Rapid Evolution in Eggs and Sperm
Sperm and eggs are ubiquitous and diverse. What drives them to diverge?
The experience of learning about the birds and the bees is almost universally awkward, bringing to mind textbook cartoons of the male and female reproductive tracts viewed with amazement, confusion or embarrassment. Although most people do not contemplate reproductive systems every day, speaking of sex and reproduction only in secretive tones, like it or not, we are surrounded by sperm and eggs. On a typical day, you might enjoy a delectable breakfast of fried eggs (a chicken egg and the accompanying yolk) and take a walk outside that triggers a sneeze caused by pollen (plant sperm). You might decide to take a dip in the ocean, where external fertilizers such as sea urchins and abalone are spawning their eggs and sperm throughout the water.
Sexual reproduction is ubiquitous across plants and animals—more than 90 percent of vertebrates reproduce sexually. Sex is largely responsible for the biological diversity that first fascinated and continues to occupy biologists. Differences in shape, size and coloration, among other characteristics, are often attributed to the evolutionary influences of sex. But why did sex evolve in the first place? Observing sperm and egg diversity across species brings more questions to mind: Why are there more sperm than eggs? How does sexual promiscuity affect the evolution of the sperm and egg? One might think that the same mechanisms, sexual traits and reproductive genes are involved in sperm–egg composition and interactions. But is this the case?