Cyrtosomum penneri

The Atractidae is a family of nematodes (roundworms) that are found in the intestines or lungs of various vertebrate animals. Instead of producing eggs, the adults produce larvae that are ready to infect as soon as they leave their mother's womb. While it is well-known that a host already parasitised by an atractid nematode can infect themselves again thanks to the infective larvae, it was not entirely clear how this parasite get passed between different hosts.

Photo taken by Charles R. Bursery
Used with permission from Gerrut Norval

For the paper we are looking at today, a team of scientists studied a species of atractid nematode that infects lizards - Cyrtosomum penneri - and conducted a series of experiments to figure out how this parasite is transmitted in Brown Anoles. They did this by administering larval worms in a number of different ways to lizards that they had previously de-wormed with anti-parasite drugs. To ensure that they could tell afterwards if the worms they found in the anoles were the ones they administered, the scientists labelled them with a fluorescent dye (the same type as the ones I used in experimental infections to track larval flukes in bivalves: see here and here)

The larvae of many parasitic nematodes infect their hosts through being accidentally swallowed, usually while their hosts are feeding (this is commonly how mammalian herbivores like sheep, cattle, and horses become infected). However, when these scientists tried to do the same with C. penneri larvae by pippetting larval worms down the lizards' throats, none of the larvae were successful in establishing in the host.

Instead, they found the feces of anoles that had been fed parasite larvae were full of dead worms - presumably they were killed by gastric acid. Indeed, the environment in which C. penneri is usually found, down in the lower intestine, is pretty benign comparing with the acidic milieu of the stomach. But when they pipetted larval C. penneri into the cloaca of the lizard, it worked every time. Instead of having a separate opening, lizards have a cloaca - a common opening for their intestinal, urinary, and reproductive tracts. When lizards mate, they bring their cloacae together - and this is when C. penneri gets transmitted.

Yes, that's right - C. penneri is a sexually transmitted infection.

In the mating trials run by those scientists, male lizards infected with the nematode passed it on to the female they mated with every single time, whereas the female lizard only passed the STI to the male in seven out of the ten trials they ran. In addition to anoles, C. penneri is also found in a few other lizard species such as the Mediterranean House Gecko and Eastern Fence Lizard. But while it infects those other lizards in addition to the Brown Anole, for some reason it does not appear to parasitise the Carolina Anole... which means that it would be interesting to consider what happens to the parasite when something like what you see at this link happens...

Reference:
Langford, G. J., Willobee, B. A., & Isidoro, L. F. (2013). Transmission, host specificity, and seasonal occurrence of Cyrtosomum penneri (Nematoda: Atractidae) in lizards from Florida. Journal of Parasitology 99: 241-246

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.

Reference:
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

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.

Reference:
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.

November 30 - Pneunonema tiliquae

Our parasite for today is a nematode called Pneunonema tiliquae and it is the only species within its genus. It is found in the lungs of the Eastern blue-tongue lizard (Tiliqua scincoides), a cute-looking skink from Australia which can grow to 30 cm (about a foot) long or more. Nothing is known about this parasite's life-cycle or how it enters the host. Based on what is known about other species of lung-dwelling nematodes in reptiles, it is likely that the blue-tongue lizard becomes infected through the oral route, when infective larvae in the environment are accidentally ingested by the lizard alongside its food.

A second parasite found by Tommy Leung in a roadkill skink he found. Click here to see the first.

November 15 - Pharyngodon australis

I was on my way home from grocery shopping when I spotted something in the middle of the road near where I live. As I got closer I saw that it was a dead lizard. So like any good parasitologist, I quickly got home, parked my car, grabbed some plastic bags and dashed across the road, scooping up the lizard in the process. It must have only just been recently killed because rigor mortis hasn't even set in. So I thought I'd make something worthwhile out of an otherwise senseless death, drove to work and started dissecting the dead lizard, and sure enough, found this parasite! Pharyngodon australis is a species of nematode found in the large intestine of Eastern blue-tongue lizard (Tiliqua scincoides), a large ominvorous skink from Australia. Thousands of nematodes live in the gastrointestinal tract of skinks and other lizards. Stable isotope studies have indicate that some of these nematodes might be consuming microbes living in the host's gut, while other experiments showed that they might even contribute to gut fermentation. So this might be a case of what would normally be assumed to be a parasitic organism actually being a welcome guest!

Contributed by Tommy Leung.

June 18 - Plasmodium mexicanum

Plasmodium mexicanum is, without a doubt, the best-studied species of lizard malaria parasite, and that is thanks to decades of work by Dr. Joseph Schall and his students. This parasite infects western fence lizards (Sceloporus occidentalis), in northern California and Oregon, but unlike most other Plasmodium species, it doesn't use a mosquito as its vector; it uses a phlebotomine sandfly. Work by Schall and others demonstrated that this parasite does have fitness consequences for its hosts - females lay fewer eggs and males have trouble defending a territory from other males. The parasite also seems to affect the bright coloring on the bellies of these lizards.

Photo by Schall himself.

Happy birthday, Joe.

May 18 - Parapharyngodon cubensis

Today’s parasite is the pinworm Parapharyngodon cubensis. Species in this genus are parasites of reptiles and amphibians, with over 40 taxa known worldwide. These are characterized by a direct life cycle and fecal-oral transmission. Except for this brief transmission period, the parasite spends its entire life within and dependent on the host (usually its large intestine). P. cubensis is broadly distributed in the Caribbean and the only known non-Caribbean population is found on Bermuda in introduced populations of Caribbean anole lizards.

Like other pinworms, P. cubensis is haplodiploid. In low densities, the worms reproduce asexually, and have just one set of chromosomes (i.e., they’re haploid). Once the populations get bigger, and the parasites begin encountering one another, they have sex. Their offspring have two sets of chromosomes – one from each parent (i.e., they’re diploid, like us). This is a pretty good reproductive strategy for an otherwise lonely animal that may not encounter another of it’s kind in its lifetime.

Contributed by Bryan Falk.

January 11 - Plasmodium minuoviride

Humans are not the only animals that get malaria. In fact, there are over 100 species of Plasmodium that have been described from lizards worldwide. This is one of the newest species, Plasmodium minuoviride, reported last year by Perkins and Austin (link to paper). Its Latin name means "to draw green blood" - so given to this parasite, because the host in which it lives, the skink Prasinohaema prehensicauda, has green blood due to the presence of biliverdin in it.