original size once its gel coat has inflated. Podolsky realised that the advantages of the size increase would contribute to the development of the gel, but how much of an effect did it have relative to the chemical benefits?
Podolsky set about resolving the physical and chemical components from the evolutionary equation by looking at the effect on the egg's fertility. He removed the gel from freshly laid eggs, and watched to see how many eggs were successfully fertilized by the sperm. As expected, the fertility dropped, but Podolsky was concerned that the eggs might have been damaged when he removed the gel. But after trying an alternative gel stripping technique, he was convinced that the decreased fertility was due to the lost gel.
But were the eggs less fertile because they had lost the sperm enhancing chemicals, or had the eggs' slimmed down size had made it harder for the sperm to collide with them? Podolsky did a calculation, which measured the collision rate for a small and large egg, with the amazing result that the egg's chance of fertilization dropped to half of the value he'd measured experimentally! So size was a significant fraction of the gel's evolutionary driving force.
Podolsky also realised that the gel increased the egg's buoyancy, increasing the length of time an egg remains suspended in a cloud of sperm. When Podolsky factored that into his equation, the egg's increased size and greater buoyancy contributed 75% of the total benefit the egg gained from the coat, proving that big is beautiful if you're a sand dollar egg.
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