Heligmosomoides polygyrus: a twirly taxonomic customer.

Heligmosomoides polygyrus (or is it H. bakeri?) Image via Wikipedia.

(post edited 6 October)

Last week, I taught in an undergraduate parasitology prac class. My job was to remove nematodes from lab mice so the students could examine them. The nematodes were were using were Heligmosomoides polygyrus, and they’re quite cool, because they  are coiled, and look a bit like twirly telephone cords. As I dissected the worms out the mice, I explained to the students how special these worms are. Why, you ask? Well, I’ll tell you.

Heligmosoloides polygyrus is a really nice model for exploring host-parasite relationships, for example in understanding  Th1/Th2 immune responses, and other aspects of immune system function. It’s also good as a model for drug targets for parasites of livestock. It is easy to work with, but it’s name is a problem.

As synthesised by Behnke and Harris (2010), the nematode was first described in 1842 from wood mice, Apodemus spp. as Strongylus polygyrus. It was synonomised with another species as Heligmosomoides polygyrus in 1922. Not long after, another species from Apodemus wood mice was described as Nematospiroides dubius. Between the late 1920s and the late 1960s, there was much confusion over the name of this nematode(s), and its range of hosts. During the 1930s, the parasite had been collected from Peromyscus maniculatus (an American mouse species) and was begun to be used in laboratory experiments.  This served to cause more confusion over what name should be used for the lab strains and the wild strains, because its host was not the usual one. To complicate the story even further, H. polygyrus had also been isolated from wild house mice, Mus musculus in America (as opposed to Europe where the wood mice lived). Were they the same species? They looked the same, but people still wrote about the worms using both names.

The name N. dubius was officially abandoned in 1991, and later in 2001 the species was split into two subspecies – H. p. polygyrus (wild strain) and H. p. bakeri (lab strain). Essentially, people were getting hung up on the host species (Apodemus vs. Mus) and the distribution (Europe vs. North America). To complicate it just a little more (as if it wasn’t confusing enough already), the lab strains were also being compared to the wild strains. Molecular evidence presented in 2006 (Cable et al. 2006) showed that there was indeed a difference between the two taxa, enabling them to be named as separate species – this creating two new species from one: H. polygyrus in Apodemus and H. bakeri in Mus (including lab mice).

But it doesn’t end there. Maizels et al (2011) contended that the methods used by Behnke and Harris were not that great, and that differences in infectivity of lab strains vs. wild strains of Heligmosomoides polygyrus/bakeri could not reasonably be used to split species. The issue of infectivity (or lack thereof) was most likely because the number of generations of nematodes that had passed through mice had probably resulted in selection of infectivity to lab mice over the years. They recommended the use of two subspecies of H. polygyrus as per the previous idea. But as they had no taxonomic evidence for this assertion, the current taxonomy of the two species is retained. Therefore, according to the current evidence, I was showing students H. bakeri, not H. polygyrus. Unfortunately, most undergraduate students don’t care for taxonomy, and when I explained the history of the names of these worms to them, several pairs of eyes glazed over…. But when they saw the worms down the microscopes, they seemed genuinely excited, so I’ll take that as a win.

And that’s where we’re at today. A complicated and convoluted early history has led to an ongoing complicated and convoluted taxonomy for this parasite(s). Chances are, we’re actually seeing evolution in action, as the lab strains become more distant genetically from the wild strain due to manipulation through many, many generations of host mice.

References:

Behnke and Harris (2010) Heligmosomoides bakeri: new name for an old worm? Trends Parasitol 26, 524-529.

Cable, Harris, Lewis, Behnke (2006) Molecular evidence that Heligmosomoides polygyrus from laboratory mice and wood mice are separate species. Parasitology 133, 111-122.

Maizels, Hewitson and Gause (2011) Heligmosomoides polygyrus: one species still. Trends Parasitol 27, 100-101.

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