In some species she found hundreds or even thousands of sperm cells stacked up like badminton birdies, forming a long, fairly rigid rod of sperm resembling a fuzzy worm as it moves around inside the female. In other species, sperm are connected by their heads with some kind of glue.
Higginson then set out to investigate what evolutionary forces are the most likely to drive all this unexpected diversity. In an unprecedented effort, she measured many different parameters such as overall sperm length, head length, tail length, length of the female ducts, area of the sperm storage chambers, presence of conjugates, head shape and so on. Using statistical tools to analyze evolutionary relationships among traits, she then reconstructed the most likely evolutionary relationships between sperm and female reproductive tracts.
"We can reconstruct what the ancestors of these diving beetles looked like," Higginson explained. "We find they had conjugated sperm and rather compact female reproductive tracts. The female morphology undergoes evolutionary change, and then sperm compensates for that. We can't say from this study that the males are catching up but it's suggesting there could be an arms race."
Pitnick said: "When you look at the intricate morphology of the reproductive tracts, you can't help but think that sperm needs Swiss army knives and compasses to make it through there. The females make it really complicated."
"In most cases of sexual selection, we expect co-evolution between a female preference for a male trait, like flashy tails in peacocks," Higginson said. "Females prefer males with flashy tails, and then females want bigger and flashier tails after that, setting up a co-evolutionary cycle. We are not positive that this is happening here, but is possible that all this diversity we see in the sperm is the equi
|Contact: Daniel Stolte|
University of Arizona