Big fleas have little fleas, Upon their backs to bite ’em, And little fleas have lesser fleas, and so, ad infinitum.

One of the many problems farmers of various kinds of legumes need to deal with is the pea aphid, which reproduce incredibly fast and live by sucking the sap out of the plants. However, while they are terrifying parasites of legumes, they have their own yet more horrific parasites, a parasitoid wasp. Below is a really nice close up picture of one doing its thing, here is a video of the act, and here is a brain meltingly horrific video of a dissection of the mummified aftermath 8 days later. Essentially, these wasps deposit their eggs in a pea aphid and the growing larva feeds on it, developing there for about a week, and then consuming the host from the inside out like a Xenomorph. When it’s done, the wasp larva dries the aphid’s cuticle into a papery brittle shell and an adult wasp emerges from the aphid mummy. Legume farmers love these wasps as much as they despise the aphids that destroy their crops, however, when farmers noticed that the wasps didn’t work as effectively on all of aphids infestations, Nancy Moran’s group at the University of Texas in Austin went to work figuring out why. It turns out that all aphids have a primary bacterial endosymbiont living inside their cells, in addition to and much like a mitochondria, and that many have some combination of five other known secondary endosymbionts. Interestingly, two of those other five, Hamiltonella defensa and Serratia symbiotica have been shown to confer varying levels of resistance to the parasitoid wasp, allowing the aphid to survive infection. However, it turns out that there is yet one more layer to this story,

The relationship these endosymbionts have with the aphid, as well as the primary endosymbiont, is hard to classify as they confer a fitness cost in the absence of the wasp but a significant fitness boost when the wasps are around and trying to infect the aphids. At least for H. defensa, the reason why some strains are fully parasitic and provide no protection against the wasps while others are at least plausibly commensal and do provide protection, is a bacterial virus that infects the endosymbiont, even while it is inside the eukaryotic aphid cell. To understand why it will require a bit of knowledge of how some bacteriophages work. Most bacterial viruses, also known as bacteriophages, have a clear dividing line between two strategies. The simplest and most virulent phages will always immediately shut down their host’s metabolism upon infection and replace it with their own. Within a short period of time, generally between 20 and 80 minutes, the phage will have used the host cell to replicate its genome, build new viral particles, packed those particles with the genome and lysed the cell; setting loose 30-3000 new inert infectious particles. These are known as obligately lytic phages. Most phages however, use a mix of this strategy and another one known as lysogeny. These temperate phages will, at the beginning, decide to either virulently infect, producing particles at the total expense of the host, or hide in the host’s genome and inactivate all of its many host lethal genes. Generally it does this by expressing a transcriptional repressor that prevents expression of everything but the repressor, which incidentally protects the host from subsequent infection by related phages. However, some temperate phages will allow for expression of a genomic cassette that will perform some function of benefit to their host – they might as well since they are completely dependent on their host’s wellbeing while in this stage of their life cycle.
It turns out that there is a temperate bacteriophage called APSE, which is common in H. defensa populations in the aphids, that encodes for a cassette of genes that causes H. defensa to attack the wasp larvae with vicious toxins while the phage hides in the genome of its bacterial host. This makes for a really fascinatingly complex system of interdependencies for each of the agents involved. The phage, the bacterial symbiont, and the aphid are all each united in their interdependent need to combat the wasp that kills all three when it succeeds. However, at the same time, both the phage and the bacteria are dependent on the wasp to apply pressure on the aphid to keep them around – otherwise the aphid would cure itself of both creatures that would then be free-loading. Additionally, the wasp the bacteria, and the phage are all completely dependent on the aphid’s sap sucking ability to sustain them, and the aphid is totally dependent on the farmer to continue growing legumes in massive vulnerable monocultures. Furthermore the farmer and the legumes are dependent on the wasp to combat the aphid and largely helpless against the bacteria and the phage. At the same time, despite all of the interlocking incentives toward cooperation, there are also incentives towards each of these agents cheating each other. The farmer has an incentive to ‘cheat’ and save money by neglecting to buy aphid mummies every so often, because they still benefit from the fitness cost caused by the aphid not rejecting the bacteria or the bacteria rejecting the phage. Similarly, the aphid has an incentive to cheat both the bacteria and the phage to cure itself of them and bet on the farmer not buying aphid mummies full of wasps that year. The bacteria also has an incentive to cheat the aphid by curing itself of the phage, and also bet on the farmer not buying mummies that year itself.
What I love most about this story is that complex series of interdependent yet competing evolutionary interests, which forms as an emergent property of the Siphonaptera – this blog’s namesake,

Big fleas have little fleas, Upon their backs to bite ’em, And little fleas have lesser fleas, and so, ad infinitum. And the great fleas, themselves, in turn Have greater fleas to go on; While these again have greater still, And greater still, and so on.

It is much like another wonderful paper, where a woman’s eye is a big flea bitten the smaller flea of an Acanthamoeba polyphaga parasite,  which is in turn bitten by its Mimiviridae (Lentille virus), which is bitten by its virophage (Sputnik 2), which is itself in a sense bitten by its mobile genetic elements. We have a situation where the farmer is a big flea bitten by their legume, which is bitten by its aphid, which is then bitten by both its parasitoid wasp and its secondary endosymbiont, which is in turn then bitten by its temperate bacteriophage.

This post is deeply indebted to one made on Moselio Schaechter’s excellent blog Small Things Considered, which is slightly more technical and no doubt more clearly written.

Bacteriophages encode factors required for protection in a symbiotic mutualism

Oliver KM, Degnan PH, et al. Published 2009 in Science doi: 10.1126/science.1174463  [REQUIRES FREE REGISTRATION]

Bacteriophages are known to carry key virulence factors for pathogenic bacteria, but their roles in symbiotic bacteria are less well understood. The heritable symbiont Hamiltonella defensa protects the aphid Acyrthosiphon pisum from attack by the parasitoid Aphidius ervi by killing developing wasp larvae. In a controlled genetic background, we show that a toxin-encoding bacteriophage is required to produce the protective phenotype. Phage loss occurs repeatedly in laboratory-held H. defensa–infected aphid clonal lines, resulting in increased susceptibility to parasitism in each instance. Our results show that these mobile genetic elements can endow a bacterial symbiont with benefits that extend to the animal host. Thus, phages vector ecologically important traits, such as defense against parasitoids, within and among symbiont and animal host lineages.

The players in a mutualistic symbiosis: insects, bacteria, viruses, and virulence genes.

Moran NA, Degnan PH, et al. Published 2005 in PNAS USA, doi:10.1073/pnas.0507029102

Aphids maintain mutualistic symbioses involving consortia of coinherited organisms. All possess a primary endosymbiont, Buchnera, which compensates for dietary deficiencies; many also contain secondary symbionts, such as Hamiltonella defensa, which confers defense against natural enemies. Genome sequences of uncultivable secondary symbionts have been refractory to analysis due to the difficulties of isolating adequate DNA samples. By amplifying DNA from hemolymph of infected pea aphids, we obtained a set of genomic sequences of H. defensa and an associated bacteriophage. H. defensa harbors two type III secretion systems, related to those that mediate host cell entry by enteric pathogens. The phage, called APSE-2, is a close relative of the previously sequenced APSE-1 but contains intact homologs of the gene encoding cytolethal distending toxin (cdtB), which interrupts the eukaryotic cell cycle and which is known from a variety of mammalian pathogens. The cdtB homolog is highly expressed, and its genomic position corresponds to that of a homolog of stx (encoding Shiga-toxin) within APSE-1. APSE-2 genomes were consistently abundant in infected pea aphids, and related phages were found in all tested isolates of H. defensa, from numerous insect species. Based on their ubiquity and abundance, these phages appear to be an obligate component of the H. defensa life cycle. We propose that, in these mutualistic symbionts, phage-borne toxin genes provide defense to the aphid host and are a basis for the observed protection against eukaryotic parasites.

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The origin of virulence, and why its important.

Virulence is an abstraction of the harm caused to hosts by a pathogen, and explaining the paradox of virulence has been an active field of study in evolution for a while. In general the harm caused to the hosts of pathogens is not great for the pathogen, after all, why hurt or lose a useful host? However, in studying the abstraction with basic research, we’ve found that virulence is almost always is part of helping the pathogen find a new host. Thus the generalized answer to the paradox is that so long as the harm to the host causes the parasite to spread effectively enough, it doesn’t really matter how much harm is caused to the host – as the parasite will have already found new hosts to spread from. At the same time, helpful bacteria don’t have nearly the same need to spread as pathogenic ones, as they keep their hosts happy and alive and can stick around for longer.

Here I’ll introduce two papers demonstrating this model and try to convince you of how important it is.

The spectrum between virulence and mutualism can be seen as a trade off between two strategies, as well as of course often a mix between the two. A critter existing in community with another one can care little for its host and work to be as infectious as possible at the host’s expense, thus increasing virulence. In this strategy it doesn’t matter so much that the host becomes quickly unsuitable so long as the parasite has already found replacement hosts sneezed on, or transmitted to, by the time that happens. Or it can do the opposite and try its best to reduce impact on the host, spread infectious particles slowly or even not at all, and thus not need to spread too quickly because it will last a while in each host. Most of the critters that live in our guts and on our skin are at this end of the spectrum, and have become so adept at not messing up their host as to actually benefit us in some way. On the other end of the spectrum are parasitoids. These are the parasites that not only destroy their host in their race to infect as many more hosts as possible, but spend the majority of their life cycle doing so and ultimately sterilize or kill, and sometimes consume the host in the process. The Xenomorphs from the movie Alien are a beautiful example of a bunch of these sorts of parasitiod strategies, each inspired by real terrifying stuff in nature. This might all seem uselessly theoretical, but the implications it has for public health are really cool.

Before the advent of antibiotics, we lived with Staphylococcus aureus strains on our skin that existed in a complex mixture of commensal and virulent strategies, but antibiotics suddenly applied very strong selective pressure against any vaguely virulent strategy. Thus, following the model, the observed sudden decrease in both virulence and transmissibility of virulent strains makes a lot of sense. However, the sudden increase in both virulence and transmissibility of virulent strains that we’ve seen in multi-drug resistant (MRSA) strains also makes sense. Indeed, if you look back far enough in the literature all of the crazy new and terrible virulence factors we are now seeing in MRSA strains all existed before the 1930s. For example, while the pyomyositis and necrotizing pneumonia we are now seeing is commonly associated with poverty, tropical climates and HIV, ie: things which didn’t get much attention prior to 1935, it was described. (At lest with this source you’ll need to wade your way past the kinds of phrases that start with “Africans are not different from any other humans, however, …” to page 1214) Until recently it would not be terribly remarkable, being easily addressed with a simple round of I.V. antibiotics.  Additionally, the PVL toxin which that first paper describes as now being found in pneumonia was initially discovered by Van deVelde in 1894 and was named after Sir Philip Noel Panton and Francis Valentine when they associated it with soft tissue infections in 1932. All of this makes logical sense anyhow, the mechanisms of antibiotic resistance are not associated with pathogenesis.

Timing of transmission and the evolution of virulence of an insect virus.

JC de Roode, AJ Yates, & S Altizer. Published 2002 in Proc. R. Soc. Lond. B doi:10.1098/rspb.2002.1976

We used the nuclear polyhedrosis virus of the gypsy moth, Lymantria dispar, to investigate whether the timing of transmission influences the evolution of virulence. In theory, early transmission should favour rapid replication and increase virulence, while late transmission should favour slower replication and reduce virulence. We tested this prediction by subjecting one set of 10 virus lineages to early transmission (Early viruses) and another set to late transmission (Late viruses). Each lineage of virus underwent nine cycles of transmission. Virulence assays on these lineages indicated that viruses transmitted early were significantly more lethal than those transmitted late. Increased exploitation of the host appears to come at a cost, however. While Early viruses initially produced more progeny, Late viruses were ultimately more productive over the entire duration of the infection. These results illustrate fitness trade-offs associated with the evolution of virulence and indicate that milder viruses can obtain a numerical advantage when mild and harmful strains tend to infect separate hosts.

Virulence-transmission trade-offs and population divergence in virulence in a naturally occurring butterfly parasite (PDF).

VS Cooper, MH Reiskind, et al. Published 2002 in PNAS doi:10.1073/pnas.0710909105

Why do parasites harm their hosts? Conventional wisdom holds that because parasites depend on their hosts for survival and transmission, they should evolve to become benign, yet many parasites cause harm. Theory predicts that parasites could evolve virulence (i.e., parasite-induced reductions in host fitness) by balancing the transmission benefits of parasite replication with the costs of host death. This idea has led researchers to predict how human interventions—such as vaccines—may alter virulence evolution, yet empirical support is critically lacking. We studied a protozoan parasite of monarch butterflies and found that higher levels of within-host replication resulted in both higher virulence and greater transmission, thus lending support to the idea that selection for parasite transmission can favor parasite genotypes that cause substantial harm. Parasite fitness was maximized at an intermediate level of parasite replication, beyond which the cost of increased host mortality outweighed the benefit of increased transmission. A separate experiment confirmed genetic relationships between parasite replication and virulence, and showed that parasite genotypes from two monarch populations caused different virulence. These results show that selection on parasite transmission can explain why parasites harm their hosts, and suggest that constraints imposed by host ecology can lead to population divergence in parasite virulence.