Category Archives: Sexual conflict

Fancy sperm

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.

CLICK HERE AND LOOK AT THIS RIDICULOUS SHIT.

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

SCIENCE

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.

How sexual conflict can drive the elaboration of a sexually dimorphic trait

Every so often, a scientific paper comes along that really ticks all the boxes: the science is exciting, the methods innovative, the outcome tells you something new, and it comes together to make a great story. The latest research from Locke Rowe’s lab at the University of Toronto, Canada, is one such paper. I’m a big admirer of Rowe’s work – his 1996 paper with David Houle on the ‘genic capture’ model is a great bit of theory which provides a conceptual solution to a very thorny problem, while his book on sexual conflict (with Goran Arnqvist) is a thorough examination of the subject, and is packed with enough examples of weird mating systems that I really have no excuse for being so rubbish at updating my other website. Oh, and Rowe made me explain my poster to him at the European Society for Evolutionary Biology conference in Tuebingen last year while he drank a beer and pretty much just laughed at my nervous, sweaty mumblings. So, in all: great guy.

The paper in question furthers Rowe’s research into sexual conflict, which is the idea that males and females of a single species can have conflicting strategies when it comes to optimising fitness. The most obvious example of sexual conflict in action is mating rate. Females often need to mate only once to fertilise eggs (in insects, females can often store sperm away for future fertilisation); not only do additional matings bring no fitness benefits, but they can actually have detrimental effects on the immune system. By way of contrast, males should gain fitness with each mating, as it should fertilise more eggs (and thus produce more offspring). Anyone with a passing interest in reading science blogs is likely to be aware of the kind of morphological terror that can occur when there’s conflict over the frequency of reproduction: that’s right, it’s time to link to Ed Yong’s seminal (hah) duck-genitalia article. Again.

This time, we’re taking a closer look at the murky world of those poster children for sexual conflict: water striders. Females don’t just resist their suitors due to the usual costs of mating; there is an additional reason for their vigorous struggles. Because males try to mount females atop the surface of the water, the female in particular is susceptible to death from below – a situation that has led to something of an evolutionary ‘arms race’ in terms of morphology and tactics in water striders as each sex aims for a fitness advantage.

Image by Chang S Han. Read Han’s paper with Piotr Jablonski – or just watch the video – about males intimidating females into sex by attracting the attention of predators here. Alternatively, check out the feature in Nature!Sex!TopTips!

The Rheumatobates genus of water striders is known for strange structural modifications in males, used as ‘graspers’ in order to overcome the vigorous resistance they come up against in females. This female resistance is so strong that only around 12% of mating attempts by males are actually successful, indicating just how much of a force sexual conflict is in this species. In this paper, Rowe and colleagues Abderrahman Khila and Ehab Abouheif were studying a particular species named Rheumatobates rileyi, in which the antennae of males are curious and elaborate structures. Through the use of high-speed video, flash-freezing mating pairs, and scanning electron microscopy, they were able to show that there are four composite traits in these antennae, perfectly adapted to grasp the female head. Just in case that passed you by: mating pairs of water striders were lobbed unceremoniously into liquid nitrogen in order to study exactly how these male graspers fit to female heads. Science is AWESOME. This was the first piece of the puzzle: to find some evidence suggesting that the structures are driven by sexual conflict, and that they are not simply for general grasping use by males (hint: look at the picture below).

HERE IS SOME EVIDENCE, NICE HIGH-RES EVIDENCE.
A: close-up of male head
B: super-intense-close-up of the hook
C,D: female’s head, colour-coded to match where male grasping traits ‘lock’ in
E,F: male antennae grasping female head (two different positions)

Using superfancypants transcriptome sequencing technology, they determined that the gene ‘distal-less’ (dll) was responsible for causing males to develop all four grasping traits on their antennae. While dll is expressed in both male and female antennae, it has no such effect in females. By using a technique called RNA interference (RNAi) to disrupt the expression of this dll gene, they produced male water striders which had varying expression of these traits, from “complete loss to a subtle reduction”. They then put these males to work in trying to mate with normal females.

The results are clear to see: just as the antennal grasping traits are reduced, so follows the mating success. RNAi males with no difference from ‘wild type’ had similar success to the average water strider; those with mild reduction in grasping traits failed significantly more, and those with moderate reductions failed on more occasions (and more quickly) than that. These ‘moderate’ RNAi males failed so quickly because their antennae failed to keep hold of the female’s head during the initial flip.

Taken together, this shows how the elaboration of the antennae in one sex of a species has arisen: the conflict between male and female fitness ‘strategies’, the fitness advantage which drives the need for grasping traits, and the genetic basis underlying the antennal elaboration. As the authors conclude, the graded effect of dll RNAi and the corresponding consequences show that even slight modifications to an unmodified ancestral state (like the female antennae) should result in higher mating success, and thus higher fitness. As such elaborate graspers are found in multiple species within this genus, there is a good possibility that variation in the expression and function of the dll gene, combined with sexually antagonistic behaviour, underlies this diversity.

Read the full paper here:

Abderrahman Khila, Ehab Abouheif, Locke Rowe (2012) Function, Developmental Genetics, and Fitness Consequences of a Sexually Antagonistic Trait. Science 336, 585

Image 1 copyright Chang S Han
Image 2 copyright Science magazine
Image 3 copyright Science magazine; modified by me (that’s right, they didn’t actually publish it like that)