Tag Archives: “sexual conflict”

Twisted wings, twisted sex

Some time ago, I wrote a post on here. It was reasonably popular, but I deleted it for foolish reasons. However, I no longer care about those reasons, so now I’ve edited it slightly and it’s back! Enjoy? ENJOY!

If you’re the type of person who frequents animal behaviour blogs, you probably love yourself some animal sex posts (I sometimes feel like I should put some sort of “it’s science so it’s not weird OKAY” disclaimer or something here, but you should be used to it by now… and also I get so many hits from people googling ‘dolphin rape’ that I don’t think it would really make any difference). Such a predilection for tales of animal mating systems will mean you’re most likely well acquainted with the ornaments, weapons and displays that males (for the most part) of a huge variety of shapes, sizes, and species use to improve their chances of mating. Perhaps you’ve tired of pictures of peacock trains and scarab beetle horns; those videos of jumping spiders shaking their curiously colourful buttflaps (erm, you should get used to this level of technical terminology) or bowerbirds prancing around their ornately decorated nests just don’t cut it for you these days.

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(I’m really sorry to the people who took the original photographs that I have ruined there)

Even Stephen Stearns doing his sage grouse impression isn’t enough.

Give us more, you cry: we need more sexual dimorphism! More weird behaviours! Different body shapes! Life histories! Displays, weapons, ornaments, EVERYTHING!

Well, I was flicking through my copy of Thornhill and Alcock’s seminal (hurr hurr) work, ‘The Evolution of Insect Mating Systems’, and happened upon a short passage describing the Strepsipteran order of insects. ‘Strepsiptera’ translates as ‘twisted wing’, but the curious wing shape that gives these insects their name is not the main reason that they piqued my interest. No, it’s because the sub-order Stylopidia has some extreme sexual dimorphism going on: only the males actually grow wings, legs, antennae, mouthparts, eyes, or any of the traits that we associate with adult insects; the females, meanwhile, have none of these features. Male flight is required because they need to find a female to mate with, and quick, because these guys only live for a few hours after emerging as adults.

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Male (left) and female (right) Xenos vesparum.

So while males are flying around in a desperate sexual frenzy, what are their rather curious female counterparts doing? And where are the females, if they have no means of locomotion? The Strepsiptera are ‘obligate parasites’, meaning that some part of their life cycle must take place within a host animal. Hosts include a whole variety of different insects, including silverfish, crickets, stink bugs, wasps, bees, flies… In one particular family, the Myrmecolacidae, males parasitise ants while females parasitise Orthopteran insects. Female Strepsipterans never leave their hosts, instead sitting pretty – at least, about as pretty as a wingless, eyeless, mouthless parasite can get – and waiting for a male to come along. To move things along, virgin females help out by releasing a pheromone that males can use to home in on a potential mate before suffering an early death.

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Females poking out from betwixt the thoracic segments of a Polistes wasp.

Well, you may say, that is some fairly intense sexual dimorphism. But is that all? Males have wings and fly about? It’s not really setting the world on fire. I mean, there are those huge beetles which climb giant trees and throw each other off branches in order to reach the demure, hornless female at the top – it’s not even as good as that, is it?

Oh, ok. I get it. People are immune to weird sexual dimorphisms in insects these days. Fur and feathers, that’s what you want. Maybe if it were a lion with wings flying around briefly in search of a weird giant worm thing to hump, then you’d be impressed? Also, the giant no-face lion-worm would probably live in a giraffe’s bum. Then you’d care. Then you’d ALL care. You want to know something else about the Stylopidae? Well, there’s some controversy over how they mate, but one of the main hypotheses is that mating occurs via TRAUMATIC INSEMINATION.

…also known as HYPODERMIC INSEMINATION.

Why is it called that? Well, males have a pointed, hook-like aedaegus, which is an appendage used to transfer sperm to the female. It’s a bit like a penis, although in deference to this particular method of reproduction, let’s just call it a STABBYCOCKDAGGER*. The male, without so much as a by-your-leave, simply shunts his STABBYCOCKDAGGER straight into the female, releasing sperm into her body cavity.

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STABBYCOCKDAGGER

This isn’t controversial because of the process itself – after all, traumatic insemination is well-characterised in various other species (in particular, the Cimicidae – or bedbug – family: one of the most interesting talks I’ve ever seen was Mike Siva-Jothy presenting some of his work on bedbugs at the ESEB conference in Tuebingen, 2010) – but more due to the lack of detail as to how or why this might have evolved. The reasons for such an adaptation include the following: bypassing ‘mating plugs’ (in many species, a male can inject a secretion into the female’s reproductive tract, ‘gluing’ it closed, or can even break off its penis – or STABBYCOCKDAGGER – in the female so as to block access by rivals); getting round female resistance to mating; eliminating any time that would otherwise be required for courtship; or even in terms of sperm competition, by enabling males to deposit their sperm closer to female ovaries. However, studies indicate that short-lived Stylops males are unlikely to encounter many competitors, and the females stop producing the attractive pheromone just a few days after mating (so the period during which she may attract males is reasonably brief). A study using scanning and transmission electron microscopy in Xenos vesparum failed to either support or rule out traumatic insemination as a mating strategy, but did provide further evidence (adding to studies dating back to the 1840s) that males could simply be using their STABBYCOCKDAGGER to spread sperm fluid into the female’s ‘ventral canal’, which sounds like a much more soothing process. In fact, a ‘spread into a ventral canal’ sounds like a nice holiday that you might take in the Cotswolds. It is possible that this latter method was actually the ancestral form of mating, and that traumatic insemination has developed more recently – potentially so as to bias paternity.

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Yes, I know that isn’t a face, it’s the female’s ventral canal. POETIC LICENCE, YEAH?

Are you happy now?

Happy?, you might ask, happy that you just did the text-based equivalent of screaming STABBYCOCKDAGGER in my face, over and over again?

Oh. Well, then you might be interested to hear about ‘hemocelous viviparity’. Doesn’t sound so bad, right? It’s just that the eggs hatch inside the female, and the offspring eat their mother from the inside out; the larvae then escape from the host and use tiny little legs to run around and find new hosts.

Also, in the case of X. vesparum, the host is a wasp named Polistes dominulus; parasitised female wasps become sterile, inactive, and leave their colony to form aggregations where the parasites can perform their curious mating. Cappa et al. describe them memorably as “idle, gregarious ‘zombies’”. There is also evidence that ants parasitised by other Strepsipterans tend to linger on the tips of grass stems, even in bright sunlight, which may increase the chances of males finding a mate, or even just give the males a good start when emerging from their own host. Such behaviours may be due to our twisted little parasites somehow manipulating their hosts to their own ends.

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To conclude: extreme sexual dimorphism, traumatic insemination, cannibalisation of their own parents, and turning hosts into sterile zombies? Safe to say these strange little flies do a little bit of everything. And maybe, just maybe, you’re glad that creatures the size of lions don’t behave like this after all.

*I asked on Twitter whether people had a preference towards either ‘STABBYCOCKDAGGER’ or ‘STABBYCOCKNEEDLE’. The results were overwhelmingly in favour of ‘STABBYCOCKDAGGER’.

References

Wojcik, D.P. (1989). Behavioral interactions between ants and their parasites. Florida Entomologist, 72(1), 43-51.

Hughes, D. P., Kathirithamby, J., Turillazzi, S., Beani, L. (2004). Social wasps desert the colony and aggregate outside if parasitized: parasite manipulation? Behavioral Ecology, 15(6) 1037-1043.

Beani, L., Giusti, F., Mercati, D., Lupetti, P., Paccagnini, E., Turillazzi, S. and Dallai, R. (2005). Mating of Xenos vesparum (Rossi) (Strepsiptera, Insecta) revisited. J. Morphol., 265: 291–303. doi: 10.1002/jmor.10359

Beani, L. (2006). Crazy wasps: when parasites manipulate the Polistes phenotype. Annales Zoologici Fennici., 43(5-6), 564-574.

Cappa, F., Manfredini, F., Dallai, R., Gottardo, M., Beani, L. (2014). Parasitic castration by Xenos vesparum depends on host gender. Parasitology, 141(8), 10808-7.

All strepsiptera images modified from Beani et al., 2005.

Other images used under a creative commons licence.

Additional links

There is a flickr group for all things Strepsiptera; not many pics, but they are quite wonderful.

Gregory Paulson has a nice bunch of SEM images of strepsiptera here.

Immediately prior to my posting this (well, the first time around), Sam Evans asked on Twitter whether I was writing about bed bugs, and sent me this cartoon. It’s basically a ‘Simpsons did it!’ for anyone blogging about traumatic insemination. THANKS GUY.

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)

Check Mate

Gryllodes sigillatus mating
A female decorated cricket, with spermatophore attached, dismounts male

Think of a duck. Chances are, you’re picturing a mallard – and, if you weren’t, you probably are now. Imagine the male with his striking green-blue head, and the comparatively drab, mottled brown female. If you are a regular reader of science blogs, or simply a fan of waterbird genitalia, your mind may wander to his 20cm-long explosive corkscrew penis, or her cavernous vagina, riddled with dead ends and hairpin bends.

The father of modern taxonomy himself, Carl Linnaeus – admittedly, without having gained privy to these reproductive organs – first classified the male and female mallards mistakenly as separate species. We accept these differences readily nowadays, but how and why have they occurred? Even stranger, why should any species evolve such elaborate reproductive organs?

The answer stems from ‘gametes’, or reproductive cells. Males are defined by carrying sperm, millions of pared-down parcels transporting genetic cargo at high speed. Meanwhile, females harbour relatively few eggs, sluggish monoliths packed with nutrients and protection, waiting patiently for one battling sperm to fuse the genetic information and produce a new organism.

This imbalance in the resources ploughed into gametes results in conflicting desires in the sexes. A male could incur little cost by mating with as many females as possible, increasing the probability of his genes passing to the next generation. His female counterpart, having invested so much in her eggs, gains more from exercising restraint to ensure her offspring are of the highest possible genetic quality.

By what means, then, can a female determine her perfect match? Darwin outlined such characteristics in his 1871 book ‘Selection in Relation to Sex’:

“…the greater size, strength, and pugnacity of the male, his weapons of offence or means of defence against rivals, his gaudy colouring and various ornaments, his power of song, and other such characters.”

This system can be as straightforward as the bull elephant seal using his great strength to drive rivals from a harem of females. Other traits proved far more puzzling – Darwin’s frustration in the years prior to this publication is summarised in an 1860 letter containing the epithet:

“The sight of a feather in a peacock’s tail, whenever I gaze at it, makes me sick!”

One theory is that such traits indicate genetic quality by acting like handicaps. Bright colours, displays and songs attract predators, while a cumbersome tail hinders a quick getaway. A male may be indicating, “Hey, if I have such a big handicap and I’m still here, doesn’t that show you how great I am? You should probably have my babies.”

What happens, then, to those that cannot compete at the highest level – should they simply give up? No, instead they engage in some downright sneaky behaviour, such as yellow dung flies waiting at the edge of a dung heap, ready to pounce on unsuspecting females travelling to meet Mr. Right atop a delicious pile of excrement. On the Hawaiian island of Kauai, Professor Marlene Zuk discovered that the mate attraction ‘song’ of the field cricket – produced by males rubbing their wings together – had enabled an invading parasitic fly to target the species ruthlessly. Rather than face extinction, however, this cricket population rapidly evolved to favour a mutation that removed rough edges from the wings. These smooth, silent crickets possess no real powers of attraction, instead surrounding the remaining chirpers in the hope of engaging a prowling female. While she would prefer a personal serenade from her potential mate, waiting too long exposes her to greater danger – or the risk of being ‘left on the shelf’. There is such a thing as being too choosy, after all.

Another tactic is to offer a ‘nuptial gift’, which can be an edible treat that the male produces himself. This has led to the synthesis of offerings that lower the female’s desire to remate before his sperm have completed fertilisation. ‘Aggressive sperm’ may outcompete or kill off rival gametes, while a dragonfly penis can scoop out existing seminal fluid. The bumblebee, meanwhile, breaks off its penis in the queen to form a ‘vaginal plug’ – an extreme case of putting all your eggs in one basket, if that’s not too confusing an analogy given the topic at hand.

It would be foolish to presume, however, that females have been standing by idly in evolutionary terms. Insects such as water striders and seaweed flies are locked in a race whereby females evolve anti-grasping functions just as the males develop claspers. Indeed, the mallard’s complex vagina has evolved as a defence mechanism. As the female is victim to frequent attempts at forced copulation, this not only helps to prevent entry, but also diverts unwanted sperm away from those precious eggs.

This arms race has raged ever since sex first evolved. As long as males and females continue searching for the tiniest advantage over one another, the battle of the sexes shows no sign of slowing down.