Reviews Archives - Page 3 of 4

18/02/2013

Palaeo-poetry and placental mammals

Recently Science published O’Leary et al.’s – new load of oil to fuel the burning debate on the origins of placental mammals.

Just to be clear: there is an important distinction between mammals in general that includes many fossils from the Jurassic as well as the extant platypus, kangaroo and your grandma; and placental mammals that includes your grandma and the armadillo (but not kangaroos or the platypus) and no fossil before 65Myr. For readers that are not used to the debate concerning the first placental mammal here’s the main question: did the first placental mammals diversify before or after 65Myr (the important KT boundary)?

(1) After Katie

One view suggests that early mammals lived in the shadow of dinosaurs and the demise of these mighty creatures allowed our rat-like ancestors to take over the earth, the seas and the skies “Save yourself mammals”. This idea was proposed by Simpson in the 1950’s and is supported by the fossil record; many dinosaurs (both big and small) were present before the 65 Myr KT boundary, then a catastrophic meteorite impact marked the KT limit and placental mammals radiated after that. This sequence of events seems to be very straightforward but reality appears not to be so simple. Increasing numbers of mammal species from the late Cretaceous are being discovered, (including rather big ones feeding on dinosaurs) and not many species in general are found in the fossil record before 55Myr when all groups of placental mammals seem to suddenly appear (for a full story see Luo’s 2007 Nature review).

(2) Before Katie

In contrast, another group of people, mainly assisted by molecular dating methods, found out that post-KT placental mammalian diversification may just be an artefact of the fossil record (like Meredith et al 2011 in Science again). Their DNA evidence seems to say that placental mammals evolved before the KT limit and that either palaeontologists failed to find them or else the fossil record failed to preserve them. One major criticism that moderate people argue is that there are still problems associated with molecular dating methods. I won’t go into the details (yes I’m trying hard not to) but molecular dating relies on DNA on the one hand (sampling quality and modelling) and on the fossil record on the other hand. So if the people using DNA criticise the fossil record and want to improve the DNA dating estimates, they have to rely on the same fossil record that they are criticising. The snake bites his own tail.

So what about O’Leary and colleague’s paper? They basically support the first theory (placental mammals evolved after KT). Fair enough, it was led by a number of great palaeontologists and based on a massive morphological data set (~4500 characters introduced as phenomics (from the phenotype) as opposed to genomics (from the genome) data) collected on 40 unambiguous fossils and 46 extant placental mammals. Genomic data based on 26 genes of these extant placental mammals was also included. This paper is the result of an impressive and unique collaborative work, but – Ned Stark from Games of Thrones said “nothing someone says before the word “but” really counts” – but this paper is criticisable…

First of all, the data set: although the morphological data is impressive, the taxa sampling effort seems a bit weak, especially for extant placental mammals. Meredith et al used the same genomic data (26 genes) but based on ~164 mammals to answer the same question. Why couldn’t O’Leary use all of this already published mammal DNA? For the second criticism, I’m just going to quote Yoder’s review published in the same issue “Today, sophisticated theoretical and computational methods are used to estimate and calibrate molecular phylogenetic branch lengths (which represent time). Together with improved methods for integrating fossil and molecular data, dates derived from molecular phylogenies have inched closer to those implied by the fossil record. Is the approach used in the O’Leary et al. study directly comparable to these recent molecular phylogenetic studies? Not really, as it turns out.”

No wonder this paper supports the first theory, it is just a precise and massive analysis of the 40 species of the placental mammals fossil record. Personally, I’m really frustrated by how they managed to publish this paper. Since it’s part of my PhD research, I automatically get excited when I see fossils mixing with extant species so I really hoped this paper would link the two approaches instead of supporting the old fashioned view of evolution (the dinosaurs dying and the mammals taking over). I’d like to think that the history of life is a bit more complex and exciting…

A last comment to justify my title and which will be my main critique to this paper is that O’Leary et al. tried to recreate the “hypothetical placental mammal ancestor”.

As I said, this paper could be seen as a summary of the placental mammal fossil record. So why did they break the first rule that keeps palaeontology away from palaeo-poetry (i.e. going too far with palaeontological hypotheses)? Here they reconstructed a whole creature using their morphological data. What they made was essentially a mean (average) placental mammal (a primitive rat-like creature) – a throw-back to the early stages of palaeontological views of mammalian evolution. What did the ancestor of a duck and a beaver look like? Something in between – a platypus for example? As Olaf Bininda-Edmonds said on Ed-Yong’s Nature post “comparing the two estimates is like comparing “apples and oranges”, they haven’t really done anything to resolve this on-going dispute”.

This paper has also caused controversy on twitter. I’ll just cite two opinions.

Gavin Thomas (@Phalaropus)

“The reconstruction is fun – I’d love to see a picture based on 95% CIs for the ancestral states.”

and Rich Grenyer’s answer (@rich_)

“yes indeed. Something like this” (see our title image).

Many parts of the online science community got excited about this paper, you can see further discussions on Jerry A Coyne’s blog (here and here), on Ed Yong’s one (here and here or there) or else on the twitter feed #placental.

Author

Thomas Guillerme: guillert@tcd.ie

Photo credit

http://aerox21.deviantart.com/

04/02/201304/02/2013

Apocalypse Meow

Cats eh? You either love them or you hate them it seems. Well the latest research published in Nature by Loss et al. (2013) will give those who hate them plenty more reason to do so. While those who love their cats may just sit that little bit less comfortably next to their feline companions.

Let me start by making a few things clear. Cats are predators, they are an invasive species which have been introduced to islands all over the world, by man. In many places domestic cats have become feral, i.e. reverted to living in the wild, which has led to huge increases in their numbers in some places, which can have a devastating effect on indigenous wildlife populations. For a more detailed and somewhat depressing example of where this has occurred read about Stephens Island in New Zealand. Famously a lighthouse keeper’s cat had been blamed for the extinction of an entire species on this island though it seems that reports may have been somewhat exaggerated in this case.

The report in Nature is more scientifically robust than urban legends about a lighthouse keeper’s cat though. Figure 1 below shows just how devastating domestic cats can be on local wildlife populations. The graphs show the estimates of predation by domestic cats on (a) birds and (b) small mammals.

Figure 1. Estimates of cat predation on US birds and mammals (from Loss et al. 2013)

The numbers are startling, an estimated 2.4 billion birds are killed by cats every year in the US and 12.3 billion mammals. Incredible numbers I’m sure you will agree, there is however a caveat; only 31% and 11% for birds and mammals respectively are caused by what the writers class as “owned cats”, cats which are regularly kept indoors and well fed. While the majority of the mortality is thought to be caused by free roaming “unowned” cats. Incidentally there has recently been some debate about wind farms and their impact on local bird populations but this excellent blog and another recent Nature piece put the numbers into perspective in terms of other anthropogenic causes of bird deaths.

As those responsible for the domestication and introduction of cats, we can’t lay all the blame at the feet of our feline friends. First of all we need to somehow effectively manage the populations of feral and “unowned” cats and while this has been attempted with the Trap-Neuter-Return movement, it has been viewed as a response based on regarding feral cats as part of the native fauna rather than the invasive aliens that they are , therefore largely unscientific and ineffective. Secondly pet owners can take several measures of their own, neuter or spay your cat while keeping your cat indoors at night can vastly reduce their impact on local wildlife.

Author

Keith McMahon: mcmahok[at]tcd.ie

Photo credit

wikimedia commons

30/01/2013

Intelligent Design: Part One – a brief explanation and history

Trinity College Theological Society recently held a talk by Dr Alistair Noble titled ‘A Scientific Case for Intelligent Design’ which I attended as, possibly, the only biologist in the room. It was a fascinating, if deeply frustrating, experience. Before I get into the details of the talk, a brief explanation of intelligent design may be necessary. . .

Intelligent design (ID) is the ‘theory’ that certain features of the universe, including life, are best explained by invoking a creator. I put ‘theory’ in quotes because in a scientific theory is a very particular beast. It must have both explanatory and predictive powers. For example, the theory of evolution by natural selection explains how life evolved and can also be used to make predications about life that can be tested. The ‘theory’ of intelligent design has little explanatory power (“the designer did it”) and makes no predictions. As such, it is held with little esteem within the scientific community.

Outside the scientific community, however, there are some who hold ID in very high esteem. They think that it is a credible scientific theory and there have been many attempts, particularly in the U.S., to have ID taught in schools as a counter to evolution. This is deeply worrying to those who care about scientific literacy but has to be tackled carefully.

The reason for such caution is that ID is most loudly promoted by religious groups who feel that the theory of evolution is anathema to their beliefs and as such must be countered. In the past they countered with Creationism, but in recent years they have tried to remove the explicit religious overtones of Creationism, removing God, replacing him with an unspecified ‘designer’ and calling the new theory ‘intelligent design’. Thus the debate around ID is not just a scientific debate but is also a religious debate involving deeply held personal beliefs.

I hold the opinion that your personal beliefs are yours, and are no concern of mine, but when you try and mess with science, well, that’s another story! I went to the talk as I was curious to hear the scientific evidence for ID. Would it persuade me that there was a case for ID? . . .

Author

hearnes[at]tcd.ie

Photo credit

wikimedia commons

23/01/2013

Men are from Earth and women are from Earth

We love to explore and our adventures into outer space represent the acme of our derring-do. But when we leave our cozy planet we put an awful lot of stress on our minds and bodies. The billions of years of evolutionary pressures exerted on our ancestors all took place within the confines of Earth so a sudden dose of zero gravity is completely alien to us.

Some of the effects of space travel will give even those among you with the right stuff cause for pause.

There are the obvious perils like the terrifying oxygen-less vacuum of space but other, less obvious, afflictions abound.

Okay, so our skeletal system allows us to saunter around this planet quite comfortably. The whole point of the system is to provide some structure and locomotory ability against the force of gravity. But remove the pull and the bones start to wither away. There’s no longer any strain for the bones to resist. It happens at quite an alarming rate too. An average (?) astronaut can expect to lose 1% of his bone mass per month due to spaceflight osteopenia.

Still there’s no shortage of people who’d jump at the chance to be a star voyager for a few months.

But with longer flights, like a mission to Mars, there are even more insidious problems to consider. Back in 2010, six astronauts were selected to simulate such a mission (I was rejected for being too tall). They were locked in a room modeled on a spacecraft and given tasks that would be typical of such a journey. The whole ‘trip’ took 520 days and was an effort to better understand what happens to a person during a period of prolonged isolation.

While not quite space madness the six developed a range of symptoms. Chief among them were hypokinesis and disturbed sleep-wake cycles. The authors of the study describing the effects believe that the cause of these problems was a disruption to the circadian rhythms of the people involved. On Earth, we have our 24 hour day with its predictable light and dark cycle. But in space there is no such thing. Subtle changes in light can throw off your internal clock. This would be quite problematic. If one person has changed to a 25 hour day this can destroy the working ability of the team because he’ll find himself sleeping when everyone else is up.

It’s quite frustrating that we don’t have a biological blank slate that can adapt to all conditions. When we blast off from Earth, one thing we don’t leave behind is our evolutionary past.

Author

Adam Kane: kaned[at]tcd.ie

Photo credit

wikimedia commons

11/01/201311/01/2013

Hey Tree of Life! How’s it growing?

Following the influence of science writers such as S.J. Gould, I always try to look back at the historical perspectives of what I’m studying. These days I’m playing with 3Gb trees so I was delighted by Mindell’s 2013 Systematic Biology publication about the Tree of Life.

The idea of placing species into the so called Tree of Life emerged before the Origin of Species with works such as Augier’s Arbre Botanique (1801) (Fig. 1) and Eichwald’s tree (1829 – possibly inspired by Pallas’s 1766 work) (Fig. 1). But the spreading of such trees began only after publications of Lamarck’s scheme (1809,Fig. 2), Darwin’s famous sketched drawing (1859 – Fig. 3) and Haeckel’s beautiful tree (1866 – Fig. 2). It is only within an evolutionary framework that these representations of the relationships among organisms make sense: the idea of descent with modification.

Figure 1: Augier’s Arbre Botanique (1801) & Eichwald’s tree (1829) – from Mindell 2013 (Fig.1)

Figure 2: Lamarck’s scheme (1809) & Haeckel’s tree (1866) – from Wikimedia Commons

Figure 3: Darwin’s Origin of Species unique figure (1859) – from Wikimedia Commons

From that point, we all know how the story continued; from Darwin’s sketch (Fig. 3) to modern phylogenomics (fig. 4). Our understanding of the Tree of Life progressed from Simpson’s (this one, not this one) cladistic methods for looking at morphological relations among vertebrates, through to the discovery of DNA, the first molecular clock and, eventually, the use of complicated Bayesian stuff. Depictions of the Tree of Life evolved from something like a cypress (a nice, straight tree with Neil Armstrong at the top surrounded by monkeys and mosses and jelly fish near the roots) to a three-rooted shrub full of immense dead branches near the centre. If you look at the figures included in this blog, the changes in our understanding of the tree are clear: a gradual reduction of anthropocentrism and inclusion of microscopic organisms.

Figure 4: Tree of Life from David and Alm (2011) – from David and Alm 2011 (Sup. Fig. 15)

So what should we do next? Should we just expand the dataset until we have all the species and all their genome plotted in the Tree of Life? Hopefully there are still lots of less boring things left to do for researchers working in this area today… Two questions are (in my mind) really important to look at: is the Tree of Life only the result of descent with modification and what should we put in the tree?

For the first question, it appears more and more clear nowadays that the Tree of Life is not really a tree but rather something along the lines of a tree-shaped web. Regarding Archaea and Bacteria alone (the majority of organisms in the tree), it is estimated that at least 81% of their genes have been laterally transferred among lineages at some time in the past. This phenomenon is also becoming increasingly evident among Eukaryotes (even among vertebrates!) and recognition of these events should lead to a more web-shaped “Tree” of Life. Incidentally, it is interesting to note that Batsch recognised this web structure in plants as early as 1802 (Fig. 5).

Figure 5: Batsch’s web of plants (1802) – from Mindell 2013 (Fig.1)

Regarding the second question, asking what can be included in the tree of life comes down to how we determine what is living. I remember one question that a classmates had in my phylogenetic lectures; “What is the out group of the tree of life?” The lecturer had just said that a tree without an out group is not valid. The resulting discussion turned into a really long (and interesting) debate about viruses – the question being whether we should put the viruses in the tree of life? We might define living organisms by entities that can replicate their own DNA. So you could argue that if viruses cannot achieve this independent replication then we should prune them out of our tree. Haha ! But wait, it’s not so easy: although most viruses require host cells for reproduction, so do many other “living” organisms like Richettsia or Chlamydia. In addition, some viruses have many genes involved in DNA replication so how should their self-replication abilities be classed ?

Mindell conclude by quoting Brooks and van Veller (2008): “There are two choices. Do we classify a tree with [lateral transfers], or do we try to classify a [lateral transfer] network? If we wish our classifications to reflect what we think we know about evolution, it seems that we will have to opt for the first alternative.” Does this mean that we should go for a tree shaped web including viruses? Let see how the debate will go on…

Author

Thomas Guillerme : guillert[at]tcd.ie

Photo credits

wikimedia commons

Mindell 2013

David & Alm 2011

03/01/2013

You’re grounded!

Pterosaurs are the largest animals to have ever flown. Some species had wingspans exceeding 10 metres dwarfing the largest avian challenger. It must have been quite a sight to see one of these things blocking out the Mesozoic sun. But there have been niggling doubts about the ability of the larger representatives to fly. Will we have to re-evaluate our mental image of the Mesozoic and ground our pterosaurs?

Flight is no easy thing for an animal. It makes all sorts of demands on the physiology, morphology and ecology of the creature trying to take to the air for a living. With every added kilo a bit more lift has to be generated, for every extra wing flap more energy is required. Still, most pterosaurs look like they fit the bill. Their skeletons were heavily pneumatized and they had a hyper-elongated fourth finger from which they could support a membranous wing.

The problem arises when we look at the giant pterosaurs especially the Azhdarchidae family which houses the biggest species like Quetzalcoatlus northropi and Arambourgiania philadelphiae. One analysis gave a mass estimate of half a tonne for Quetzalcoatlus n., which would almost certainly render it flightless. Other researchers point to the terrestrial adaptations seen in this family and of course we can see many instances of birds who have become secondarily flightless. A size gap was pointed out where there exist small pterosaurs and giant ones but no intermediates which was said to mirror the pattern of flying birds and flightless ratites. Then there is the taphonomic bias seen in the fossil record whereby most of the Azhdarchid skeletons are found in terrestrial environments.

But not all palaeontologists are convinced by these arguments, pterosaur specialists Mark Witton and Michael Habib have taken each one of these lines of evidence to task and found them wanting.

Firstly, while most of the fossils have been found on land this doesn’t mean the animals were terrestrial, many bird species fly exclusively over land, so that bias is neither for nor against.

Secondly, the suggested size gap looks like an artefact in the fossil record which has been filled with intermediate forms.

Perhaps the most convincing piece of evidence in favour of flightlessness are the huge mass estimates. A half tonne reptile is going to struggle to get airborne. But this figure is beginning to look like an overestimate, the result of distorted fossils and inappropriate scaling techniques. A more lightweight figure of 240 kg looks to be more realistic when these biases are accounted for.

What of the terrestrial adaptations? Well, there is no issue with the animals being adept on the land while still being able to fly. Indeed the authors above argue that large Azhdarchids occupied the niche of modern day ground horn bills or storks both of which are well adapted to the land while still being able to fly.

In the end it looks like giant pterosaurs did take to the skies. Piecing together the mode of life of long extinct species is never easy but it’s not impossible.

Author

Adam Kane: kanead[at]tcd.ie

Photo credit

Witton MP, Naish D (2008) A Reappraisal of Azhdarchid Pterosaur Functional Morphology and Paleoecology. PLoS ONE 3(5): e2271. doi:10.1371/journal.pone.0002271

31/12/2012

No animal is an island

No man is an island; the same could be said for the millions of life forms that populate our planet. Think of all the ways in which organisms interact with each other through predation, parasitism and the countless symbioses. Sometimes a pair of interacting partners can become inextricably linked such is their mutual dependence. Each one may provide the other with a resource it’s unable to obtain on its own.

A recent collaboration explored instances when these interactions lead to the loss of a trait and showed the fragility of this situation. One of the examples the authors use is an ant species that farms fungus. The fungus provides the ants with all the arginine (an amino acid) they need so they have lost the ability to synthesise it themselves. Thus anytime an ecological interaction involves some provision of a resource by one partner to another the evolutionary pressure is removed and the trait can be lost in the species receiving the goods. In other words we end up getting ‘compensated trait loss’ due to the ecological interaction. This can tighten a symbiosis from a facultative to an obligatory one.

But the fragility of compensated trait loss should be obvious now. In the ant example, were the fungus to go extinct the ant would disappear along with it. It’s like the ecological interaction is undermining all the good work done by natural selection in providing the ancestral ants with all the traits they need. The authors reckon that trait loss is “grossly underestimated” which puts many species in a precarious position in this age of mass extinction. Although there have been some instances where the trait was recovered, in flagrant disregard for Dollo’s law. Some of these law breakers include parasitic insects who regained their ability to synthesise lipids once the provision was lost.

A difficulty in studying these systems is how to detect when trait loss is taking place. A decreased expression of some gene in some members of a population would probably be reported as natural variation. But with ever improving molecular techniques we will be able to get a better estimate of the number of compensated trait loss interactions.

Author

Adam Kane: kanead[at]tcd.ie

Photo credit

wikimedia commons

27/12/201227/12/2012

Friends with benefits

With the apocalypse come and gone we can still theorise about how our downfall will come about. E. O. Wilson wrote that ‘‘sex is an antisocial force in evolution’’. Charlie Cornwallis, his colleagues at Oxford and their promiscuous birds illustrated why this is so. As is so often the case in evolutionary theory the question centres on sociality, in this instance it takes the form of cooperative breeding. Why would an individual help someone else raise their young rather than having progeny of their own?

The authors ask us to consider the case in birds of a mother and her offspring. If the offspring remains at his natal site and his mother is monogamous he can help in rearing his siblings who are all as related to him as he would be to any of his own potential children. This makes evolutionary sense in terms of relatedness. However, if his mother is promiscuous his level of relatedness to the resultant other birds will be less than that of his own offspring. The sensible thing for him to do here is to disperse and start a family rather than helping out his half siblings.
The authors of the study collected data on almost 300 bird species recording levels of promiscuity and degree of cooperative breeding to test the monogamy hypothesis. This states that species with high levels of promiscuity will tend towards societies with low levels of cooperative breeding and vice versa.

They were able to show that highly promiscuous birds tend not to have cooperative breeding whereas species like the White-winged Chough, which are completely dependent on others for help in raising their young, are faithful.

But more than this their results demonstrated that, over evolutionary time, transitions to cooperative breeding were associated with low promiscuity. The reverse is also true and echoes Wilson’s statement, because highly promiscuous groups saw a breakdown in cooperation.

There are some exceptions to the rule in that birds can be promiscuous and still have a society of cooperative breeders but this is offset by kin discrimination which involves “directing aid preferentially towards relatives.” So there you have it, sex has the power to destroy societies.

Author

Adam Kane: kanead[at]tcd.ie

Photo credit

wikimedia commons

19/12/201203/01/2013

Smoking nests

We all know that cigarettes and smoking are bad for you. However, a recent Biology Letters paper which was featured on BBC Nature suggests that the discarded remains of cigarette butts may help to maintain the health of some urban birds.

Researchers from the National Autonomous University of Mexico found that smoked cigarette butts incorporated into house sparrow and house finch nests may act as parasite repellents. Nests which included high numbers of cigarette butts had fewer parasites. A further experiment involving heat traps to attract parasites indicated that the anti-parasitic properties of the cigarettes seemed to be related to their nicotine content which is only released after the cigarettes have been smoked. This doesn’t necessarily mean that the birds use cigarette butts as an adaptive anti-parasite strategy. Cellulose in the butts is an effective thermal insulator so any anti-parasite effects may be a fortuitous coincidence. The authors suggest that future behavioural choice tests could be used to determine whether birds can distinguish and preferentially include smoked cigarette butts in their constructions.

The paper is an interesting contribution to the growing fields of urban ecological and wildlife research. More than half of the world’s population now lives in urban areas so it is increasingly important to understand the ecological effects of this changing environment. Wildlife and urban areas are not always a happy mix – we have all seen the squished remains of a hedgehog’s attempt to cross a busy road. However, as this paper shows, sometimes urban animals can adopt novel behaviours which appear to have positive consequences.

However, I can’t see the paper being used to counteract anti-cigarette butt litter campaigns any time soon!

Author

Sive Finlay: sfinlay[at]tcd.ie

Photo credit

wikimedia commons

30/11/2012

War of the worms

398px-Cerithidea_californica_001 — A battlefield

Some of the most successful animals on earth live in societies characterised by a division of labour between reproducing and non-reproducing castes. One role non-reproducing members may undertake is defence. Spectacular examples include the heavily armoured termites and ants. Recently a soldier caste was discovered in an entirely new and unexpected battleground, inside the bodies of snails. The soldiers? Tiny parasitic flatworms.

Flatworms, or trematodes, have complicated life cycles, involving several different stages infecting a variety of host species. In one host, often a snail, a single trematode undergoes repeated clonal reproduction. Clones produce more clones or go on to produce the next infective stage, which leaves the snail to infect the final host. While working with trematode colonies of Himasthla sp. infecting the Californian horn snail Cerithidea californica, researchers at the University California Santa Barbara observed that the trematode occurred in two distinct morphological forms. There was a large reproducing primary morph, which appeared to be the form typically described in the literature, and a secondary smaller, thinner morph.

These secondary morphs had a number of other features which set them apart. They rarely showed any signs of reproduction and were far more active. They also had huge muscular pharynxes and guts relative to their larger sisters. When researchers preformed behavioural tests, they discovered just what those large mouth parts were for. The secondary morphs attacked and killed other trematode species and unrelated conspecifics. This behaviour is not unknown in trematodes; a number of species attack and kill other trematodes. What was novel was that the smaller morphs appeared to be doing all attacking. The behaviour was rarely observed in the primary morphs. There was also a spatial segregation of morphs. Primary morphs were located in the visceral mass, mainly in the region of the gonads. The secondary morphs were more widely distributed though mainly found within the mantle. The snail mantle is the main entry point for trematodes, a strategic area to defend against invading armies. Finally, the researcher found very few intermediate morphs, suggesting that the smaller morphs were not simply juvenile stages of the primary morphs. They were a distinct, permanent caste whose function appeared to be defence – soldiers. The researchers had discovered eusociality in a completely new taxonomic group. Previously, eusocial systems consisting of morphologically distinct, specialised reproductive and non-reproductive castes had only been recognised in insects, snapping shrimp, a sea anemone and mole rats. The researchers have already suggested a further five species of trematodes that may have soldier castes.

Work from New Zealand, published this year, on another species (Philophthalmus sp.) has expanded the list of trematodes with soldier castes. The authors also showed that interspecific competition has a heavy impact on colony numbers. This is just the sort of pressure that favours adaptive strategies to reduce competition, such as a permanent soldier caste. However, competition may not be the only selective pressure driving or maintaining caste differentiation in trematodes. In the absence of competition, the presence non-reproducing morphs were found to provide a benefit to the colony, as measured by the number of infective stages produced. Precisely how this benefit comes about is not yet known. The authors suggest some form of communication or nutrient exchange may be taking place between the two morphs. This gives tantalising hints that these colonies are even more complex and interactive than previously thought.

Not only has the discovery of the eusociality in trematodes widened the taxonomic range of this phenomenon, it has also provided researchers with an exciting new tool to study its evolution. The Trematoda class contains at least 20, 000 species with a wide variety of life-histories and ecologies. The discovery is also a great example of how new and unexpected results can still come from well-studied animals. The Himasthla sp. /Californian horn snail system had been studied for over 65 years.

Author

Karen Loxton: loxtonk[at]tcd.ie

Photo credits

wikimedia commons