Category Archives: Evolution 2012

#Evol2012: Evolution 2012 review, special Doug Emlen edition

I’m going to skip ahead in my review of the talks which I enjoyed at Evolution 2012 in Ottawa, as Doug Emlen‘s latest research has just been published in the latest issue of the prestigious journal Science. This gives me an excuse to write about his talk and the new paper, as well as to engage in gratuitous posting of beetle photos.

It makes me a bit sick when I think about how awesome this lab is.

I have a real soft spot for research on beetle horns, as followers of Nature!Sex!TopTips! may be aware, so I was really excited to see Emlen’s talk – even more so after the taster that was Erin McCullough’s presentation earlier in the week (McCullough is a PhD student co-supervised by Emlen and Bret Tobalske at the University of Montana’s ‘Flight Lab’). Research into animal weaponry often goes hand-in-hand with studies of ornaments because there is direct sexual selection upon them; females use ornaments as a basis on which to select a mate, while weapons are used by males to defeat rivals (or to assess their condition and status) and so gain access to females. Together, these exaggerated, elaborate structures are some of the most incredible sights we see in nature.

The rhinoceros beetle Trypoxylus dichotomus, used in both Erin McCullough’s work on how horn size affects flight and Doug Emlen’s research into the mechanistic basis of exaggerated trait expression. Photo copyright flickr user Mushimizu (note: open the link from this image in a new window to see the animated gif in action!)

It’s no surprise that a lot of research investigates these amazing traits, but there are still some big questions to grapple with. For example, they seem to be very reliable indicators of male quality – why should this be so? Can’t some males ‘cheat’ by somehow investing more into ornament or weapon growth than other things? Also, if females select upon a particular heritable trait, then shouldn’t we see very little variation by now, with all males having pretty much the same size of trait? Consider the range of deer antler size in comparison to, say, the range of deer leg length. Antlers are much, much more variable – but why?

Brief non-beetle interlude: red deer, showing the variability in antler size and shape. Image copyright David J Slater.

I’ve written about the maintenance of genetic variation in such traits before, both here and over at the Nothing in Biology Makes Sense blog, using the ‘genic capture’ model proposed by Rowe and Houle. This model posits that the continued evolution of sexually selected ornaments and weapons is enabled by these traits ‘capturing’ the underlying condition of the animals. An individual’s condition is affected by its general health, nutrition, parasite resistance, competitive ability, etc…  essentially, the genetic variation among males in terms of all these factors underlies the variation in these amazing traits. It’s this ‘condition-dependence’ of traits, a close association with the individual’s condition, which means that the expression level should be ‘unfakeable’ and thus a reliable indicator of male quality. Not only this, but it also allows the evolution of ever-more exaggerated ornaments and armaments. So, these traits have some particular characteristics which have triggered huge interest from an evolutionary point of view: extreme size, heightened sensitivity to condition, and much more variability than we see in other morphological traits. We often think of condition-dependence as a kind of ‘black box’ – environmental and genetic factors go in, and traits come out. Emlen’s current research asks the question of, well, what mechanism enables this to happen? What’s inside the black box that creates these incredible, extreme biological structures?

Emlen proposes that there is a developmental explanation for this, and it lies within the insulin / insulin-like growth factor (IGF) pathway. This pathway has emerged as the central mechanism in animals for integrating physiological condition with growth; insulin and IGFs not only regulate tissue growth and body size, but they are also sensitive to factors such as nutrition, stress and infection. The levels of insulin / IGF circulating in an individual would cause a graded response via this particular pathway, with growth speeding up or slowing down in response to changes in nutritional or physiological state – i.e., the same kind of factors which affect what we term ‘condition’. So far, so straightforward, you might think: there’s a pathway which controls tissue growth that depends on how healthy and well-nourished you are. But how might this lead to the evolution of highly exaggerated weapons and ornaments?

Well, here comes the even cooler bit: traits differ in how they respond to these signals. This can have a truly profound effect on the amount and nature of their growth. Some traits, like Drosophila genitalia size, are not particularly sensitive to insulin / IGF signalling, meaning that they tend to be around the same size in all individuals, no matter their nutritional state. Wings, meanwhile, are more sensitive to these signals. Within a variable population of fruit flies, with a normal range of body sizes, we would see variation in wing size approximately equal to variation in body size, while genitalia size would hardly vary at all. So, just as wings are more sensitive to insulin signalling in Drosophila than are genitals, Emlen predicted that exaggerated weapons or ornaments are even more sensitive than that. Such heightened sensitivity to insulin / IGF levels would explain how such traits grow to extreme sizes, why there is such huge variation within populations, and why such traits seem to be reliable indicators of underlying quality.

Normal beetle service has resumed: the male rhinoceros beetle Trypoxylus dichotomus. Image copyright flickr user golbenge.

Emlen and his colleagues tested this hypothesis in male rhinoceros beetles (Trypoxylus dichotomus), which have a large forked horn on the top of their head. They used RNA interference (RNAi) to perturb transcription of the insulin receptor (InR) – that is, they simply stopped this particular signalling pathway from working properly. They did this at the beginning of metamorphosis, a point when body size is no longer growing, but adult structures – such as genitalia, wings, and the huge sexually-selected horn – are. If increased cellular sensitivity to insulin / IGF signalling is at least partly responsible for the evolution of this exaggerated horn in these beetles, then horns should be more sensitive than wings to the experimental manipulation of the pathway activity via RNAi. Furthermore, Emlen and his team predicted that – just as with fruit flies – genitalia should be relatively insensitive to this disruption of insulin / IGF signalling.

Results showed that the genitalia of males whose InR pathway activity was disrupted did not show a significant reduction in size when compared to control males (which did not undergo the RNA interference treatment). Meanwhile, the wings of RNAi treatment males showed a significant reduction in size that measured around 2% in comparison to control males. This is typical of the majority of ‘metric’ traits, such as eyes, legs, etc. Horns, however, predicted to be the most sensitive to nutritional state, suffered a significant reduction of around 16% in RNAi treated males relative to control animals. This eight-fold increase in sensitivity of horns in comparison to wings is highly consistent with Emlen’s model of the evolution of exaggerated trait size from heightened sensitivity to this particular pathway – giving us a real insight into the black box of condition-dependence, and how such incredible traits evolved.

Note: I highly recommend reading the paper itself, not only because it’s very well-written, but also because Emlen does a great job of summarising models of sexual selection and condition-dependent traits, and the impact of this latest research on those models. Plus there’s some nice beetle pictures in there, and you love nice beetle pictures. DON’T YOU?

Read the Science paper here:

Emlen, D.J., Warren, I. A., Johns, A., Dworkin, I. and Corley-Lavine, L. (2012) A mechanism of extreme growth and reliable signaling in sexually selected ornaments and weapons. Science (in press).

Other references:

Rowe, L., & Houle, D. (1996). The lek paradox and the capture of genetic variance by condition dependent traits. Proc. Royal Soc. B, 263 (1375), 1415-1421.

Shingleton, A.W., Das, J., Vinicius, L. and Stern, D.L. (2005) The temporal requirements for insulin signaling during development in Drosophila. PLoS Biol. 3, e289.

Further reading:

Cotton, S., Fowler, K., Pomiankowski, A. (2004) Do sexual ornaments demonstrate heightened condition-dependent expression as predicted by the handicap hypothesis? Proc. Biol. Sci. 271, 771.

Emlen, D.J. (2008) The evolution of animal weapons. Annual Review of Ecology Evolution and Systematics 39: 387–413.

Houslay, T.M., Bussiere, L.F. (2012) Sexual Selection and Life History Allocation. In: eLS 2012, John Wiley & Sons, Ltd: Chichester.

Shingleton, A.W., Frankino, W.A., Flatt, T., Nijhout, T.H., Emlen, D.J. (2007) Size and shape: The developmental regulation of static allometry in insects. Bioessays 29, 536.

That’s right, I did just recommend a review paper that I wrote. You should know by now that I’m absolutely shameless.

#Evol2012: Evolution 2012 review, part 2

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Here are some more notes on talks that I enjoyed at Evolution Ottawa 2012, with links to finding out more if you are so inclined…

Emilie Snell-Rood – Changing nutrient dynamics and the relaxation of sexual selection: effects of human-altered nitrogen inputs on butterfly mate choice

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Image copyright Emilie Snell-Rood

I was really blown away by Snell-Rood’s talk, and a look at her research interests on her lab page shows that she’s working on some really interesting stuff and with very cool systems (including Onthophagus nigriventis, which have a very cool male dimorphism, work on which I wrote about previously on Nature!Sex!TopTips!). This talk looked at how the increased availability of a once-limited nutrient was affecting mate choice, using the cabbage white butterfly Pieris rapae. I’ve written in greater detail before on how variation in resource acquisition is important for the maintenance of genetic variation in sexually-selected ornaments, and it turns out that the acquisition and processing of nitrogen, a once-scarce resource, is important for the P. rapae male’s secondary sexual trait (ultraviolet signalling). Since around the end of World War II, fertiliser use in the USA has skyrocketed, flooding the system with nitrogen; Snell-Rood and her lab were interested in whether P. rapae ornamentation has lost ‘condition-dependence’ over time, given that this important resource is now widely available to all individuals?

One of the ways which we can use to figure out if a signal is condition-dependent is to look at allometry, or the scaling relationship between two traits (for more detail, see this great article by Alexander Shingleton, or this rather more in-depth paper by Russell Bonduriansky). A typical study would be to plot the trait of interest against body size; on the log-log scale, we’d expect a standard trait to have a slope of 1, so it is always in proportion to body size. Condition-dependence is a special type of plasticity, however, meaning that those in higher condition can afford the costs of increased investment in such traits. We’d therefore expect to see a slope greater than 1, i.e., larger individuals have proportionally larger traits. This is called positive allometry. Using museum specimens and previously collected data (if my memory serves me correctly), Snell-Rood found that after WWII – and thus after the environment became flooded with high levels of nitrogen – these butterflies no longer exhibited a greater body size : ornament correlation. This is a really neat study, and I’m very excited to read the paper when it arrives – Snell-Rood also mentioned a few other experiment underway in her lab that sound very cool, using irradiated fathers (so likely more deleterious mutations) and differing levels of nitrogen to investigate ornamentation and female choice. This is extremely awesome! Until then, you’ll have to make do with this interesting review she wrote recently on adaptive phenotypic plasticity

Devin Arbuthnott – Ecology and sexual conflict in Drosophila melanogaster

This talk centred around the idea that sexual conflict and ecology may not be as separate as one might think, and that adaptation to different environments may promote parallel changes in sexual conflict and population interactions. I don’t have much in the way of notes from this talk, but Arbuthnott is working on some very cool stuff, and published a paper in Evolution recently with Howard Rundle that has some very interesting implications: ‘Sexual selection is ineffectual or inhibits the purging of deleterious mutations in Drosophila melanogaster‘.

Christopher Chandler – Runaway sexual selection leads to good genes

Chandler presented some work on how runaway sexual selection and good genes are not mutually exclusive, using the ‘Avida‘ platform. This is a computational system which uses self-replicating digital organisms to investigate questions that we can’t address with natural organisms. As someone with a computing background, I’m really interested to see whether this type of work gains more acceptance in the mainstream literature. Avida is currently used in the ‘DevoLab‘ for teaching evolution at Michigan State University; the 2004 Ofria & Wilkes paper describing the system can be found here.

Erin McCullough – Elaborate weapons: the costs of producing and carrying horns in a giant rhinoceros beetle

I’ll be honest: I pretty much just went to this talk as, after a couple of theory-heavy talks, I wanted to see some cool pictures of giant horned beetles. McCullough didn’t disappoint in that regard, but also presented some very cool work that also seems to challenge our ideas of the costs involved in the carrying and production of oversized secondary sexual characters. She found that, in the species Trypoxylus dichotomus, horns are not costly in terms of body mass as they are hollow and air-filled. McCullough is interested in how such horns affect flying, which is important for mate-searching, and found that any issues in regards to drag are more likely due to body angle than horn size, with horns really having only a trivial effect on flight. She found no evidence for allocation trade-offs, merely some evidence for compensation in terms of male flight apparatus (but not, for example, changes in muscle mass). McCullough’s recent paper in Behavioural Ecology discusses whether horns were costly in the past, and how this may have led to compensatory changes in terms of wing and muscle morphology. I’m excited to see what further research tells us about the evolution of horns in this particular species.

That’s all for now, but plenty more to come…

#Evol2012: Evolution 2012 review, part 1

Last week, I went to Evolution 2012 in Ottawa, the first joint congress on Evolutionary Biology – bringing together 5 top societies (American Society of Naturalists, Canadian Society for Ecology and Evolution, European Society for Evolutionary Biology, Society for the Study of Evolution, and Society of Systematic Biologists) for one massive meeting. Kudos is very much due to Howard Rundle and Andrew Simons, of the universities of Ottawa and Carleton respectively, for organising such a huge and fantastic event: it lasted 5 days, where the 8.30am-5pm sessions featured 3 concurrent symposia and 13 concurrent talk sessions, around which there were a variety of workshops, meetings, presidential addresses, talks from invited guests, poster sessions, and even a live performance of ‘The Rap Guide to Evolution‘ from Baba Brinkman! Of course, I could only manage to attend a tiny proportion of these events (especially given that I turned up to Ottawa with my own talk unfinished), but I had a great time, learned a lot, and met a load of people. The talks which I wanted to see but failed to (for various reasons) would be way too extensive to list here, but I thought I’d give brief run-downs of some of those which I did get to. This is probably going to take me a while, so I’ll spread it out over a few days.

Note that a lot of this is based on my frankly indecipherable notes which I scrawled on a tiny notebook, so the details are extremely patchy; at some point over the next year, I really need to drum up the cash for a little tablet computer. Anyone with any ideas as to how I can trick my supervisor into buying one for me, just let me know…

Susan Bertram – Integrating behavioural and nutritional ecology

I was really excited about this talk, as Sue Bertram does a lot of work in condition-dependent acoustic signalling in crickets, so there’s a pretty obvious overlap between her research and mine. A fair amount of her presentation was about how the Geometric Framework (GF), a modelling approach developed by Steve Simpson and David Raubenheimer to investigate an animal’s balancing of nutrients, can be used to further investigate the ‘black box’ that we term condition. She used some interesting examples to discuss how ‘nutrient space’ affects fitness – and provided the week’s best cricket impression – in a talk where the central message was (unsurprisingly) to propose better amalgamation of nutritional ecology with behavioural ecology. It’s certainly an important concept, and a lot of people are working on this (I managed to miss my friend Felix Zajitschek‘s talk, but he did a lot of cool stuff with the likes of Alexei Maklakov on a similar angle before turning to the dark side of drosophila *boos*). There’s some interesting stuff going on as regards the effects of phosphorus, as well as the usual suspects of protein and carbohydrate, on cricket calling effort; they also have the bonus of having the EARS (electronic acoustic recording system) II setup, which measures a whole lot more than the original EARS (which I have) does. I’m jealous. I had a chat with Sue, and her student Sarah Harrison, later in  the week at Sarah’s poster (which was also great), and I’ll definitely be keeping an eye on what’s coming out of her lab…

Russell Bonduriansky – The ecology of sexual conflict: background mortality rate can modulate male effects on female fitness

It’s always worth going to a Bonduriansky talk, because he is really, really smart. This was a pretty great one – he basically just presented something he’d been ‘thinking about a bit’, showed a model he’d made, and then had some time for questions and comments. As far as I can tell, it was an extension of a paper he published last year in the American Naturalist journal on ‘Sexual Selection and Conflict as Engines of Ecological Diversification‘; I’d recommend reading it, as I’d do a terrible job of summarising it. There were a few throwaway comments which could also have implications for lab studies: the central theme was that background mortality varies, and this modulates male effects on female fitness, but when we bring animals in to the lab then we extend lifespan (by reducing predation, some forms of stress, etc), and so we could be massively increasing sexual conflict. I’m going to go ahead and assume that Russell will have a paper out on this fairly soon, as he’s a bit of a publication machine, so definitely look out for that…

Kevin Judge – Evidence for hunger-driven hybridisation in a genus of sexually cannibalistic insects (Cyphoderis spp.)

Full disclosure: Kevin did his PhD in the great Darryl Gwynne‘s lab in Toronto alongside my supervisor, and I first met him last year when he visited Stirling before presenting at the ASAB winter meeting in London. In addition to being a great, friendly guy, and a fantastic naturalist (especially with regards to crickets and cricket-like insects), Kevin is doing some incredibly interesting work on linking micro- and macroevolution. He also has some great videos of bizarre insect sexual behaviour to show in his talks, prompting online responses such as this:

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In this talk, Kevin presented some research he’s been doing with various collaborators on several species of hump-winged grigs, cricket-like insects living in the Rocky Mountains. As with crickets, male grigs stridulate to create a calling song which attracts females; during mating, the females actually feast on the male’s fleshy hindwings… this provides them with extra nutrition, but the male cannot regrow these, so it’s in his interests to mate successfully without giving away too much (although the nutrition from this nuptial meal will help his offspring, he’s likely to achieve higher levels of fitness if he can mate multiply). Kevin showed a great video in which the male allows the female to climb on and start chomping away at him, while he attaches his ‘gin trap’: a weird genital morphology which basically clamps onto the female, clutching her to him while he transfers his spermatophore. As soon as this is done, he starts pushing her off with his hind legs, while she’s desperately trying to eat more. It’s amazing, and – via the magic of the intertrons – you can watch it here:

But it’s not just the weird mating behaviour that’s interesting here. In two species of grigs, Cyphoderris buckelli and Cyphoderris monstrosa, species which diverged a long time ago, the calling song is very similar. Kevin and colleagues discovered what appeared to be hybrids, identified by a different genital morphology from both C. buckelli and C. monstrosa, and used experiments to show that hungry females of one species would indeed mate with males of another species in order to get a meal. Perhaps, in areas harbouring both species, females of one may be attracted by the calling song of another, and will seize the opportunity to eat…  are males then making the best of this by using the gin trap to ensure that they at least get a mating from this? There’s a whole heap of really cool questions to answer with this system, and it’s definitely one to watch; check out this excellent BBC Nature article which features a bunch of Kevin’s great photos as well.

To be continued…