Bacterial Thunderdome: Decoding virulence, spiteful interactions, and diversity

What happens when two parasites infect the same host individual? Is the outcome similar to the Thunderdome: two parasites enter, one parasite leaves? Host-parasite interactions are rarely so simple. While a reductionist approach to understanding the interaction of a parasite or pathogen with its host may decompose the system to a single infection, nature is full of much more complex puzzles. Within the host, the battle itself raging between parasites (within-host competition) may have cascading effects on the host.

A recent paper on virulence caught my eye (Bashey et al., 2012) which provides an update to a very interesting result from the group a few years ago. The system includes bacterial parasites, along with parasitic nematodes, that infect insect larvae and eat/digest them from the inside out. Vigneux et al. (2008) found that when multiple parasite isolates are mixed in a host, the host mortality decreased. However, this only occurred when the isolates were not related. In the experiment, the researchers created low relatedness by mixing populations with migration. I reviewed the 2008 paper previously. The hypothesis was that chemical warfare among the parasites decreased the parasite load and reduced the negative effects on the host, virulence.

The bacteria produce chemical weapons, bateriocins, which can broadly harm other isolates, but relatives are left unharmed. These chemical weapons can be classified as spiteful: in the process of harming others they also harm the focal individual. This self-harm comes from the cost of making the chemical weapon. Others have labeled this antagonistic trait a greenbeard gene.

Greenbeards are genes that can identify the presence of copies of themselves in other individuals, and cause their bearer to behave nepotistically toward those individuals (Gardner and West, 2010).

Gardner and West explain that the origin of this term comes from Richard Dawkins illustrative example where individuals bearing this trait had green beards.

Experimental results: Recently, natural system specific isolates of the parasite have been cultured in the lab allowing more specific tests of the within-host competition (Hawlena et al., 2010a; Hawlena et al., 2010b). In the most recent paper, Bashey et al. (2012) found that the bacterial isolates that produce the toxin have a higher growth rate in coinfections (where within-host competition might be important). However, in the absence of coninfections, there was no relative growth rate advantage of the toxin producing, inhibitory, isolates. In coninfections, even though the toxin producing isolate was winning the internal host battle, lower host mortality rate emerged as an outcome.

A beautiful world: As an evolutionary biologist, I’m interested not only in the diversity of the natural world, but also the mechanisms that keep that diversity around. We often think of natural selection as favoring the fittest. If a single type is better than the rest, then over time, diversity will decrease. Often the analogy of hill climbing is used. On the other hand, if the fittest depends on the context of the other players involved, than diversity might be maintained. That is, what if the shape of the mountain range is constantly changing. In relation to this research,

bacteriocins might promote local diversity when producer, sensitive, and resistant strains are engaged in a version of the rock‐paper‐scissors game (i.e., the producer can kill the sensitive strain, the resistant strain outcompetes the producer, and the sensitive strain outcompetes the resistant strain) in a spatially structured environment (Hawlena et al., 2010b).

If bacteriocins are costly to produce, than they must provide some benefit in some contexts. Bashey et al (2012) suggest that this mechanism, where the fitness of a particular parasite isolate is context dependent, may explain the high frequency of bacteriocin production found in the natural populations surveyed in their earlier work (Hawlena et al., 2010a).

Stay tuned for future research by Dr. Farrar Bashey as she assures me more pieces to this puzzle will be revealed.

References

• Bashey F, Young SK, Hawlena H, Lively CM (2012) Spiteful Interactions between Sympatric Natural Isolates of Xenorhabdus Bovienii Benefit Kin and Reduce Virulence. Journal of Evolutionary Biology 25: 431-437. DOI: 10.1111/j.1420-9101.2011.02441.x
• Gardner A, West SA (2010) Greenbeards. Evolution 64: 25-38. DOI: 10.1111/j.1558-5646.2009.00842.x
• Hawlena H, Bashey F, Lively CM (2010a) The Evolution of Spite: Population Structure and Bacteriocin-Mediate Antagonism in Two Natural Populations of Xenorhabdus Bacteria. Evolution 64: 3198-3204. DOI: 10.1111/j.1558-5646.2010.01070.x
• Hawlena H, Bashey F, Mendes Soares H, Lively CM (2010b) Spiteful Interactions in a Natural Population of the Bacterium Xenorhabdus Bovienii. The American Naturalist 175: 374-381. DOI: 10.1086/650375
• Vigneux F, Bashey F, Sicard M, Lively CM (2008) Low Migration Decreases Interference Competition among Parasites and Increases Virulence. Journal of Evolutionary Biology 21: 1245-1251. DOI: 10.1111/j.1420-9101.2008.01576.x

Main Paper
BASHEY, F., YOUNG, S., HAWLENA, H., & LIVELY, C. (2012). Spiteful interactions between sympatric natural isolates of Xenorhabdus bovienii benefit kin and reduce virulence Journal of Evolutionary Biology, 25 (3), 431-437 DOI: 10.1111/j.1420-9101.2011.02441.x

Double, double toil and trouble: a tale of two infections

What are the evolutionary consequences of parasite superinfection (i.e. simultaneous infection by multiple parasites)? When parasites are genetically distinct, coexistence within a host generates conflict because of limited resources. How this conflict is resolved is the source of evolutionary research on the evolution of parasite life history traits such as virulence, the negative effects on the host caused by infection, and transmission mode, how parasites infect a new host. The transmission mode of a parasite is often characterized as occurring in one of two different modes: vertical or horizontal. With vertical transmission, an offspring obtains its parasites directly from its parents. In contrast, with horizontal transmission, infections occur either directly from the environment or contagiously by infection from other individuals.

My interest in the evolution of transmission mode in parasites and symbionts led me to a recent paper (Ben-Ami et al. 2011), which addresses the consequences of superinfection by two different parasites with different transmission modes of the waterflea, Daphnia magna, on virulence and parasite fecundity. Pasteuria ramosa is a castrating, horizontally transmitted, blood-infecting bacterium where spores are produced from the cadaver of the host Daphnia. Octosporea bayeri, a microsporidium, utilizes both vertical transmission to eggs and horizontal transmission via waterborne spores.

Photo by Paul Herbert in Gewin (2005)

Conflict resolution: The difference in the transmission strategies among the parasites generates an extreme conflict. O. bayeri needs the host to produce offspring for vertical transmission, that is the host and parasite have an aligned interest in producing offspring. On the other hand, P. ramosa is using host resources, including the reproductive tissues, to produce spores for infecting other hosts. Because of the alignment of interests between host and the vertically transmitting parasite, the question becomes: does infection by O. bayeri provide host protection from future infection by P. ramosa? In contrast, virulent parasites are expected to be more competitive by exploiting host resources more quickly than less virulent parasites. Here, P. ramosa may reduce infection by O. bayeri by competitive exclusion but at the cost of additionally reducing host survival.

To test these hypotheses, Ben-Ami et al. (2011) used two different infection experiments. The first tested the impact of horizontally occurring superinfection on host and parasite life history. These infections occurred simultaneously or sequentially (separated by 7 days). The second experiment used vertically infected hosts with O. bayeri parasites which were then exposed to P. ramosa for secondary infection. P. ramosa competitively excluded O. bayeri in double infections. Additionally, host fecundity was lower with super infections than with P. ramosa infection alone indicating an increase in virulence due to the interaction. The authors also found that vertical infection by O. bayeri provided no significant protection from future horizontal infection by P. ramosa. In fact, they found that P. ramosa was able to clear O. bayeri vertical infections and was clearly the superior competitor.

The part of the paper that I found most interesting was how the authors related their results to previous theoretical predictions. Many authors have addressed the interaction of parasites with different transmission modes (Altizer and Augustine 1997; Faeth et al. 2007; Haine et al. 2005; Jones et al. 2007, 2010; Lipsitch et al. 1996; Lively et al. 2005). Most of these previous models make assumptions about the lack of superinfection, suggesting one infection protects against a second. The authors of this paper point out that no one has specifically modeled the combination of a vertically transmitted parasite with one that can use both strategies and allows for superinfection.

In summary, I found that this paper and the results contained are clearly presented. While the authors did not find the support for the protective hypothesis, they did find evidence of increased virulence with coinfections as predicted. The authors do point out that these two parasites have a very narrow range of coexistence, in southwestern Finland, and suggest that coexistence may be a difficult or transient dynamic for this system. I would agree.

Interested in more? In addition to this interesting paper, Dieter Ebert’s group has recently published exciting research on the specificity and mechanism of infection by one of the parasites discussed the above paper, P. ramosa.: (Luickx et al. 2011; Duneau et al. 2011).

References

• Altizer SM, Augustine DJ (1997) Interactions between frequency-dependent and vertical transmission in host-parasite systems. Proceedings of the Royal Society of London Series B-Biological Sciences 264: 807-814. http://dx.doi.org/10.1098/rspb.1997.0113
• Ben-Ami F, Rigaud T, Ebert D (2011) The expression of virulence during double infections by different parasites with conflicting host exploitation and transmission strategies. Journal of Evolutionary Biology 24: 1307-1316. http://dx.doi.org/10.1111/j.1420-9101.2011.02264.x
• Duneau, D, Luijckx P, Ben-Ami F, Laforsch C, Ebert D (2011) Resolving the infection process reveals striking differences in the contribution of environment, genetics and phylogeny to host-parasite interactions. BMC Biology, 9:11. http://dx.doi.org/10.1186/1741-7007-9-11
• Faeth SH, Hadeler KP, Thieme HR (2007) An apparent paradox of horizontal and vertical disease transmission. Journal of Biological Dynamics 1: 45-62. http://dx.doi.org/10.1080/17513750601040367
• Haine ER, Boucansaud K, Rigaud T (2005) Conflict between parasites with different transmission strategies infecting an amphipod host. Proceedings of the Royal Society B-Biological Sciences 272: 2505-2510. http://dx.doi.org/10.1098/rspb.2005.3244
• Jones EO, White A, Boots M (2007) Interference and the persistence of vertically transmitted parasites. Journal of Theoretical Biology 246: 10-17. http://dx.doi.org/10.1016/j.jtbi.2006.12.007
• Jones EO, White A, Boots M (2010) The evolutionary implications of conflict between parasites with different transmission modes. Evolution 64: 2408-2416. http://dx.doi.org/10.1111/j.1558-5646.2010.00992.x
• Lipsitch M, Siller S, Nowak MA (1996) The evolution of virulence in pathogens with vertical and horizontal transmission. Evolution 50: 1729-1741. http://dx.doi.org/10.2307/2410731
• Lively CM, Clay K, Wade MJ, Fuqua C (2005) Competitive co-existence of vertically and horizontally transmitted parasites. Evolutionary Ecology Research 7: 1183-1190. http://www.evolutionary-ecology.com/issues/v07n08/iiar1894.pdf
• Luijckx P, Ben-Ami F, Mouton L, Pasquier L, Ebert D (2011) Cloning of the unculturable parasite Pasteuria ramosa and its Daphnia host reveals extreme genotype-genotype interactions. Ecology Letters 14:125-131. http://dx.doi.org/10.1111/j.1461-0248.2010.01561.x

Paper read

BEN-AMI, F., RIGAUD, T., & EBERT, D. (2011). The expression of virulence during double infections by different parasites with conflicting host exploitation and transmission strategies Journal of Evolutionary Biology, 24 (6), 1307-1316 DOI: 10.1111/j.1420-9101.2011.02264.x

Sex and death: a model of density-dependent virulence

Providing evidence that supports the role of parasites driving the maintenance of sex (i.e. the Red Queen hypothesis) has been a challenge ever since it was proposed. Both theoreticians and empiricists have tackled this hypothesis with vigor to mixed results. This week we read Lively (2009) which focuses on a singular effect to help build a theoretical argument for the Red Queen, density-dependent virulence. Here virulence is defined as the effect of the parasite on the host population growth rate. The density-dependent part comes into play in that the virulence increases with host population size.

The main argument of the paper is that as an asexual population invades a sexual population, the level of virulence changes and this can in turn change the outcome of the overall winner. Parasites with large density-dependent effects can change the balance and allow the maintenance of sexual populations. Presented in several graphs, virulence is a population measure of the effect of the parasites on the hosts. I'm still curious about the magnitude of selection on the individual genotypes in the model. When interpreting the results of this model, I was only able to see what happens when a group of asexual organisms invades a sexual one.

Lively provides an excellent ion description and understanding of the cost of sex. Of course the cost of sex has been detailed before, but the mathematical explanation helps with a basic intuition. The model described in the paper identifies two populations of hosts: asexual and sexually reproducing individuals. What he identifies is that in a sexual population, males provide little and females must produce at least two offspring to replace themselves. These males are using up resources. They are also decreasing the overall density of hosts that could be achieved in a complete female (or asexual) population.

One of the topics that came up during out discussion was how sex ratio may change or evolve during the evolution of sex. The simulation results presented in Lively (2009) assumes a sex ratio of 50/50 which makes sense in an evolutionary context. This has the effect of setting the advantage of the asexual population to be two fold over the sexual population. What happens when instead of two separate populations that do not interbreed, we have females choosing to produce offspring via sex or parthenogenesis? Will rare males in such a population change the early dynamics enough to produce different results?

References

Lively, C. M. 2009. The maintenance of sex: host-parasite coevolution with density-dependent virulence. J Evolution Biol 22:2086-2093.

LIVELY, C. (2009). The maintenance of sex: host-parasite coevolution with density-dependent virulence Journal of Evolutionary Biology, 22 (10), 2086-2093 DOI: 10.1111/j.1420-9101.2009.01824.x

Why doesn’t this pathogen kill me and why is it taking so long to clear?

This week the Coevolvers read a brand new paper by King et al (2009). The authors present a pathogen model that incorporates within host dynamics of pathogen growth as well as multiple forms of transmission among hosts which depend on pathogen load. The authors do motivate the study by telling us about two human disease pathogens, Bordetella pertussis and Bordetella parapertussis (which can cause whooping cough), but model is not meant to be a predictive model of future outbreaks. The main message of the paper is that including within host dynamics in conjunction with SIR models of populations leads to a better understand of disease evolution. Mideo et al (2008) wrote a recent review on including within host dynamics in evolutionary epidemiological models for more general information on this approach.

While the outline of the model was well written, how they combined the multiple different parts was unclear. The model consisted of three components: 1) within host pathogen replication 2) dose dependent transmission and 3) between host/SIR type model. What we found hard to understand was how the model incorporated the variation in pathogen loads among the hosts into the overall transmission rate. It appeared as if the model integrates over a number of classes of hosts (depending on their age of infection), but we felt that this then removed quite a bit of the variation that was being captured by including within host dynamics. A simplifying assumption that the authors made also was that each host was always infected with the same dose of pathogens and that their immune system had to be restarted each time. The authors do state that they have already worked on a stochastic model of this system which hasn't yet been published. We are very interested on the quantitative results from that analysis since some of these problems could be addressed there.

Why not make a population genetics model to address the questions posed by the authors at the beginning of the paper. This was question stimulated by our previous reading of Boots et al (2009) and Day and Gandon (2007) that provide detailed reviews of different modeling approaches as well as addressing specific problems in evolutionary epidemiology. King et al (2009) present their results of how intermediate within host pathogen growth rates can maximize R₀ under some transmission models, but what they don't do is present an analysis where they look at how different pathogens might evolve. Is the intermediate growth rate a stable strategy? Given the model framework, there might be complex interactions between different pathogens mediated through hosts. Higher growth rates of an aggressive pathogen could lead to a tragedy of the commons.

References

Boots, M., A. Best, M. R. Miller, and A. White. 2009. The role of ecological feedbacks in the evolution of host defence: what does theory tell us? Philos. Trans. R. Soc. B-Biol. Sci. 364:27-36.

Day, T and S Gandon. 2007. Applying population-genetic models in theoretical evolutionary epidemiology. Ecology Letters 10 (10), 876–888.

King, A. A., S. Shrestha, E. T. Harvill, and O. N. Bjørnstad. 2009. Evolution of Acute Infections and the Invasion-Persistence Trade-Off. The American Naturalist 173:446-455.

Mideo, N., S. Alizon, and T. Day. 2008. Linking within- and between-host dynamics in the evolutionary epidemiology of infectious diseases. Trends in Ecology and Evolution 23(9): 511-517.

Paper read:

King, A., Shrestha, S., Harvill, E., & Bjørnstad, O. (2009). Evolution of Acute Infections and the Invasion‐Persistence Trade‐Off The American Naturalist, 173 (4), 446-455 DOI: 10.1086/597217

Evolution of virulence revisited

In their recent paper, Vigneux et al (2008) address a classic idea in the evolution of virulence. When multiple genotypes of a parasite infect a single host, competition can influence the overall virulence. The paper is examines the interaction of relatedness and virulence. One viewpoint is that with a low level of relatedness, virulence should increase as competition among genotypes overexploits the host. Another hypothesis that the authors test is that different genotypes may engage in a "chemical warfare" inside the host. This would lead to a decrease in virulence as relatedness decreases.

Their overall results are completely consistent with their second hypothesis, increases in virulence with increases in relatedness as mediated through limited migration. Their evidence is that the host shows a quicker mortality in the low migration treatment. More compelling at least in gaining evidence for the role of interference competition is their growth inhibition assay. Bacterial clones from the low migration treatment did not inhibit the growth of other clones from the same host. When the authors tested clones from different hosts did still possessed some ability to inhibit growth.

While the details on the infection protocol in this paper seemed to make the results a little harder to understand, they did gain evidence that clearly support the role of interference competition on virulence. The proposed mechanism seems sound, but obviously could use further investigation. I initially misunderstood the role of migration in this experiment. To my understanding the effect of their different treatments was to reduce the variation among genotypes and increase the relatedness. Previous arguments about the role of transmission and virulence are not completely appropriate in the context of this experiment. Some of the discussion among our group focused on the role of kin selection in the evolution of greater virulence.

Some extra details: This experiment uses a rather complex host-parasite interaction consisting of a nematode (Steinernema carpocapsae) that is a parasite in insect larvae. However, unlike a previous paper (Bashey et al 2007) focusing on the nematode, here the main focus is a symbiotic bacterium of the nematode (Xenorhabdus nematophila) that along with the nematode induces mortality in the insect host. X. nematophila is also known to produce bacteriocins which inhibit the growth of other genotypes. Over the course of 20 host passages, the authors construct two types of experimental treatments. In one treatment (high migration), parasites from several lines are mixed together creating an infection containing bacteria. In the second treatment (low migration), the majority of parasite were transferred from a single host line. These two treatment setup a contrast of the potential relatedness of the bacteria in the current host.

References

Bashey, F., Morran, L.T. & Lively, C.M. 2007. Coinfection, kin selection, and the rate of host exploitation by a parasitic nematode. Evol. Ecol. Res. 9: 947-958.

VIGNEUX, F., BASHEY, F., SICARD, M., & LIVELY, C. (2008). Low migration decreases interference competition among parasites and increases virulence Journal of Evolutionary Biology, 21 (5), 1245-1251 DOI: 10.1111/j.1420-9101.2008.01576.x