Sloths, their moths, and poo

Besides my typical holiday posts, this is going to be the last post of 2014. Where did the time go?! To celebrate the end of another awesome year of parasite ecology, I’m going to finish with one of my favorite symbiont stories of the year: sloth, their moths, algae, and poo.

I bet you guys have heard about this one, because it was in the mainstream news a bunch when the paper came out. Therefore, instead of summarizing the paper for you, I’m just going to say that it’s awesome, and you can read a summary here. You can also see a cute cartoon version of the story here. And of course, the citation for the paper is below.

sloths

Reference:
Pauli JN, Mendoza JE, Steffan SA, Carey CC, Weimer PJ, and Peery MZ. 2014. A syndrome of mutualism reinforces the lifestyle of a sloth. Proceedings of the Royal Society B – Biological Sciences 281(1778): 20133006

Helminthic Therapy for the Treatment of Autoimmune Diseases

(Ladies and Gentlemen, today marks the first guest post on Parasite Ecology!! We’re going to go a bit more medical this week, with this freelance article from Gemma Corden.)

Parasites are incredibly ubiquitous but to us humans they’re still mysterious creatures in many ways. And on the whole, autoimmune diseases and their specific triggers and causes are as yet not entirely understood, so it’s somehow fitting that a very specific kind of parasitic infection can be an effective therapy for certain types of autoimmune disease. Specifically, helminthic treatment for autoimmune disease has been under investigation for more than a decade by several research groups, with some promising results that are interesting for more than just their therapeutic implications.

Helminthic Therapy Under Investigation

Just a couple of decades ago, helminthic therapy as a field of investigation was limited to purely observational studies, and relatively speaking, it’s been a rapid advancement from those beginnings to the clinical trials that are now underway and already showing promising results. In part this is due to the fact that the sum of immunological knowledge has vastly increased in the same amount of time.

Helminthic therapy is being tested in patients with a range of inflammatory and immunological disorders, including multiple sclerosis, Crohn’s disease, Celiac disease, inflammatory bowel disease, ulcerative colitis, and allergic rhinitis. The results have been mixed; in Crohn’s disease, for example, helminth therapy has been highly successful for many people, effectively treating many of the symptoms, and even reducing the need for and reliance on pain medications. In one study, patients with Crohn’s disease were treated with a single dose of Trichuris suis; twelve weeks later, 75% were in remission. However, with a 66% relapse rate for this single-dose therapy, there is still more information to be uncovered. Another study recorded a 100% remission and 33% relapse rate for patients with ulcerative colitis following a single-dose therapy. Results have been largely negative in the case of allergic rhinitis, with most studies recording no significant improvements in symptoms such as airway responsiveness, asthma control, and allergen tests. And while positive results are scarce for Celiac disease, one study on people with MS showed that T. suis therapy was able to temporarily reduce CNS and neurological lesions.

Despite the fact that helminthic therapy is still experimental, unregulated, and unapproved for use in any country, organisms are already being sold for therapeutic use; the relative ease with which this mode of treatment can be manufactured outside of a laboratory setting unfortunately means it’s ripe for exploitation.

Why does it Work? Hygiene Theory and Immunome Selection

In the last four or five decades, most industrialized countries have seen huge increases in the development of allergies, inflammatory disorders of idiopathic origin, and autoimmune diseases. Similar trends develop in modernized regions of countries that are less industrialized. Previously it was thought that this trend might be accounted for in large part as the result of lifestyle and environmental changes—in particular the reduced exposure to infectious agents that is experienced by people in industrialized countries. This is the “hygiene theory,” which posits that lack of exposure to pathogens, particularly in childhood, prevents the immune system from developing normally, and increases the likelihood that a given individual might develop an allergy or autoimmune disorder. Essentially, the immune system needs this early pathogen exposure to become primed to react normally to infectious agents and to differentiate correctly between pathogen and self.

Recent research indicates that there may be more to the story than this, however, as it seems likely that underlying the hygiene theory, there are highly influential genetic factors at play—specifically, that as humans and their pathogens have evolved alongside one another, human pathogens have exerted evolutionary pressure on the human genome. In other words, the immunome—the genes that code for immune system components—that humans have today is the result of coexistence with pathogenic agents, and as such it can be inferred that pathogenic exposure is necessary for normal immunome expression, and normal immune system function.

Interestingly, it seems that helminths in particular may be “master selectors of the immunome,” that have exerted more evolutionary pressure on the human immunome than any other type of pathogen, in part due to the incredibly ubiquity of these organisms. Another contributing factor is that helminths tend to infect humans prior to puberty and most often cause chronic infections, meaning that people who survive to adulthood and parenthood with such infections are more likely to have immunomes with advantageous mutations in terms of resistance to helminth infection.

References

Jabr, Ferris. “For the Good of the Gut: Can Parasitic Worms Treat Autoimmune Diseases?” Scientific American.Accessed 28 November, 2014.

Lisenborg, Jori. “Helminthic Therapy.” Accessed 28 November, 2014.

Matisz, Chelsea E., McDougall, Jason J., Sharkey, Keith A., and McKay, Derek M. “Helminth Parasites and the Modulation of Joint Inflammation.” Journal of Parasitology Research. Volume 2011 (2011). doi: 10.1155/2011/942616 Accessed 28 November, 2014.

Wamms, Linda J., Mpairwe, Harriet, Elliot, Allison M., and Yazdanbaksh, Maria. “Helminth therapy or elimination: epidemiological, immunological, and clinical considerations.” The Lancet Infectious DiseasesVolume 14, Issue 11, Pages 1150 – 1162, November 2014. doi: 10.1016/S1473-3099(14)70771-6Accessed 28 November, 2014.

Caterpillar Campfire

(Prepare yourselves. This post is about the sexiest paper that I have read in a long time.)

Many plants are covered in tiny hairs, called trichomes, which defend the plant by trapping insects. These can be hooked trichomes that snag insects, or glandular trichomes that produce secretions that the insects stick to. These trichomes may be beneficial because they trap and kill insects that might harm the plant. But they also seem somewhat detrimental, because predatory insects (e.g., lady beetles) that might benefit the plant by killing herbivorous insects (e.g., aphids) might also get stuck in the trichomes and die (Eisner et al. 1998).

However, some predatory arthropods can avoid getting stuck in the trichomes. For instance, the common tarweed has glandular trichomes, but it is still visited by five types of predatory arthropods (Krimmel and Pearse 2013). Interestingly, those predatory arthropods will eat both dead and living insect prey. AND GUESS WHAT?! Just like plants use extrafloral nectaries and food bodies to attract ant defenders, the dead insects trapped in the sticky trichomes of the tarweed attract those five predatory arthropods. Those predators reduce herbivory from a caterpillar by 60%, and that reduced herbivory leads to greater plant fitness.  A. Maze. Ing. Go read the paper. It’s beautiful.

Fun facts from the paper:

20-30% of vascular plants have glandular trichomes. So, this specialized predator attraction may be a widespread phenomenon.

Individual plants had up to 40 insect corpses at a time!!

caterpillarcampfire

References:

Eisner T, Eisner M, Hoebeke ER. 1998. When defense backfires: Detrimental effect of a plant’s protective trichomes on an insect beneficial to the plant. PNAS 95(8): 4410-4414.

Krimmel BA, Pearse IS. 2013. Sticky plant traps insects to enhance indirect defense. Ecology Letters 16: 219–224.

The benefits of attracting many symbiont species

Textbook examples of mutualisms usually involve two interacting species; for instance, honey bees and clover. But of course, one host may have many mutualists (e.g., bees, butterflies) and one mutualist may have many hosts (e.g., clover, blueberries). In many cases, the benefits of having multiple possible host species are obvious. For instance, for ants that eat tasty elaiosomes and disperse plant seeds, it would be hard to get by while specializing on the seeds of only one plant species, because those seeds would only be available for part of the year. But how does the host benefit from having many symbiont species?

In ant-plant interactions, one plant species may have individual visitors from many ant species. For instance, the shrub Urera baccifera receives visitors from 22 facultative ant species, which visit to harvest food bodies and/or fruits (Dutra et al. 2006). Similarly, the yellow alder (Turnera ulmifolia) receives visitors from 24 facultative ant species, which visit to harvest seeds with elaisomes and/or to feed at extrafloral nectaries (Cuautle et al. 2005). While they’re on the plant, these ants can protect the plant from herbivores, like caterpillars. And finally, there may be multiple ant species visitors to a given plant species even when the associations are obligate: for instance, Acacia drepanolobium may be inhabitated by one of four ant species at any given time (Palmer et al. 2010).

Ok, but why so many species of ant visitors? Why shouldn’t the plant sanction any partners except the most beneficial species? This is still an active area of research, but at least part of the answer is that different ant species vary in the services that they provide to the plant. For instance, ant species may be more of less aggressive defenders, more or less likely to disperse seeds, more or less likely to farm scale insects, etc. (You might remember that this is true for guard crabs, too, where big crab species are good at protecting corals against starfish, while small crab species are good at protecting against vermetid snails.) Ant species also vary in how costly they are to harbor. For instance, one species may be a good defender, but it may also sterilize the host plant (e.g., Stanton 1999). So, a plant’s lifetime fitness may be determined not only by the guild of ant species that visits the plant during its life, but also on the timing and order in which those species visit the plant (Palmer et al. 2010). For instance, having sterilizing symbionts when you’re young can actually increase lifetime plant fitness. Isn’t that wild?

antacacialongtermfitness

References:

Cuautle M, Rico-Gray V, Diaz-Castelazo C (2005) Effects of ant behaviour and presence of extrafloral nectaries on seed dispersal of the Neotropical myrmecochore Turnera ulmifolia L. (Turneraceae). Biological journal of the Linnean Society 86: 67-77.

Dutra HP, Freitas VL, Oliveira PS (2006) Dual Ant Attraction in the Neotropical Shrub Urera baccifera (Urticaceae): The Role of Ant Visitation to Pearl Bodies and Fruits in Herbivore Deterrence and Leaf Longevity. Functional Ecology 20(2): 252-260.

Palmera TM, Doak DF, Stanton ML, Bronstein JL, Kiers T (2010) Synergy of multiple partners, including freeloaders,increases host fitness in a multispecies mutualism. PNAS 107(40): 17234–17239.

Stanton ML, Palmer TM, Young TP, Evans A, Turner ML (1999) Sterilization and canopy modification of a swollen thorn acacia tree by a plant-ant. Nature 401:578–581.

Elepunts

Are elephants afraid of mice? Well, maybe. There isn’t much evidence, unless you like sample sizes of 1 individual. However, elephants are definitely afraid of ants, and that is a much more interesting ecological story.

Last week, I posted an introduction to the symbiosis between ants and plants. One of the services provided by ants is protection from herbivores. Those herbivores may be insects, like caterpillars and grasshoppers, but they may also be megafauna, like elephants.

Elephants love munching on Acacia trees, but some Acacia species are protected by ants. With the ants removed, elephants will gladly eat species that typically have ants. But when the ants are present, the elephants avoid defended trees (Goheen and Palmer 2010). This decision to avoid getting viciously stung by hordes of ants may have far-reaching consequences in savanna ecosystems: tree community composition is affected, because defended tree species are more likely to survive in areas with many elephants (Goheen and Palmer 2010).  Tiny symbionts can play big roles in ecological communities!

antsandelephant

References:

Goheen JR, Palmer TM (2010) Defensive plant-ants stabilize megaherbivore-driven landscape change in an African savanna. Curr Biol 20:1768–72.