"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

August 30, 2011

Skrjabinoptera phrynosoma

Life isn't easy as a parasite with a complex life-cycle. In order to grow up and reproduce, you often need to make your way through the bodies of at least two very different host animals - a very haphazard process that depends largely on timing and luck. In the case of today's parasite - a nematode worm called Skrjabinoptera phrynosoma - it has to make its way between a lizard and an ant. The adult S. phrynosoma lives inside the stomach of the desert horned lizard Phrynosoma platyrhinos. However, when the female becomes filled with mature eggs, she migrates to the lizard's cloaca (a nice, technical way of describing a lizard's butt).

Unlike most parasitic nematodes, which often lay eggs that are cast out of their host and left exposed to the elements, S. phrynosoma is a very maternal parasite - in a slightly morbid way. The female S. phrynosoma makes the ultimate sacrifice by casting her egg-filled body out of the lizard via the host's feces. She will die outside of the host - but in addition to protecting her eggs by doing so, it is also her strategy for helping her eggs reach the next host. For some reason, ants find the shriveled, egg-filled cadavers of female S. phrynosoma to be a tasty treat, a meal fit to feed to their brood of growing ant larvae - which then become infected with the parasite's own larvae. The life-cycle is complete when the infected larvae mature into workers, emerge from the colony, and become lizard food - horned lizards are specialists on ants.

Researchers at Georgia Southern University discovered that to ensure that this sequence of events occurs, S. phrynosoma has evolved to synchronise its life-cycle with the seasonal behaviour of both its lizard and ant hosts. They found that the number of egg-filled females (all ready to evacuate) reach peak abundance during the middle of the lizard's mating season. This is also the period when there are the greatest number of ants out busily foraging and when the colonies are packed to capacity with broods of growing ant larvae. By timing its life-cycle in such a manner, S. phrynosoma ensures that when next season rolls around, when those broods of larvae are ready to emerge as a new generation of workers ants, they will be doing so pre-infected with nematodes and just in time to welcome the hungry lizards coming out of hibernation.

Hilsing, K.C., Anderson, R.A. and Nayduch, D. (2011) Seasonal dynamics of Skrjabinoptera phrynosoma (Nematoda) infection in horned lizards from the Alvord Basin: temporal components of a unique life-cycle. Journal of Parasitology 97: 559-564.

August 21, 2011

Caenorhabditis briggsae (KT0001)

Today's parasite is in the same genus as the famous and well-studied model lab nematode worm Caenorhabditis elegans. Caenorhabditis briggsae is a relative of C. elegans and is often used in comparative studies with its more famous counterpart because many of the tools developed for C. elegans can also be used on C. briggsae. While C. elegans is the darling lab worm due to its usefulness in studying genetics and developmental biology, until very recently, very little is known about its natural ecology.

Worms in the genus Caenorhabditis are often associated with invertebrates, hitching a ride on them as a way of traveling between food sources, or even opportunistically feeding on their ride if it happens to drop dead for whatever reason. In a paper published last year, a group of researchers reported on a strain of C. briggsae (KT0001) from South Africa displaying an ability not previously known for any Caenorhabditis species - it is capable of infecting and killing wax moth larvae. This strain of C. briggsae was found to be in a symbiosis with the pathogenic bacteria Serratia which presumably allows C. briggsae (KT0001) to become a parasitic killer.

Furthermore, when the researchers tested 10 wild strains of Caenorhabditis species which had not previously displayed any ability to infect insects - including a strain of C. elegans - and cultured them with Serratia, all but one strain gained the ability to infect, kill, and reproduce in insects, including the famous C. elegans. It seems that Serratia gives Caenorhabditis a license to kill - upon forming a partnership with the bacteria, these worms turn from mere passengers into deadly killers.

Abebe, E., Jumba, M., Bonner, K., Gray, V., Morris, K., Thomas, W.K. (2010) An entomopathogenic Caenorhabditis briggsae. Journal of Experimental Biology 213: 3223-3229.

August 10, 2011

Cytinus hypocistis

For a change of pace today the blog is going to feature a parasitic plant. Cytinus hypocistis is a holoparasitic plant, which means that unlike ordinary plants it does not perform photosynthesis, but obtains all the nutrients that it needs from its host. Cytinus hypocistis is embedded entirely within the the root of its host plant, but in spring, it pokes flowers out of the ground, which are then pollinated by ants and ripen into berry-like fruits. Each of these fruits contains thousands of tiny seeds, each about 0.2 mm in length.

What makes C. hypocistis unusual is that while most fruit-bearing plants rely upon vertebrate animals to disperse their seeds, C. hypocistis mainly uses a beetle. Researchers found that the seeds collected from beetle frass (fancy name for insect poop) are just as viable as seeds which are collected directly from the fruit. While rodents and rabbits also frequently consume C. hypocistis fruits, because they have a tendency to eat immature fruits and deposit their dung (with any viable seeds) at ground level, they are not as effective as the beetles. Not only do the beetles consume only fully-ripened fruits, they also have a tendency to bury themselves into the sand during midday, which can bring the seeds closer to the roots of the host plant.

This is one of the few known case of endozoochory (where the seed is consumed and pass through the gut of an animal) which involves an insect. The researchers of this study pointed out that this type of ecological interaction may in fact be quite widespread and common, especially for plants with very small seeds. However, they have simply been overlooked because all those involved were, quite literally, lurking meekly underneath our feet.

de Vega C, Arista M, Ortiz PL, Herrera CM, Talavera S (2011) Endozoochory by beetles: a novel seed dispersal mechanism. Annals of Botany 107: 629-637.

August 4, 2011

Isospora plectrophenaxia

Today's parasite is Isospora plectrophenaxia. A few weeks ago, you met a related species - Isospora lesouefi - the coccidian parasite found in the Regent Honeyeater which keeps a daily timetable, shedding most of its oocysts (the parasite's infective stage) in the afternoon. This is a well-described phenomenon among different species of Isospora - the parasite's shedding schedule appears to be calibrated by the light-dark cycle experienced by the bird host throughout the day. Indeed, experiments conducted on Isospora in house sparrow shows that if you disrupt the circadian rhythm of the host, you also mess up the parasite's shedding schedule.

Under natural condition, the usual light-dark cycle works just fine for most species of Isospora. But I. plectrophenaxia is found in the Snow Bunting (Plectrophenax nivalis) - a bird living in the High Arctic where there is perpetual sunlight during summer. So you'd think the shedding schedule of I. plectrophenaxia would be all messed up, right? Not so, researchers found that the parasite continues to stick to its regular regime of late afternoon shedding, just like all the other Isospora. At the moment researchers are unsure how I. plectrophenaxia is able to perform this feat. Perhaps this species is more sensitive to very low concentration of melatonin - the chemical secreted by the pineal organ which coordinates the bird's circadian rhythm, or perhaps it sets its timetable on different level of UV (ultraviolet) radiation exposure, which still varies throughout the Arctic summer day. Hopefully, ongoing research on this host-parasite system will shed further light on this little mystery, so watch this space!

Dolnik O.V., Metzger B.J., Loonen M.J. (2011) Keeping the clock set under the midnight sun: diurnal periodicity and synchrony of avian Isospora parasites cycle in the High Arctic. Parasitology 138:1077-1081.