Home » Evolution » Host-pathogen coevolution in a test tube: C. elegans running with the red queen

Host-pathogen coevolution in a test tube: C. elegans running with the red queen

Posted by Kasra

 

“Well, in our country,” said Alice, still panting a little, “you’d generally get to somewhere else — if you run very fast for a long time, as we’ve been doing.”

“A slow sort of country!” said the Queen. “Now, here, you see, it takes all the running you can do, to keep in the same place. If you want to get somewhere else, you must run at least twice as fast as that!”

***

The Red queen hypothesis was first proposed to explain why the fitness of some species stays the same, although they are constantly evolving. Arms race between a predator and prey, or a host and a pathogen are classic examples. Constant evolution occurs when the species is in a constantly changing environment. For instance, the host is infected by more virulent species of the pathogen each generation; therefore it is under constant selection for having a better and better immune system.  Similar selection pressure holds for the pathogen that is facing stronger and stronger immune systems, so it is under selection for more virulence. In such situations, the host and the pathogen coevolve, but their fitness doesn’t change, ‘it is as the Red queen says it takes all the running you can do to keep in the same place’.

There is another important implementation of the Red Queen hypothesis and that is in explaining sexual reproduction. In species that reproduce sexually, males charge an extra cost. They contribute little or nothing to the offspring, compared to females or hermaphrodites. They consume resources that could be otherwise consumed by offspring or females that could actually produce offspring. Theoretically, an asexually reproducing population of females could grow faster and produce more offspring than a sexually reproducing one. But it doesn’t just end there. Sexual reproduction creates a new selective force called sexual selection. This selective pressure is exerted on one sex that is being chosen by the other. It can result in selection of very bizzare traits that might actually reduce the fitness of the individual (think of the Peacock’s mganificant tail). Then why are higher eukaroytes reproducing sexually? What are the males for?

The big advantage of sexual reproduction is generation of genetic diversity. Without shuffling of genes through sexual reproduction, it would take a lot longer (or never) for a new advantageous mutation to be fixed in the population. In addition, new genotypes are created because different array of alleles are put together with each round of shuffling. Without this shuffling, these potentially advantageous allele combinations might never occure. Yeasts switch from asexual reproduction to sexual reproduction when they are under stress or in an unhospitable environment. In other words, although they are well fit to live in that environment, they hope that by shuffling their genes, some of the offspring would have better chances. Obviously if they continue reproducing asexually, their offspring would be identical to them and doomed for extinction.

However, this sexual-asexual switch does not exist in animals and plants (there are always exceptions in biology). This would make you infer that they are in a constantly changing environment that requires constant evolution or it would lead to extinction. In other words, it is like running with the red queen to keep fitness at the same place. But what is it that is constantly changing and imposing this strong selective pressure for such a high costly method of reproduction? It was proposed that presence of constantly evolving pathogens and parasites puts their hosts under selective pressure for adaptation. Sexual reproduction offers the selective advantage of genetic diversity and possibility of more resistance in the offspring. Same argument stands for selfing versus outcrossing strategies. Outcrossing is more costly. It requires actively seeking (dating!) another outcrossing partner. Also, outcrossing populations may have mixed subpopulations of hermaphrodites and males. In this case again, the males are an extra cost, while selfing limits the genetic diversity of the offspring.

It is difficult to create controlled evolutionary systems in a lab environment, thus evolutionary hypotheses are difficult to test. This is what makes the paper by Morran et al. published in Science in July 2011 very exciting. They tested the validity of the Red Queen hypothesis in the coevolution of hosts and pathogens.

The question was whether outcrossing offers selective advantage over selfing in the presence of a coevolving pathogen. They used Caenohabditis elegans and the deadly bacteria Serratia marcescenes as the host-pathogen couple. Three different strains of C. elegans were used:  strictly selfing, strictly outcrossing and wildtype (a mixture of both). ‘Coevolution’ of C. elegans and S. marcescenes was done by sequentially infecting surviving C. elegans worms with bacteria that were isolated from dead worms in the same culture. This was in comparison with ‘evolution’ of C. elegans where it was being reinfected with the ancestral strain that both cultures had started with. In the coevolution model, the bacteria were also supposedly under selection for more infectivity since the surviving worms were more resistant to infection. This experiment was carried over for 30 generations.

What would the red queen hypothesis predict for these conditions? Infection with deadly bacteria is supposed to induce more outcrossing as opposed to selfing in order to increase the chances of resistance in the offspring. However, once resistance has been acheieved, we would expect the worms to revert back to selfing as they are no longer under selective pressure. However, if resistance never occurs, outcrossing rates are expected to remain high compared to control. And this is what Morran et al. observed with the wildtype C. elegans worms. While evolving C. elegans worms that were exposed to a nonevolving strain of S. mersescenes peaked in outcrossing rate within 8 generations, by generation 30 the outcrossing rate was down back to control levels. However, the outcrossing coevolving worms showed continuously increasing outcrossing rates plateauing after 16 generations.

This clearly supports the red queen hypothesis in explaining the underlying reason for outcrossing and presence of non-offspring-bearing males in a population. However, the authors did not stop here and further backed up their argument:

They showed that the coevolving bacteria indeed have higher infectivity in strictly selfing, or non-coevolving C. elegans. Furthermore, the strictly selfing worms did not adapt to infection. Higher mortality rates were observed after multiple generations and also the coevolving bacteria were shown to be more infectious compared to the ancestral strain; this was not the case with the strictly outcrossing or wildtype strains.

So all together, this is an elegantly performed evolution experiment that highlights the importance of outcrossing in adaptation to pathogens. As it is beautifully put in Science2.0: The Red Queen is not dead. Long live the Red Queen.

Genes are not the only things that get better with sex. Matt Ridley, the author of The Red Queen , discusses how technology advances in the human society when ideas have sex in a TED talk.

Oh, and happy 2012!

Morran LT, Schmidt OG, Gelarden IA, Parrish RC 2nd, & Lively CM (2011). Running with the Red Queen: host-parasite coevolution selects for biparental sex. Science (New York, N.Y.), 333 (6039), 216-8 PMID: 21737739

ResearchBlogging.org

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