"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

June 27, 2012

Xenopsylla ramesis

There is no parasite that is universally infective, even generalist parasites that can infect many different host species are usually limited to a particular taxonomic group - such as fish, insects, or mammals. Some parasites may infect a broad spectrum of hosts during one stage of their life-cycle, but are very specific in another. For parasites that are host specialists, this can be taken to an extreme level where they are found exclusively on just one particular host species. Just how parasites evolve from generalist to become so specialised is one of the enduring questions in studies of the evolutionary ecology of parasites.

Image from of the related and more well-known
Xenopsylla cheopis (also known as the plague flea)
from the NHM
To investigate this question, a group of scientists from Israel carried out an experiment on Xenopsylla ramesis - a species of flea that infects a number of different desert rodents. For their experiment, the scientists raised separate populations of fleas on two species of desert rodents - Wagner's Gerbil and Sundeval's Jird - both of which are commonly infested with X. ramesis in the wild. Each of the experimental flea populations were assigned to either gerbils or jirds, and raised for nine consecutive generations on their specifically assigned rodent species. Out of every generation, the scientists also took a subset of 30-50 fleas from each of the experimental populations, and transferred them onto the other host species to see how those fleas performed compared with their counterparts that got to stay with the specific rodent host that they have been assigned with.

For the first three generations, there were no noticeable differences when the fleas were switched from either gerbils or jirds onto the alternative host. But by the sixth generation, the fleas have become so attenuated to the specifically assigned rodent species that when they were transferred to a host that was different to the one that their parents were raised on, they suffered drastically. Female fleas which had been transferred to the alternative host produced far fewer eggs (only about a quarter of the number produced by fleas that got to stay with their assigned host), and out of those eggs which were laid, fewer of them actually hatched, and out of the larvae that hatched, very only one-quarter to one-fifth reached full maturity.

The scientists who conducted this study suggested that this came about through what is known as "relaxed selection". When the fleas had been infecting multiple host species, there was selection pressure on them to maintain whatever full suite of adaptations that had allowed them to feed off a broader spectrum of hosts. But when the population is restricted to a single host species, there is no longer any selective advantage in maintaining the full suite of traits. Thus the adaptation(s) associated with infecting those other hosts (which they are no longer exposed to) were discarded, leaving only the specific adaptation(s) that are relevant to exploiting the available host species.

Another thing to note is that the natural ability of X. ramesis to live off multiple rodent hosts deteriorated very rapidly - within just a few generations - and the effects were drastic. The authors suggested it might have occurred through epigenetic modifications - inheritable changes in gene expressions which do not involve any changes in organism's DNA sequences (instead of mutations, which alter the underlying structure of the DNA). Another possibility, which the scientists who conducted this study did not raise, is whether the gut microbes of the fleas played a role in their ability to exploit different host, as it has been shown to be the case for some plant pests. However, little is known about the microbes that inhabit flea guts, apart from pathogens that are known to be vectored by fleas such as the bacteria that causes the plague.

Arbiv, A., Khokhlova, I.S., Ovadia, O., Novoplansky, A. and Krasnov, B.R. (2012) Use it or lose it: reproductive implications of ecological specialization in a haematophagous ectoparasite. Journal of Evolutionary Biology 25: 1140-1148.

P.S. Don't forget, both Susan and I will be attending parasitology conferences happening on our respective continents in July and we will be tweeting about them - you can find me on Twitter @The_Episiarch and Susan @NYCuratrix. I will be tweeting the Australian Society for Parasitology conference 2-5 July, while Susan will be tweeting the American Society of Parasitologists conference 13-16 July. Follow the hashtag #ASP2012 for relevant tweets. On the 2 July, there will also be a livestream public talk call "Parasite Encounters in the Wild" - Twitter participation is encouraged so feel free to tweet your question with the hashtag #ParasWild during the talk.

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