"So, naturalists observe, a flea has smaller fleas that on him prey; and these have smaller still to bite ’em; and so proceed ad infinitum."
- Jonathan Swift

January 28, 2012

Pasteuria ramosa

Parasitic infections can severely debilitate the host in many ways, sometimes this manifests itself as the loss of some, or even all, of the host's reproductive ability. Evolutionary speaking, an organism that cannot reproduce is as good as dead. However, it's not entirely clear who (if anyone) is benefiting from this outcome - is it; (1) a survival strategy by the host to temporarily free up resources to compensate for the parasite's presence? Or is it (2) an adaptive strategy by the parasites to divert as many resources as possible to themselves without compromising the host's ability for self-maintenance and survival? Or is it (3) merely an unintended side-effect of infection? Of course, (1) and (2) are not mutually exclusive, and in the case of (3), even if it had started out as an unintended side-effect of infection, if host castration resulted in higher reproductive fitness for the parasite, then that trait will be positively selected for and become part of the its repertoire of host-exploitation strategies.

Waterfleas (Daphnia) are infected by all manner of parasites (we featured one of them during the early days of the blog: Caullerya mesnili) ; most of them are pretty nasty - they often end up castrating and/or killing the host. Pasteuria ramosa is no different - it is a spore-forming bacterium that infects waterfleas, makes them bloated, darkening their body (see the right waterflea in the photo) and castrates them in the process. While it was previously thought that any waterfleas infected by P. ramosa were permanently castrated, it turns out that some lucky Daphnia can actually recover from their infection.

So do these little crustaceans adjust their reproductive output in response to the parasites and is castration a way for them to compensate for a (potentially) temporary hiccup in their baby-making ability? To find out, a team of scientists from Norway set out to see just who benefits the most from host castration. Their logic is that if it is an adaptive strategy by the parasite, then we should see higher spore output from a permanently castrated host. Whereas, if castration is an adaptive coping mechanism by the waterflea, then there should be a jump in reproduction upon the onset of infection as the waterflea tries to make as many baby Daphnia as possible before P. ramosa put a stop to it, then store up reserves during the infection to "wait it out".

To correct for any potential sex differences (there are many documented case of sex-bias in parasitism), these scientists used only female waterfleas for the experiment. During the course of the study, about half the waterfleas they infected with P. ramosa managed to regain their reproductive capacity. In those lucky ones, the parasite produced many fewer spores than in waterfleas that had been permanently castrated. So evidently, P. ramosa benefits from having permanently castrated hosts. But what about the waterfleas themselves? Were they able to compensate by adjusting their reproductive output in the parasite's presence?

The scientists found that by far, the strongest predictor for the lifetime reproductive output of a parasitised waterflea is the age at which it becomes infected - the later that it became infected, the more time it had to churn out babies before it came down with a severe case of P. ramosa. So it's pretty much a case of "use it or lose it". They did not find evidence to suggest the waterfleas made any effort to increase their reproductive output before they are castrated by their parasites. This is unlike other systems where parasite-castration occurs. In trematode-snail systems, the infected snails are less likely to recover from their infection. The strategy which has evolved among snails in areas with high parasite prevalence is to reach sexual maturity as quickly as possible (For example: see this study) so they can eke as many baby snails as they can before they inevitably become infected and be taken over by squirming body snatchers.

It should be noted that the waterfleas used in the experiment were from Southern Finland, whereas the parasites were isolated from a pond in Northern Germany. So perhaps the reproductive strategy of the Daphnia population used in that experiment has evolved in response to their local parasite(s) population instead. Other studies have found waterfleas to be locked in a close evolutionary race with their parasites across space and time, so the outcome of any host-parasite interaction will be dependent on the genetic identity of both host and parasite.

Image credit: Jensen et al./PLoS Biology

Magerøy, J.H., Grepperud, E.J. and Jensen, K.H. (2011) Who benefits from reduced reproduction in parasitized hosts? An experimental test using the Pasteuria ramosa-Daphnia magna system. Parasitology 138: 1910-1915

January 15, 2012

Spauligodon atlanticus

Today, we look at a paper showing how data from DNA sequences can help resolve the evolutionary relationship of different parasite species, and even find new species where we least expected it. Traditionally, parasites - like other organisms - are classified based on key characteristics of their anatomy. However, many parasites have simplified morphology (an extreme example is the parasitic snail which has evolved into nothing but a bag of genitalia) and often the few key characters that can be examined are heavily reduced. Therefore, any conclusions about relationships between different parasite species that are based upon anatomical characteristics can lead to misleading or, at best, incomplete conclusions.

Spauligodon atlanticus is a species of nematode that parasitises Gallotia, a genus of lizards living on the Canary Islands (see image). Spauligodon atlanticus was initially described in 1987 using traditional methods, i.e. based solely on its anatomical features. In the case of parasitic nematodes, the key characteristic for distinguishing different species is the shape of the genitalia and tail appendages of the male specimen (such features are too indistinct in the females across different species).

For this particular study, a group of biologist from Portugal and Spain went to the Canary Islands to collect S. atlanticus from Gallotia lizards, as well as sampling for other species of Spauligodon from lizards of southern Spain, Morocco, and Armenia. They compared the DNA sequences of the worms and found that nematodes that had been identified as S. atlanticus (based on their anatomy) actually consisted of two distinct species. While they looked the same, their molecular signature revealed two separate lineages; an eastern lineage that is specific to the lizard species Gallotia atlanticus, and the western lineage that is found in 4 different Gallotia species. They also differ in their evolutionary relationships with other nematodes in the Spauligodon genus. The eastern lineage is more closely related to nematodes in wall lizards (Podacris spp.) from southern Spain and Morocco while the western lineage is more related to worms in green lizards (Lacerta spp.) from Armenia.

These two genetically separate lineages of S. atlanticus are what are known as a cryptic species complex (something that we have previously covered on this blog). Recent studies in the last ten years have shown that some parasite species which had previously been thought to be a single generalist species infecting multiple hosts, are in fact composed of multiple specialised species in disguise.

Meanwhile, this study raises another question - how did these two genetically separate lineages, living in different lizards, evolve such similar anatomical characteristics? The authors of the paper raised the possibility that the anatomy of the two lineages had evolved to convergence due to similar conditions they encounter inside the gut of their respective lizard hosts, or that even sexual selection was responsible, since the key anatomical difference use to distinguish these nematode species is the shape of the male genitalia. But this is a question that will only be resolved with further analyses of related Spauligodon species. As the authors wrote in the title of their paper, there are "no simple answers".

Image from the Wikipedia.

Jorge, F., Roca, V., Perera, A., Harris, D.J. and Carretero, A. (2011) A phylogenetic assessment of the colonisation patterns in Spauligodon atlanticus Astasio-Arbiza et al., 1987 (Nematoda: Oxyurida: Pharyngodonidae), a parasite of lizards of the genus Gallotia Boulenger: no simple answer. Systematic Parasitology 80:53-66

January 3, 2012

Apocephalus borealis

Many of you have heard of the very scary phenomenon called "Colony Collapse Disorder" - and if you haven't, you should, because it could be a major threat to the food we eat. CCD is when the worker honey bees abandon their hives and die, which, if widespread, can mean drastic decreases in pollination of crops. This phenomenon was first reported in the U.S. in 2006 and ever since that time, scientists have struggled to uncover what was responsible. Everything from cell phone radiation to genetically modified crops to a variety of parasites of honey bees were suggested to be the cause. Then, today, a new paper in PLoS One showed data suggesting that another kind of parasite is linked to CCD. Apocephalus borealis is a parasitoid fly that was known to attack bumblebees and paper wasps, but now has been demonstrated to also attack honeybees in the U.S. - in fact, 77% of the colonies sampled near San Francisco were parasitized by A. borealis. The authors used DNA barcoding to confirm that the flies in the honey bees were genetically indistinguishable from those parasitizing bumble bees.

The authors of the new study also found that bees that were found flying around at night (something honey bees don't normally do) were significantly more likely to be parasitized by the fly and furthermore, the sick bees also seemed disoriented. It is not currently known whether or not the tendency for the parasitized bees to fly at night away from their colonies is another example of manipulation of the host by a parasite or whether this might be an act of altruism by the bee, carrying its parasite away from its colony and thus protecting the others.

Although these new results are very exciting, many questions remain to be answered about the history and impact of A. borealis. First, when did the switch into honey bees occur? Honey bees are not native to the U.S., but since they are so well monitored and studied, the authors believe that the switch must have happened recently - otherwise it would have been noticed by apiculturists. Second, could these flies also be serving as vectors for other bee pathogens? Two known bee pathogens, Deformed Wing Virus and Nosema ceranae, a microsporidian were found in the A. borealis flies. And finally, could the invasion of honey bees by this parasite mean that CCD is going to increase? The natural hosts of A. borealis are bumble bees, which live in small colonies where only the queen herself survives the winter, but honey bee colonies have thousands of bees and their activity maintains some amount of heat, even in colder winter months. This increase in host resources and more generations per year could spell a population explosion of A. borealis...and that won't be good for those of us who depend on pollination - like all of us.

The image is from the paper. Look closely at the abdomen of the bee - that's a little parasitic fly laying eggs into it. Soon the larvae will emerge from the dead host. (You can see a photo of this in the original paper as well.)

Source: Core A, Runckel C, Ivers J, Quock C, Siapno T, et al. (2012) A New Threat to Honey Bees, the Parasitic Phorid Fly Apocephalus borealis. PLoS ONE 7(1): e29639. doi:10.1371/journal.pone.0029639.