If you’ve perused the other posts on this site, you may have noticed that I have a thing for seminal fluid. That’s because semen is awesome: it’s full of proteins, lipids, sugars, and who knows what else, all of which plays some role in fertility.
Exactly what all that stuff is doing is still a total mystery (except for a very few rare cases).
Unfortunately, just picking out one protein and asking, “what happens if I break this?” doesn’t seem to work in most cases. One reason for this could be that there are tons of fail-safes built in. After all, if destroying the function of any one protein could cause infertility, making babies would be a lot more difficult.
My own research is on a type of proteins, called proteases, in the seminal fluid of fruit flies. These proteases act very quickly during and after mating to cut up other seminal fluid proteins to do… something related to reproduction. Nailing down exactly what all these proteases do inside the female is definitely not easy.
That’s why I was very excited to see this paper in PLoS Genetics recently. A protease called TRY-5 in male worms (C. elegans) activates sperm as they are transferred to a hermaphrodite. So, such an important protease must be required for fertility, right?
Wrong. The hermaphrodite uses a completely different (still unknown) activator for its sperm, and this can also activate male sperm if something were to happen to TRY-5. And if the hermaphrodite’s activator is broken? No problem, she can use TRY-5 from the male.
Of course, in the wild, most hermaphrodites probably never encounter a male, so I guess if their own activator was broken, they’d be screwed (or not, as the case may be). But males would be extremely unlikely to encounter a hermaphrodite with a broken sperm activation system. So why have TRY-5 at all?
This is the sort of question that pops up over and over again in seminal fluid research. The male goes through all the trouble to make tons of proteins that, individually, appear to do nothing all that important. But making all those proteins takes energy. And, if you look very closely at the evolutionary history of the proteins, you find again and again that they’re not lost over time. In fact, they seem to be picked out by natural selection to keep changing in ways that may affect their function, but never quite break them.
So, what is the answer? Why have TRY-5, or any of those little moochers hanging out in the seminal fluid? My best guess is that no matter how hard we try, we can never quite exactly mimic the natural environment of our favorite experimental animals in the lab. Worms (and flies, too, for that matter) experience situations in the wild that we probably can’t even imagine. In the lab, a difference of a couple of minutes may not affect anything, whereas in nature, it could be the difference between fertilizing eggs and being kicked out of the gene pool for good.
In the case of TRY-5, the authors suggest that it may be a more efficient activator of sperm and that this somehow works in the male’s favor. The difference in efficiency, though, if it exists, is too small to see under their laboratory conditions. They speculate that a difference in efficiency could benefit the male by decreasing the chance that his sperm is lost before they can fertilize the hermaphrodite’s eggs:
“For example, activation by TRY-5 might occur more rapidly than that mediated by the hermaphrodite activator. If so, its transfer would decrease the chance that transferred sperm would be lost before they have the opportunity to migrate away from the vulva.” (Smith & Stanfield, 2011 PLoS Genetics)
It may seem silly to try to pick away at the functions of all these proteins in the seminal fluid, but I think it’s very important (though I may be biased). Everyone knows that you need a sperm and an egg to make a baby, but the process is much, much more complex than that. And we still have no idea how all of these proteins, genes, and the environment fit together to determine how and when a new life will be formed.
Smith, J., & Stanfield, G. (2011). TRY-5 Is a Sperm-Activating Protease in Caenorhabditis elegans Seminal Fluid PLoS Genetics, 7 (11) DOI: 10.1371/journal.pgen.1002375