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Posts Tagged evolution

picking out the drunks, and other interesting tales Alison Campbell Sep 01

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A week or so back, one of the weekend papers ran a story on just how many beers someone needed to drink before they’d be legally too drunk to drive. The Significant Other & I were staggered to find that the answer was, A Lot. (Around 9, as I recall.) Speaking for myself, about 2 would do it for me – after that, I wouldn’t feel safe to drive. And yet, as Christian Jarrett points out in BPS Research Digest, most people are hopelessly bad at recognising the signs of inebriation in others.

Those of you preparing for Level 3 or Scholarship exams at the end of the year will (among other things) be learning about human cultural evolution. Some of the evidence for the development of culture comes in the form of carvings, including of the human form – the various ‘Venus’ figurines are a good example. Over at Gambler’s House, teofilo presents information on another type of representation: human effigy vases.

And on Deep-Sea News,  Kevin Zelnio writes about a beautiful arthropod fossil, new to science but very old in the scale of arthropod evolution. Just occasionally palaeontologists find spots (lagerstatten) where the fossil assemblages are rich and amazingly well-preserved. From one such site in China comes Yicaris, an ancient crustacean, and one that’s probably very close to the point at which crustacea diverged from the other arthropod lineages. (The late Stephen Jay Gould would have loved this one!)

I do like being on leave – it’s nice to have the chance to roam the science blogs more widely :)

how a fungus avoids a plant’s immune system Alison Campbell Aug 26

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ResearchBlogging.org

Your immune system is a wonderful, complex, multipartite mechanism that usually allows you to fight off the attentions of the various pathogenic organisms (bacterial, fungal, and viral) that you’ll meet during your life. I say ‘usually’ because it’s not always successful on its own, and even where it is, you can be laid low for quite some time – think of flu, but also think of measles, mumps, smallpox, polio… This is where vaccination comes in: this ‘primes’ your immune system so that it can react far more rapidly when it encounters the actual pathogens themselves. NB for a taste of some ‘alternative’ thinking on this concept, try this thread over on SciBlogsNZ.

Now, all multicellular animals have some form of immune system. Ours offers two modes of defence: an ‘innate’ immune system, plus the ‘adaptive’ system involving antibody production in response to the multitude of antigens we face each day. At the other end of the scale, things like jellyfish & sea anemones have only the innate component. For example, Hydra (a freshwater version of the more familiar sea anemones, greenish in colour due to the presence of green algae in the cells lining its gut) lacks any physical mechanisms to keep out pathogens – no thick skin, or anything along those lines. But its epithelial cells release antimicrobial chemicals & antiproteinase enzymes when they detect external antigens.

What about plants? They too have innate defence systems, including mechanical barriers against infection – waxy cuticles, and bark (cork), and also the trichomes (hairs) that you find on many leaves - that . But bark can split, & cuticles can be pierced eg by insect mouthparts – what do plants do then? It seems that when plants detect an invading organism, they release high levels of salicylic acid (the active ingredient in aspirin) in the affected tissues. This induces programmed cell death in the affected tissues, which restricts the spread of the pathogen, and also activates immune responses elsewhere in the plant – this in turn means the plant is now primed to resist futher attacks on other tissues. Salicylic acid isn’t the only chemical resonse to infection; it turns out that plants also produce an enzyme called nitric oxide synthase, which catalyses production of nitric oxide (NO) after an infection.

Now, a pathogen that can evade an organism’s immune system for any length of time is going to be at a selective advantage, and so you get a form of arms race, where hosts with the ability to detect & respond to such a pathogen are in turn likely to have better odds of survival, & so on. Some strains of the bacterium Staphylococcus, for example, are able to wrap themselves in strands of the protein fibrin (which they obtain from the host’s blood), which may make them much harder for the host’s immune cells to destroy. (Alas for the patient – this ability is also linked to clotting; Not Good at all.)

Like animals, plants use ‘pathogen-associated molecular patterns’, or PAMPS, as the basis for identifying pathogens (de Jonge et al., 2010), so a pathogen that can somehow hide these from a plant would be at an advantage. The range of potential PAMPS – detected by receptors on the plant cell surface - includes lipopolysaccharides, peptidoglycans, a protein called flagellin, sugars typically found in fungal cell walls – & chitin, a major constituent of cell walls in fungi. Plants with damaging mutations in these receptors would potentially be more susceptible to attack by bacteria & fungi.

De Jonge & his colleagues studied  the cause of leaf mould in tomatoes, a fungus called Cladosporium fulvan. When this fungus is moving into the inside of a leaf, among the proteins it releases is one that protects the fungal cells from plant enzymes called chitinases, which would otherwise break down the fungus cell walls. Actually there’s more to it than that – when chitinases hydrolyse fungal cell walls, this releases molecules that appear to act as PAMPs & so stimulate the plant’s immune defences.

Another protein, called Ecp6, seemed to be needed for the fungus to be really effective at infecting tomato plants. Looking this more closely, the team found that Ecp6 doesn’t affect chitinase release but appears to tidy up other proteins released by the fungus, so that they aren’t floating around & able to be detected by the plant’s defences. So, because the host’s immune system doesn’t kick in, C.fulvan is able to grow more rapidly within the plant’s tissues. And It turns out that the genes controlling Ecp6 production are widespread in fungi – perhaps one outcome of the plant-fungus arms race. (And other example of how plants are considerably more complex than many of us would think.)

de Jonge R, van Esse HP, Kombrink A, Shinya T, Desaki Y, Bours R, van der Krol S, Shibuya N, Joosten MH, & Thomma BP (2010). Conserved fungal LysM effector Ecp6 prevents chitin-triggered immunity in plants. Science (New York, N.Y.), 329 (5994), 953-5 PMID: 20724636

fungal parasites & zombie ants Alison Campbell Aug 23

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ResearchBlogging.org

Parasites are ubiquitous. I remember watching a video (years ago, while I was teaching at secondary school) about parasites that make humans their home. Lice, eyelash mites (yes, really!), various intestinal worms… I tell you, I had psychosomatic itching for days after seeing that! Then I got my hands on Carl Zimmer’s wonderful book, Parasite Rex – as well as learning all sorts of stuff about parasites & how they live, I also had it brought home to me that parasites aren’t just some sort of passive, undesirable house guest – in many cases they actively influence the host’s behaviour in ways that enhance the parasites’ ability to complete their life cycles.

I was alerted to a recent paper in this area by a blog post from another Kiwi blogger: his sub-header was ‘zombie ants controlled by parasitic fungus for 48 million years’, which reall y took my fancy (the link will take you to a story in the Guardian, of which more later in this post). The authors of this paper (Pontoppidan et al. 2010) point out that it’s not just a case of the parasite affecting individual ants – they can structure the entire host population in terms of its distribution in time and space & thus influence their own distribuiton: the parasite’s ‘extended phenotype’, if you will.

The authors kick off by listing some rather dramatic ways in which other host species are influenced by their parasites, such as behavoural changes that make them more susceptible to predation, thus enabling the parasite to move to its next host; or effectively drowning themselves, which lets the adult stage of the parasite reproduce. (Their full list’s available in the PLoSOne paper.) All this raises interesting questions about just how this manipulation of host behaviour is achieved, & the effects of such parasitism on the species’ population as a whole (it’s obviously a Bad Thing for the indivdiuals concerned). Pontoppidan & her colleagues asked a further topic: the impact of infection on the host species’ distribution in space & time. They chose to look at the fungal parasite Ophiocordyceps unilateralis , and a tropical species of carpenter ants (Camponotus leonardi.).

This is really cool stuff (in a gruesome sort of way). An ant picks up the sticky fungal spores by walking over them on the forest floor; fungal hyphae then penetrate the unfortunate animal’s cuticle & extend throughout its body. It can be just a few days from infection until death. Once the ant’s dead, the fungus grows a ‘fruiting body’ out the back of its host’s head. This produces large spores, too big & heavy to spread on the wind. Instead they fall to the forest floor, produce & release secondary spores, a hapless ant comes along… and the cycle repeats itself. So far, so good (for the fungus), but the really interesting part is that the ants don’t die just anywhere, nor do they simply turn up their toes & drop dead on the ground. 

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Ants biting the underside of leaves as a result of infection by O. unilateralis. The top panel shows the whole leaf with the dense surrounding vegetation in the background and the lower panel shows a close up view of dead ant attached to a leaf vein. The stroma of the fungus emerges from the back of the ant’s head and the perithecia, from which spores are produced, grows from one side of this stroma, hence the species epithet. The photograph has been rotated 180 degrees to aid visualization.
 
From: Pontopiddan et al. PLoS ONE. 2009; 4(3): e4835. doi: 10.1371/journal.pone.0004835
 
Instead, before an ant actually dies it bites into the surface of whatever plant it’s standing on at the time. Pontopiddan et al. identify this behaviour as the fungus’s extended phenotype: it holds the ant’s corpse in place on the plant for long enough that the fungus can secrete a ‘glue’ that will stick the body there more permanently, which in turn gives time for the fungus to develop its fruiting body (the ’stroma’ & ‘perithecia’ in the images above). What’s more, the team had heard accounts of ‘graveyards’ containing large numbers of dead carpenter ants (cue images of zombie ants staggering along to some formicine cemetery). So they decided to determine whether these graveyards really do exist and, if they do, how various biotic & abiotic factors influenced the distribution of dead ants.
 
To do this they spent more than 5 weeks & >500 person-hours in a Thai rainforest, looking for ants. (This wasn’t quite needle-in-a-haystack territory as these ants can be >4mm long, but still…) In all this time they found 2243 dead ants in their study plots (the great majority of which were Camponotus leonardi), but only 2 live C.leonardi. But there were lots of living ants from other species, doing what ants do, in the study area – which suggested that leonardi was definitely the main host for Ophiochordyceps unilateralis. It was 3 weeks before they saw an active trail of leonardi, which descended one tree & travelled only 5m on the ground before heading up another trunk, followed by yet another descent before disappearing into the canopy again. That trail led to a single leonardi nest, high in the canopy (20-25m above ground), with a network of trails running along twigs & branches & extending up to 100m from the nest.
 
On the basis of these observations, the team hypothesised that ants of this particular species actively avoid descending to the forest floor unless it’s the only way to reach a new resource. (You can see how natural selection might achieve this: a colony where too many ants go down to the ground on an everyday basis is likely to lose large numbers of foragers.  So if there’s a genetic underpinning for such behaviour, a queen passing on a ‘go to ground’ gene would end up losing lots of her daughters & thus her nest would be at a competitive disadvantage to other colonies.)  It turns out that there is some evidence supporting this hypothesis: in an area of forest where the parasitic fungus isn’t present, C.leonardi is commonly found at ground level.
 
When the research team went on to look at just where the dead ants were found, it appeared that the bodies weren’t randomly distributed. Instead they were in large aggregations (the ‘graveyards’) of up to 26/m2, separated by corpse-free zones. The now-deceased had bitten onto the undersides of leaves, on average about 30cm above the ground – an example of how the fungus influences its host’s behaviour. The distribution of dead ants appeared to be related to temperature & absolute humidity – things which could influence the survival of fungal spores & thus the chances of an individual ant picking up the infection.
 
Zombie jokes aside, this really is a fascinating example of the complexity of ecosystem interrelationships. And their longevity.  It also turns out that this particular parasitic relationship may have been in place for a very  long time indeed. The ‘death bite’ leaves a characteristic scar on a leaf, and in a separate paper David Hughes & colleagues describe finding just such a scar on a leaf dating back 48 million years, from rocks in what is now Germany.
 
 
 
 

 
Hughes, DP,  Wappler , T & Lanadeira, CC (2010) Ancient death-grip leaf scars reveal ant-fungal parasitism. Biology Letters. Published online before print August 18, 2010, doi: 10.1098/rsbl.2010.0521
 

Pontoppidan MB, Himaman W, Hywel-Jones NL, Boomsma JJ, & Hughes DP (2009). Graveyards on the move: the spatio-temporal distribution of dead ophiocordyceps-infected ants. PloS one, 4 (3) PMID: 19279680

 

a solar salamander Alison Campbell Aug 17

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This is a new story & potentially a very exciting one (& I must thank Grant for drawing this story to my attention!). A Nature News item (Petherick, 2010) describes the discovery of green algae apparently living within the cells of salamander embryos. I’ll wait with interest for the published paper, but if this finding’s confirmed then it will be the first recorded instance of endosymbiosis in a vertebrate.

The story’s based on a conference presentation by Ryan Kerney, who noticed that the salamander embryos he was studying were a bright green – and not because of algae living on the tiny animals’ skin. This was exciting stuff. Apparently scientists have known for a while that spotted salamanders & the unicellular alga Oophila amblystomatis have a symbiotic relationship. (In fact, Oophila doesn’t live anywhere else, apart from in association with the eggs of a few other amphibians.) Like most frogs, salamanders lay their eggs in water. Here the animal’s urine provides nitrogenous compounds that promote algal growth, while photosynthesising algae living on & in the jelly surrounding the embryos raise the levels of oxygen in the water, whcih would support a higher respiration rate in the tiny salamanders.

However. Kerney found that the algal cells were actually growing inside the cells of his embryo salamanders. (I suspect he would have rubbed his eyes & looked again, on first spotting this one.) This is unexpected – what you’d anticipate is that any algae somehow getting into a vertebrate’s cells would be pretty quickly picked off by the animal’s immune system, which is able to distinguish between ’self’ and ‘non-self’ cells & pick off any intruders. So an interesting question for future research would have to be, what’s the mechanism that’s allowed the algae to penetrate & survive within the amphibian’s cells – how has it overcome/avoided the animal’s immune response? 

A related question is, how do the algae actually physically get into the salamander cells? The Nature News item suggests this might happen when the cells are releasing nitrogen-rich waste products, but as the release of these compounds from individual cells would be very much on the subcellular scale, it’s hard to visualise how this would provide an opening for the algae.

And of course, do the embryo salamanders gain photosynthates from the algae living within them? Work by Hutchison & Hammen, way back in 1958, showed that salamander eggs that lack algae in their jelly casings hatch more slowly. And it’s been known for a long time (e.g. Cates, 1975) that invertebrates such as corals and jellyfish gain photosynthates from their endosymbionts, as does the sea slug Elysia chlorotica  . But if it turns out that the embryos are actually able to utilise the sugars produced by the photosynthesising algae, this would be a first. There are suggestions in Kerney’s conference paper that this might be happening: micrographs that show salamander mitochondria sitting close to the internalised algae. The next step here would be to demonstrate that sugars released by the algae were indeen being taken up and used by the animal’s mitochondria.

A solar-powered vertebrate? Perhaps – but there’s a lot of work needed here yet.

N.Cates (1975) Productivity and organic consumption in Cassiopea and Condylactus. Journal of Experimental Marine Biology and Ecology 18(1): 55-59. doi:10.1016/0022-0981(75)90016-7

V.H.Hutchison & C.S.Hammen (1958) Oxygen utilisation in the symbiosis of embryos of the salamander, Ambystoma maculatum and the alga, Oophila amblystomatis. Biological Bulletin 115: 483-489.

A.Petherick (2010) A solar salamander. Nature News published online 30 July 2010, doi:10.1038/news.2010.384  

azolla & endosymbiosis Alison Campbell Aug 01

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ResearchBlogging.org

There are other photosynthesisers besides Volvox, living in our fishpond. Bigger plants include waterlilies, various sedges, & Elodea. And at this time of year the surface is covered by a carpet of duckweed, but when summer comes the Azolla will tend to take over. Sometimes called ‘water fern’, Azolla contains an endosymbiont, a cyanobacterium (blue-green alga) that lives within the plant’s ‘body’ but not within its cells (Ran et al. 2010).

This cyanobacterium is actually a fairly recent fellow-traveller for the fern. Almost all eukaryotes (with the exception of Archaezoans like Giardia) contain intracellular endosymbionts – the mitochondria. These & the chloroplasts of plants formed as the result of endosymbiotic events that occurred perhaps two billion years ago, when the free-living ancestors of these organelles were engulfed by other prokaryote cells but for some reason weren’t digested. Instead, they continued to do their thing, churning out sugars (in the case of the proto-chloroplasts) & ATP well in excess of what the ‘host’ could generate alone (mitochondria). Lyn Margulis developed this endosymbiotic theory for the origins of mitochondria & chloroplasts on the basis of a range of observiations: both organelles contain their own, circular, DNA & (just like bacteria) are able to manufacture their own proteins; their ribosomes & tRNA molecules are like what you’d find in bacteria; and they’re enclosed in a double membrane. Interestingly, many of the genes that would once have been on that circular chromosome of a mitochondrion or chloroplast have ended up in the ‘host’ cell’s nucleus – the host can to some degree control the organelles’ functioning. (Ran et al. note that the chloroplast genome is one of the smallest known, at only 150-200,000 base pairs long.)

Ran & his colleagues were keen to delve further into the process of endosymbiosis as it relates to chloroplasts in plants. To do this, they chose to study a cyanobacterium living inside a species of Azolla – inside, but outside the actual Azolla cells, tucked away into litle ‘cavities’ in cells on the water fern’s upper surface. While there are other symbioses between plants & cyanobacteria, this one’s unusual on two counts: the cyanobacterium involved can’t grow outside the host, & it’s passed on from one generation of Azolla to the other (’vertical transmission’). The oldest fossils of Azolla date back 140 million years, & it’s possible that this endosymbiotic relationship goes back that far in time, The team hypothesised that “… genome reduction may… act on cyanobacteria in symbiosis with plants”, mirroring what appears to have happened in the evolution of chloroplasts.

Figure 1 from Ran et al. (2010): A) fronds of Azolla filiculoides;. B) Close up of the upper surface of an Azolla branch. C) Light micrograph of the cyanobiont. The larger cells represent nitrogen-fixing heterocysts. Scale bar = 5 µm. D) Transmission electron micrograph of the cyanobiont. E) A snap-shot in the vertical transmission process of the cyanobiont between Azolla plant generations, using fluorescence microscopy. Pairs of megasporocarps (blue) develop at the underside of the cyanobacterial colonized Azolla leaves. Filaments of the motile cyanobacterial cell stage (red), the hormogonia (h), are attracted to the sporocarps, gather at the base and subsequently move towards the tip, before entering the sporocarps via channels (white arrows). Once inside the sporocarp the hormogonia differentiate into individual thick walled resting spores (or akinetes; ak), seen as the intensively red fluorescing small inoculum on top of the megaspores (sp).

The team sequenced the cyanobacterium’s genome – and found it to be ‘eroding’. Thirty-one percent of its genes are pseudogenes (they’re either not transcribed, or they don’t produce functional proteins), & there are a lot of transposons – ‘jumping genes’ - in the genome. Significantly, some of the genes that are essential for a free-living bacterium have been ‘pseudogenised’, which means that the cyanobacterium must be dependent on the Azolla for things like DNA repair proteins. The ‘DNA replication initiator’ gene is also pseudogenised,  which is important as it means that Nostoc azollae can divide & grow only very slowly. And the same is true for genes involved in glycolysis and taking nutrients into the cell.

On the other hand, the cyanobacterial genes involved in nitrogen fixation are still functioning well ie the cyanobacterium is able to differentiate to produce heterocysts, where nitrogen-fixation occures, meaning that the symbiont is a key source of nitrogen for its plant host. There’d be quite strong selection pressure for continuing this endosymbiotic relationship: nitrogen is a limiting factor for plant growth, and the host (the Azolla) would gain a big selective advantage over competitors that lacked N-fixing endosymbionts.

The team conclude that over time, the Nostoc azollae genome will erode to the point that ‘ulimately may cause NoAz to resemble a plant organelle (devoted to nigtrogen fixation) more than a free-living organism.’  They also point out that for such organelles to evolve, there would initially have to have been some form of vertical transmission process (seen in this example) & eventually  the symbiont would become intracellular.

So perhaps, in the relationship between Azolla filiculoides and its cyanobacterial partner, we are looking at the evolution of a fully endosymbiotic relationship.

L.Ran, J.Larsson, T.Vigil-Stenman, J.A.A.Nylander, K.Ininbergs, W-W.Zheng, A.Lapidus, S.Lowry, R.Haselkorn & B.Bergman (2010). Genome erosion in a nitrogen-fixing vertically transmitted endosymbiotic multicellular cyanobacterium PLoS ONE, 5 (7) : 10.1371/journal.pone.0011486

from elephantiasis to sperm competition Alison Campbell Jul 20

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Well, it’s not too great a leap, is it? I thought of this post because over on the Sciblogs copy of my last item we started talking about sperm competition. We got there via Drosophila bifurca.

The male D.bifurca produces the longest sperm of any animal – an amazing 58mm. Most of that’s tail. Each sperm is rolled into a tiny ball within the male’s seminal vesicle (part of his reproductive tract) & then, when he mates, fired like a pea from a peashooter into the female’s vagina.

As you can see from the following image, in D.bifurca the male & female gametes are of broadly similar size (in technical terms, they are isogamous). 

a, Scanning electron micrograph (SEM) showing a single, 6-cm D. bifurca spermatozoon dissected from the seminal vesicle, where sperm are individually rolled into compact balls. b, SEM of a single D. bifurca sperm (copied six times) next to an SEM of a D. bifurca ovum at the same magnification. Micrographs by R. Dallai. from Bjork & Pitnick, 2006.

And these long sperm are produced in very long, relatively large testes: in D.bifurca they make up around 11% of the male’s body weight. There’s a trade-off here, though; such big sperm are costly to make & so relatively few are produced. A male bifurca produces only 6 sperm for every egg that a female makes.

The testes of Drosophila bifurca fruit flies make up 11 percent of the dry body mass of the male. In this image, a male is

The testes of Drosophila bifurca fruit flies make up 11 percent of the dry body mass of the male. In this image, a male is “surrounded” by an uncoiled testicle dissected from a male of the same size. Credit: Romano Dallai

Normally sperm are much much smaller than the eggs that they fertilise (anisogamous), as in the following example of a human sperm & egg. In animals with anisogamous gametes the male produes huge numbers of these tiny sperm, which increases the odds that one of them will actually make it to the egg. In these circumstances there’s significant male: male sexual selection, via sperm competition – the male producing sperm that swim longer, or are more active, is more likely to fertilise the female & so any genes relating to sperm size, stamina (if that’s the right word!) & activity will tend to be selected for. What’s more, in these circumstances eggs will be quite rare & thus ‘valuable’, so there’ll be strong male:male competition to be the one to fertilise them.

Bjork & Pitnick note that in an animal like D.bifurca, where sperm & egg are much closer in size, & males produce fewer sperm, you’d thus predict a reduction in competition. So bifurca provded an excellent test case for this prediction.

Now, when I first heard about D.bifurca (via an item in the book Blue genes & polyester plants, the story went that much of an individual sperm’s tail was left hanging around outside the female’s body, with only the head of the sperm making it anywhere near the egg membrane. This could be an example of sperm competition, where the sperm tail from a successful mating blocks the female’s vagina & makes subsequent fertilisation less likely. What actually happens is even more interesting: the sperm moves up into the female reproductive tract until it’s entirely housed within her body: her reproductive tract is around 60 mm long, lying like a loosely coiled spring within her abdomen.Bjork & Pitnick argue that the exceptionally long sperm of male birfuca are the result of intense sexual selectio, where only the longest sperm get the egg. Sperm competition may still be operating here, as it’s going to be harder to swim up to the egg if there’s already another sperm in there, blocking the way.

In some species sperm competition is a lot less subtle. In black-winged damselflies the tip of the male’s penis is shaped a bit like a brush. Females take multiple mates, & when the latest male comes along, before he actually inseminates the female he pumps his penis in & out of her reproductive tract, brushing out most of his competitiors’ sperm as he does so. Chimpanzees – which are promiscuous – simply produce huge amounts of sperm: the male getting the most sperm inside a female chimp will be the most likely to successfully fertilise her (always assuming that sperm motility & viability are similar in all males involved. (Those massive quantities of sperm are produced in commensurately-large testes. A pair of testes in a male chimp weigh around 120g, while in the much larger gorilla – where a single male has a harem of females & mates with them exclusively, relatively undisturbed by other randy males – the testes weigh in at only 30g.)

But wait, there’s more. The butterfly Cressida cressida takes sperm competition to even greater lengths. After mating successfully, the male applies a type of cement to his partner’s vaginal opening, blocking it up so that no other males can get in. Insect chastity belts, anyone?

A,Bjork & S.Pitnick (2006) Intensity of sexual selection along the anisogamy-isogamy continuum. Nature 441: 742-745  

positive allometry & the prehistory of sexual selection Alison Campbell Jul 06

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ResearchBlogging.org

Thanks to herr doktor bimler & the University’s science librarian, I now have my hands on two copies of the paper I mentioned a couple of posts ago: Positive allometry & the prehistory of sexual selection (Tomkins et al., 2010). The term ‘allometry’ refers to the relationship between the size of an organism & the size of various parts of that organism. When scientists study allometry, they might do this for various stages in the growth of an individual, or they might compare different organisms of the same species, or individuals from separate species. ‘Positive’ allometry means that as body size increases, so does the size of whatever other feature’s being examined. (The Panda’s Thumb has an example of this, for body size: brain size ratios in primates.) The paper by Tomkins et al. looks at the spiny ’sails’ on the backs of pelycosaurs & the crests on pterosaurs’ heads, and suggests that differences in size between male & female individuals is related to sexual selection.

When I first saw the newspaper report on this article, I wondered: just how did the authors identify male & female pelycosaurs & pterosaurs? With modern reptiles size can be a cue, as can colours & crests & inflatable throat pouches. (Sexual dimorphism is widespread in reptiles, but unfortunately it’s not consistent: in some groups the males are larger, but in others it’s the females.) And of course, hypotheses about which sex you might be looking at can be confirmed by watching to see who’s on top during mating. And also by dissection: the ‘who’s on top’ method wouldn’t work for some whiptail lizard species, where all individuals are female & reproduction is by parthenogenesis. In these species, individuals take on the ‘male’ or ‘female’ role during mating depending on the levels of oestrogen & testosterone in their blood.  So I was interested to know how the authors determined whether they were looking at a male or female in dealing with any particular indivdiual.

The ’standard’ explanation for pteranodon crests & pelycosaur sails is that they were related to thermoregulation. Pelycosaurs were unlikely to be endotherms, generating heat internally as a side-effect of a high metabolic rate, & so like modern reptiles would have had to bask in the sun to warm their bodies before dashing around chasing prey (or avoiding being prey). As Tomkins & his colleagues point out, a ’sail’ of tissue supported by vertebral spines, with blood vessels running through it, could have warmed the animal’s blood more rapidly – a bit like a solar panel used to heat water. It could equally have acted like a car radiator & shed excess heat. (I need to add that not all pelycosaurs had these sails.) Something similar’s been proposed for pteranosaur crests, although here there are other hypotheses, including acting as rudders in flight, or in courtship displays - in which case there could have been sexual selection operating. Certainly sexual selection can generate some quite extreme traits – the peacock’s tail is just one example. Tomkins et al. comment that “[p]ositive interspecific allometry occurs in the sexually selected traits of a range of [living species]“, & hypothesise that significant allometry in these two extinct taxa may also be explained by sexual selection.

For their examination of allometry in pterosaurs they used 9 skulls from Pteranodon longiceps - none of these skulls were associated with other skeletal bits & pieces from which to obtain a measure of body size. So the assumption here must be that the bigger heads/crests came from bigger-bodied individuals. Some support for this assumption came from an examination of the size of the eye socket: as you might expect this increased in size as the skulls got bigger – but in living reptiles the diameter of the eye socket is directly correlated with an increase in body size as well. However, as herr doktor pointed out in my earlier post’s comments, the individual pterosaurs are described as ‘putative’ males (N = 6) & females (N = 3) i.e. they’ve been tentatively classified as male & female on an unspecified basis. If the classification was based on size, then this is something of an a priori assumption that could colour the results. In addition, the smallest ‘putatively male’ skull is the same size as the 3 ‘putatively female’ skulls, with the same sized crest. It’s hard to see strong evidence of sexual selection in these data. I wonder if an alternate possibility could be an age series? Reptiles do increase in size as they age, within species-specific limits, so smaller skulls & crests could simply be those of younger indivdiuals.

In their analysis of Dimetrodon, the team didn’t have access to enough individuals of one species to look at intraspecific allometry. Instead, they used data from 7 Dimetrodon species, & found that the sail size did increase with an increase in body size from one species to another. But sexual selection – again, I’d like to see an explanation of just how the authors determined the sex of the individual animals concerned, & how they ruled out the possibility that they were looking at age-related size differences within particular species. Having said that, the authors do note that the sails of smaller dimetrodonts would not have had much positive effect on thermoregulation & might actually have been a thermoregulatory liability, radiating heat so fast in cooler conditions that the animals would have chilled very rapidly.

Basically, the sexual selection hypothesis as an explanation for the adornments of Dimetrodon & Pteranodon is an interesting one, but we need to see data from many more specimens, & a clear method of sexing the remains, to test it further.

Tomkins JL, Lebas NR, Witton MP, Martill DM, & Humphries S (2010). Positive Allometry and the Prehistory of Sexual Selection. The American naturalist PMID: 20565262 doi:10.1086/653001

And I see that Brian Switek has beaten me to it… 

repeat after me – pterosaurs were not dinosaurs! Alison Campbell Jul 02

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From yesterday’s RSNZ news headlines:

Size mattered for flying dinosaurs: New research into pterosaurs and pelycosaurs shows their grand headcrests and sails were developed to attract a mate, not to regulate body temperature as first thought.

Now, you may think me a palaeontological pedant (& this is not a cue for cracks about my age from the cheap seats!), but I find that loose use of the word ‘dinosaur’ really irritating. Why? – because neither pterosaurs nor pelycosaurs were dinosaurs. I know the headline’s more eye-catching than saying ’size mattered for prehistoric flying reptiles’, but still, it bugs me.

So what’s the difference?

Pelycosaurs – animals like Dimetrodon & Edaphosaurus - pre-dated the dinosaurs by a considerable length of time. They first appeared in the Carboniferous period (around 300 million years ago – here’s a link to a geological time scale) & were the dominant animal group during the Permian (280-260 mya). So what, you say. They could have been ancestral to dinosaurs. Well, no, they weren’t – & the evidence for this lies in their skulls. These splay-legged reptiles, some of which sported great ’sails’ on their backs, were synapsids – a term reflecting the presence of a single opening (behind the eye) in the dermal bone of the skull. (Have a look at the picture at that link.) Mammals are also synapsids, & this means that the mammalian lineage has its roots much further back in time than that of the dinosaurs, who first appeared on the scence in the Triassic. What’s more, dinosaurs are diapsids – a feature that they share with birds & other living reptiles. (Which is why modern phylogenetic trees see birds branching off the reptiles rather than having a branch of their own, which is the way I was taught it way back when.)

The name ‘pterosaur’ means ‘winged lizard’, but these prehistoric flying reptiles are only distantly related to modern lizards. These elegant fossils first turn up in rocks from the Triassic, but their complexity suggests that the group is probably older than that; it’s just that the fossils haven’t been found. Scientists used to think that pterosaurs were gliders, but the consensus has shifted to an acceptance that most (with the exception of the really big ones such as Quetzocoatlus) were capable of powered flight. (The evidence for this includes the fact that the bones of pterosaurs were hollow – like those of birds – and the forelimb bones had prominent muscle-attachment crests, suggesting the presence of flight muscles.) There’s also a suggestion, based on what looks to be fur on the bodies & flight membranes of some particularly well-preserved fossils, that at least some pterosaurs were endotherms ie capable of generating & maintaining their own body heat.

As for the dinosaurs, which evolved during the Triassic (around 225 mya)… I remember that Stephen Jay Gould once commented – as a throwaway line in one of his books – that people have a fascination with dinosaurs made all the more delicious by the fact that they were very fierce, very big – & very dead. (I wish I could remember the actual book I read that in; no time right now to go through my bookcase…) Of course, he would also have noted that this fascination is based on a misunderstanding of dinosaurs: they ranged in size from the giant sauropods like Diplodocus & the even larger titanosaurs, through the smaller but still large-by-human-standards tyrranosaurs, down to animals roughly the size of a turkey. I strongly suspect they’d have varied in ferocity as well :-)

If asked how to distinguish between dinosaurs & other diapsids, a palaeontologist would look at things like the anatomy of the legs. The earliest dinosaurs were bipeds, walking erect on their hind legs. In other words, the early reconstructions of some dinosaurs that showed them as quadrupeds with legs akimbo were a long way from reality. They walked on their toes, with the soles of their feet raised off the ground, and the hind legs in particular were pulled in under the body – essentially the same as in modern birds. In addition, the hip socket had a small opening in it where the 3 bones that make up each side of the pelvis come together. (Features of the hip also allow us to divide dinosaurs into 2 main groups – the ‘lizard-hipped’ saurischians & the ‘bird-hipped’ ornithischians. The latter is something of a misnomer as birds are actually most closely related to a group of saurischians.)

So – dinosaurs, pterosaurs & pelycosaurs are 3 related but distinct groups; conflating them into one ignores all their fascinating differences & oversimplifies our view of the past.

(I’d like to be able to comment on the actual paper on which that headline was based. Unfortunately, while we supposedly have an institutional subscription, at present the website keeps asking me for $US, so until that’s sorted out a review will have to wait.)

 

why an evolutionary image merits a ‘fail’ Alison Campbell Jun 29

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ResearchBlogging.org

 

 

 

Last year I commented that the following image, while funny, was a ‘fail’ in scientific terms:

evolution of the cat.jpg

A recent commenter asked, so is this image scientifically correct or incorrect? (My first thought was that teh lolcat at the end should be a clue…) 

But no, it’s not scientifically correct (lolcats aside). It’s another in the long line of images of ‘evolutionary iconography’ that portray evolution as an inexorable march towards some sort of progress – a generalisation that isn’t particularly helpful in explaining how evolution actually works.

It’s not good on the particulars of feline evolution, either…

The word ‘cats’, in its broadest sense, encompasses 38 different living species, which fall into 8 major groups comprising 11 genera (Johnson et al., 2006). All the extant species have evolved relatively recently: a combination of fossils & DNA analyses suggest their radiation began no more than 11 million years ago (mya) in the late Miocene (ibid.). The earliest divergence (10.8 mya) was between the lineage leading to the ’big cats’ (lion, tiger, leopard, jaguar, snow leopard & clouded leopard) and ‘the rest’. In other words, domestic cats are not particularly closely related to lions, despite the iconography above.

Taxonomists have found classifying the various felids a difficult problem, due to the paucity of recent fossils (notwithstanding the classic sabre-toothed cats of the Pleistocene), a shortage of distinctive skeletal features, & some confusing distirbution patterns. Johnson & his team obtained sequences from autosomal and X- and Y-linked genes, plus mitochondrial DNA, for a total of 39 gene segments, which they then compared across all living cat species. A group of 7 distantly-related species – including hyaenas, which are more closely related to cats than to dogs - made up the ‘outgroup’, something that’s used in a phylogenetic analysis in order to distinguish between ‘ancestral’ & ‘derived’ features. (Basically, if a feature is found in the outgroup as well as the group of interest, then it’s likely to be ancestral & so won’t be particularly informative about patterns of evolution in your study group.) And the molecular dates were calibrated using 16 sets of fossil remains. 

The team found that the 8 major cat lineages evolved relatively quickly, over about 4.6 million years. Between6.4 & 2.9 mya these lineages in turn underwent a fair bit of adaptive radiation, at a time when sea levels were around 100m higher than they are at the moment. There was another burst of divergence 3.1-0.7 mya which produced 27 of the extant cat species. This was at a time when sea levels were on average relatively low.

The sea level part is important, because during periods of low sea level it would have been possible for species to migrate via land bridges into previously inaccessible areas. Based on their molecular data & available information on sea level changes, Johnson et al. suggest that modern cats evolved in Asia with that divergence between the big cats (Panthera) and all other feilds.Somewhere between 8.5 & 5.6 mya the ancestors of caracals, servals & golden cats arrived in Africa. Then, between 8.5 & 8.0 mya felids arrived in North America for the first time via the Bering Strait land bridge. This immigrant group seems to have been the common ancestor to ocelots, puma, leopard cats, lynxes – and the domestic cat. When the Panamanian land bridge formed 2.7 mya this opened up more new ecological opportunities for the feline explorers.

Subsequently there were other migrations back from the Americas to Eurasia & then further west. Cheetahs, for example, are now found in Africa, but the genetic analyses by Johnson’s team indicate that their closest relatives are the North American pumas. Similarly members of the genus Felis must have crossed back into Eurasia at least once, given that the domestication of the common moggy seems to have occurred in the Near East, at about the same time that agricultural settlements were developing in the Fertile Crescent (Driscoll et al., 2007) (Other American species moved across the Bering land bridge to Eurasia, & hence Europe, at various times – most notably the various horse species. The fossil remains of this particular sequence of species migrations were interpreted by T.H.Huxley as evidence for a European origin of the horses, a view he rapidly & happily relinquished when presented with evidence of the horses’ long evolutionary history in America.)

Once more – that simple linear iconography is not a scientific representation of feline evolution, and a long way from the much more complex and fascinating reality.

C.A.Driscoll, M.MenottiRaymond, A.L.Roca, K.Hupe, W.E.Johnson, E.Geffen, E.H.Harley, M.Delibes, D.Pontier, A.C.Kitchener, n.Yamaguchi, S.J.O’Brien & D.W.Macdonald (20008) The Near Eastern origin of cat domestication. Science  317: 519-523

Johnson, W., E.Eizirik, J.Pecon-Slattery, W.J.Murphy, A.Antunes, E.Teeling & S.J.O’Brien (2006). The Late Miocene Radiation of Modern Felidae: A Genetic Assessment Science, 311 (5757), 73-77 DOI: 10.1126/science.1122277

another early hominin specimen, & other things to read Alison Campbell Jun 27

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I’m catching up on my reading of other people’s blogs, so here are some interesting posts to share with you.

At Laelaps Brian Switek has commented on the latest fossil hominin find. Dubbed ‘Kadanuumuu’ (or ‘Big Man’), this is a partial Australopithecus afarensis skeleton.Kadanuumuu was much larger than the more familiar (& more recent) ‘Lucy’, & because of this & because of features of the pelvis, the scientists who described the remains feel they were probably those of a male. There’s also the suggestion (see the comments thread for Brian’s article) that these remains may overturn the current hypothesis that afarensis’s ribcage was funnel-shaped. Or may not – we probably need more data on this one.

There’s an interesting discussion on Pharyngula  around the separation of science & belief. Part of the post, & the ensuing comments thread, focus on a post by another blogger that appears to be making an argument for students’ personal beliefs to count as valid answers in science exams. Every now & then I’ve seen a student answer a question in this way, rather than giving a reasoned scientific response to said question. In each case I have marked them down, & it’s not because I deny students the right to personal belief systems. It’s because the question has been science-based, & that’s what I expect the answer to be as well. Anyway, the post & discussion are interesting & thought-provoking.

And the Silly Beliefs team have taken a critical look at a recent item on ‘60 Minutes’ that took an extremely credulous stance on the issue of UFOs & alien visitations. I had wondered whether to watch the program but the promos made me think that this would do damage to my blood pressure. Presenting information that turns out to be at least a decade old as something new & exciting doesn’t strike me as particularly good journalism…

Enjoy :-)

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