If you want to know what it takes for a sperm to meet an egg, this is the podcast for you!
Conceived, written and presented by Hunter Fox, Tiffanie Luong and Alexander Wang
for course BIOL 4585-001 Evolution of mating systems (J-term, 2021)
Class of 2021, University of Virginia
Transcript of Audio
Tiffanie: Good morning everyone. Welcome to this week's episode of Microscopic Olympics, where no competitor is too small.
On this week’s episode, our discussion topic comes from a very special listener, who asks:
Dear Olympians,
My name is Simon Seminal and I’m a sperm cell who wants to know how I can win the great race of fertilization one day. I am perplexed as I know there are many different morphologies of sperm, and I was hoping you could shed light on why these arise and how I can increase my odds of winning.
Thank you, Simon, for such an insightful question. On today’s cast, we have several Olympians who are going to explain a little more about the background and factors influencing the evolution of different sperm morphologies. Introducing our speakers and topics: professional sperm trainer Alex will give us a brief background on sperm morphology and explain how male-to-male competition contributes to variation in morphology. Course analyst Hunter will explain how the navigation of the course can affect the odds of winning, and lastly one time fertilization winner Tiffanie myself will explain how it may be a little more complicated than all that. Alex, do you want to start us off?
Alex: Sure thing, Tiffanie. Before we get into the different perspectives that we each have to offer, here’s a quick breakdown on the structure of sperm for our listeners. Now, a sperm cell can be divided into three main components: the head, the midpiece, and the tail. As you might imagine, the head is the most crucial part of the sperm, as it contains the genetic information (in the form of DNA) that will be passed on to the goal, the egg. The midpiece is a smaller section that contains mitochondria, the engines that power the sperm’s movement. And last but not least, we have the tail, which acts as a sort of propulsion device that allows sperm to move through the female reproductive tract. All these pieces come together to make up our microscopic competitors.
So, interestingly enough what we find is that the structure of sperm, or sperm morphology, differs enormously across species. And the big question of the day is what might be the cause for all of this variation that we see? Might I propose… male-to-male sperm competition. This is the Microscopic Olympics after all, where sperm are vigorously competing against one another. To give our listeners a quick definition, sperm competition is all about males simultaneously competing in the attempt to have their sperm (and not their competitors’) successfully fertilize a female’s eggs. So, sperm morphology should evolve in response to the selective pressures of the competition, with males having sperm morphologies such that they can outcompete their foes.
Now, we know that sperm competition creates a selective pressure for increased sperm number, and while the effect on size isn’t as clear, it’s been indicated that sperm are often longer in polyandrous species, where females mate with multiple males and males with only a single female, compared to species where males and females form individual mating pairs. This is all to say that where sperm competition is strong, so is the selection on sperm morphology. To give our listeners a specific example of the effect of male-to-male competition on sperm morphology, the European bullfinch, a species of passerine birds, has been observed to have a sperm morphology that’s distinctly different from those seen in related species of birds. And this is highly likely due to the relaxed nature of sperm competition in the European bullfinch compared to their more competitive neighbors. So you see, Mr. Seminal, it’s quite simple to be the best so long as you’ve trained such that your own sperm has what it takes to outcompete those of your competitors.
Hunter: Hey, hey, hey, I think you’re forgetting a major piece of the puzzle. I mean it does still take two to tango in the act of sex, with the exception of asexual reproduction that is LOL. I think you’re undermining important abilities that suggest the female reproductive system plays a huge role in the evolution and determination of sperm traits, such as morphology. In turn, this selection has given rise to various shapes, sizes, and contours of sperm across species. There have been numerous studies to suggest that some sperm are better at navigating the female reproductive tract than others. Some of the reasons include the complexity of the female reproductive organs, the length of the oviduct, as well as the mucus in the reproductive tract. All of these are ways in which the female reproductive system has evolved and become an arena for sperm competition.
As we know, females are complex...and their reproductive system is no different. The female reproductive system can be considered like a labyrinth, and Mr. Seminal, you know just as much as I do that you can only go as fast as the obstacles will let you in any obstacle course race. How you overcome them is where you will succeed. The female reproductive system presents many obstacles. The more complex the reproductive system or the longer the reproductive system is, the better you need to be at navigating it. In a complex system, like a labyrinth, it is easy to get lost or tired out when trying to find the finish line.
In this case, it is no different. Sperms that are better at navigating and better at displacing other sperm have the advantage here. Sometimes, sperm require help from a friend in order to reach the finish line. We see that oftentimes, sperm will form conjugates, which is basically a cooperative effort where the sperm will help one another in order to have the best chance of reaching the egg. This provides more locomotive power to propel the sperm throughout the treacherously complex reproductive tract. With more energy and locomotive power, the longer it can search for the finish. It is with no doubt that the more complex the female organ, the more selection there is for only the highest quality sperm to win the race, and in many cases, the sperm that have longer tails for greater velocity and sperms that have more energy stores are those that are better at navigating the complex labyrinth of the female system.
I think the female reproductive tract is an interesting organ and since it poses such complexity and obstacles, it is easy to support that the reproductive tract drives sperm selection in the males, leading to the rise of various forms and functions of sperm with one goal: who can win the race? My answer says it depends on the course and the males’ ability to navigate and overcome obstacles within that course.
Tiffanie: Now Hunter, I think you brought up some really good points, but I think there’s something you should really consider, Simon. Every time you race, the finish line gets farther and farther away. You might think this is unfair, right, because you’re training to become better at the race you just ran? But the truth is, so is everyone else, including the finish line, making the glory even better when you are the winner, and don’t forget, there can only be one winner.
It makes sense, no matter how unfair it seems at first. If every sperm keeps getting better and faster at reaching the egg, the egg also has to get better at resisting until the right sperm comes by. This introduces a cycle of the sperm and the egg evolving to countermeasure each other, each continuously striving to best the other. The idea of coevolution comes from the Red Queen hypothesis, which actually got its name from a quote by the Red Queen in Alice in Wonderland. The quote reads: It takes all the running you can do, to keep in the same place. If you want to get somewhere else, you must run at least twice as fast as that.
So how does the sperm and the egg compete with each other? Well there’s a few different ways and it’s not really known where this cycle starts
In Drosophila melanogaster fruit flies, it has been shown that males have evolved to have longer sperm tails and females have evolved to have a longer sperm storage organ called the seminal receptacle. This possibly explains why the male sperm of the fruit fly is at least 20 times longer than its body. When comparing 100 species of Australian myobatrid frogs, longer head and tail length for sperm correlates to larger eggs with thicker walls. So boys, I think the race is a little more complicated than you think. But Alex and Hunter both made valid points that I think you should keep in mind, Simon.
Alex: So Mr. Seminal, I hope these perspectives have provided some insight to your question. It’s clear that there isn't one specific approach when it comes to winning the race, and that’s both the difficult and beautiful thing when it comes to understanding biology—there’s rarely a single answer. As you’ve heard today, there are multiple factors that can go into the creation of various sperm morphologies, and we hope that you were able to glean at least a little bit of understanding concerning what some of those factors are from what our Olympians shared. We invite you to tune in to next week’s episode of Microscopic Olympics and to keep sending in your training videos and questions! Follow us on Spotify, Instagram, and Facebook to see what our Olympians and other listeners are up to! Until next time, stay strong and keep on training to be the next great microscopic Olympian.
References
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Higginson, D. M., Miller, K. B., Segraves, K. A., & Pitnick, S. (2012). Female reproductive tract form drives the evolution of complex sperm morphology. 109(12). https://doi.org/10.1073/pnas.1111474109
Hook, K. A., David Weber, W., & Fisher, H. S. (2020). Collective sperm movements are shaped by post-copulatory sexual selection and phylogenetic history in Peromyscus mice. In bioRxiv (p. 2020.02.08.939975). bioRxiv. https://doi.org/10.1101/2020.02.08.939975
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San, M. A., Zoo, D., Dixson, A., Anderson, M. J., Dixson, A. S., & Dixson, A. F. (2006). Mammalian sperm and oviducts are sexually selected: Evidence for co-evolution. Article in Journal of Zoology. https://doi.org/10.1111/j.1469-7998.2006.00173.x
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