Thursday, October 11, 2012

When mummies attack! Why specificity matters for coevolution

Evolutionary change by means of Natural Selection needs a couple of things in order to happen: heritability and variation in fitness. That is, offspring need to resemble their parents at least a little (heritability) and individuals need to differ in their survival and offspring production (fitness). We’ll worry about heritability in another post, but variation is something that seems like it might be hard to maintain. Some forms of Natural Selection will reduce variation as more fit individuals become frequent and all the different kinds of less fit individuals are eliminated from the population. However, there is a force, common in nature, which may maintain variation, parasites.

Interactions between hosts and parasites can generate strong selective pressures on each player, especially if your life depends on infecting a host. Often, biologists make an analogy to an arms race where players are developing bigger and better defenses or weapons. Antagonistic interactions may also generate negative frequency dependence where a rare host type is favored because the parasites are adapted to a common type. You can learn more by checking out two posts over at Nothing in Biology makes Sense (CJ’s post on the Red Queen Hypothesis or Jeremy’s post on a different coevolutionary puzzle). A key component for maintaining variation via negative frequency dependent selection is specificity. There must variation in the interaction among different host genotypes and parasite genotypes. This is sometimes referred to as a GxG interaction. If parasites can infect all the hosts, there is no specificity. Specificity allows different hosts to be favored over time depending on the composition of the parasite population.

Theoreticians love to use different models of interactions between hosts and parasites, but without empirical evidence, there seems little point. In a recent paper by Rouchet and Vorburger (2012), the authors looked for evidence of just the kind of genetic specificity would result in the maintenance of genetic variation.

Thursday, April 12, 2012

Multidimensional coevolution, no oscillation overthruster required

Conventional wisdom suggests that pathogens and parasites are more rapidly evolving because of various reasons such as short generation time or stronger selection. Yet somehow, they have not completely won the battle against the host. Recently, a theoretical paper on coevolution in Nature caught my eye (Gilman et al., 2012). Here the authors address this paradox: “How do victim species survive and even thrive in the face of a continuous onslaught of more rapidly evolving enemies?

Instead of treating a coevolutionary interaction between two species as the interaction of only two traits, the authors investigate the nature of an interaction among a suite of traits in each species. It’s not hard to think of a host having a fortress of defenses against attack from a parasite with an arsenal loaded with many weapons.

Tuesday, February 28, 2012

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.

  • 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