For males, fitness depends on reproductive success: this requires that a male find a mate, ensure that his sperm fertilises her eggs, and that the subsequent offspring are viable. I have a tendency to focus on the first ‘episode’ of selection, given that I can then post photos of fancy traits, although this has led to my dabbling in sperm now and again (did I really just write that?). Thankfully, the Pitnick Lab at Syracuse University is all about sperm – to the extent that Scott Pitnick himself has a ridiculously awesome sperm-inspired tattoo, which you may have seen in Carl Zimmer’s ‘Science Ink’ book. His research includes investigation of sperm competition, where the sperm from multiple males competes within a female’s reproductive tract to fertilise her eggs.
Sperm competition has itself led to the evolution of different morphologies and tactics in both males and females (check out a preview of Leigh Simmons’ book here to find out more), but the particular research I want to look at here concentrates purely on tackling the problem of discriminating between the competing sperm of different males. Although it is now easy enough to mate a female with two males, and figure out the proportion of offspring sired by the second of these, we don’t really know what’s going on ‘under the bonnet’ (if that’s not too horrendous a euphemism). Research has shown that the last male to mate will usually sire around 80% of the offspring, but what are the mechanisms involved? Is it that the last ejaculate ‘displaces’ the previous? Do females ‘eject’ previous sperm?
The Pitnick Lab have found a way to investigate this, by using Drosophila melanogaster males which have been transformed so that they express a protamine in the sperm head that is labelled with either green or red fluorescent protein (GFP / RFP). These sperm can then be watched as they duke it out within the female’s reproductive tract, enabling researchers to figure out which of the hypothesised mechanisms are actually working in this system. Just in case, let’s recap. The video linked just below this paragraph (the website refuses to let me embed it, BOOOOoooo HISSSssss etc) is a female fruit fly’s reproductive tract. The red and green objects are sperm from male fruit flies who have mated with her. The reason that we can see that the sperm are different are because the fruit flies have been transformed so that their sperm are labelled with a particular protein. Oh, and green fluorescent protein was first isolated from a jellyfish, and is now routinely used to just, you know, show that some shit is happening. So bear with me while I repeat: SCIENCE IS AWESOME.
Here’s a different video so that I can embed something. It’s not quite as good, it only shows a FREAKING SPERM VORTEX:
I’m going to add to this post later, but for now I need to go and sit down.
WHAT THE HECK
If you actually want to learn more while I go and try to get my head around it all, you can read the Science paper here:
Mollie K. Manier, John M. Belote, Kirstin S. Berben, David Novikov, Will T. Stuart, and Scott Pitnick (2010) Resolving Mechanisms of Competitive Fertilization Success in Drosophila melanogaster. Science 328 (5976)
There are more videos on the Pitnick Lab’s youtube channel.