You may notice that this paper has been out for a while already, even though I really should have been excited to blog about it right away, given its importance for my field of study. But, lately, research has kept me too busy to actually sit down and write a decent post about it. So, instead, I guess I’ll just have to write a less-than-worthy post. However, I do encourage you to read this primer in PLoS Biology, written by my very own advisor, about the paper.
My research, and the research of many people in the Wolfner lab, is geared toward understanding how proteins in semen help sperm do their job of fertilizing eggs. These proteins aren’t part of the sperm, and they’re made in a totally different organ, but they’re essential for male fertility (yes, even in people). In our favorite animal, the fruit fly Drosophila melanogaster, these “extra” proteins are needed for all three steps of sperm success: getting into storage (in the female), staying alive in storage, and being released so they can fertilize eggs.
We’re starting to understand a great deal about the male side of things (though there are still A LOT of questions), but we basically know nothing about the female side.
That’s where this paper by Sandra Schnakenberg, Wilfredo Matias, and Mark Siegal comes in. They focused on a few cells that surround two of the female’s sperm storage organs: the paired spermathecae. They discovered that these cells (called secretory cells) pump out proteins that tell sperm how to get into storage. Without the stuff from these cells, sperm get lost and can’t enter the spermathecae. The spermathecae are attached to the uterus and store about 100 sperm each after mating. Another structure, the long, coiled seminal receptacle stores the other 400-600 sperm.
Schnakenberg and colleagues found two proteins (my favorite kind: proteases!) that are made in these secretory cells and nowhere else in the fly (the cells are shown in green in the picture above). They then identified the part of the fly’s DNA–the regulatory sequence–that tells the fly to only make these proteins in those particular cells.
With the regulatory sequence in hand, they could turn on cell-killing genes in those secretory cells, essentially deleting them from the fly’s body. The point? To figure out what the hell those cells do, other than make some generic-looking proteases. If you want to know what something does, the best way to find out is to break it, then wait to see what goes wrong. In this case, without those cells, sperm lose their sense of direction. They can’t get into storage (at least, not into the spermathecae). But, what about the seminal receptacle, you ask? Don’t tons of sperm make it in there? Well, turns out they can get in, but they don’t stay alive in there for very long.
The fun part? Proteins from the secretory cells only need to be around right before/during mating to do their job. If you get rid of them after mating, everything is still okay. So, how do they keep sperm alive for long periods of time after mating? One possibility is that they’re needed to turn on some program in the storage organs right at the time of mating, and other proteins do the heavy lifting.
The weirdest result from this experiment is that the secretory cells are needed for the female to lay eggs like a normal female. Fruit flies usually lay a bunch of eggs every day after mating, for up to two weeks. But females without their spermathecal secretory cells will lay eggs normally for a day, then not lay any the next day, then be normal, and on and on.
What happens on those days when she isn’t laying any eggs? An egg is still released from the ovary, but for some reason gets stuck in the uterus. There, it can be fertilized and, amazingly, develop and hatch into a larva. This creates a situation where flies give live birth. Kinda gnarly.
Now that we have an idea of how female flies guide sperm into storage, we can start to study the interaction between semen proteins and female proteins. And we can finally ask questions that evolutionary biologists have been wanting to ask for a very long time. Do male and female proteins generally work together, or is the “battle of the sexes” a more common theme? Which kinds of proteins tend to be in conflict? And who’s really in control over fertilization success? My guess: it’s going to be a little of everything, depending on the context. And that’s why biology is so much fun.
Schnakenberg, S., Matias, W., & Siegal, M. (2011). Sperm-Storage Defects and Live Birth in Drosophila Females Lacking Spermathecal Secretory Cells PLoS Biology, 9 (11) DOI: 10.1371/journal.pbio.1001192