Tag Archives: host

Measuring Immunity With Transparent Hosts

Host controls of within-host dynamics: insight from an invertebrate system (2021) Stewart Merrill et al., The American Naturalist. https://doi.org/10.1086/715355

This is a guest post by Dr. Tara Stewart Merrill

Image Credit: Per Harald Olsen, NTNU, CC BY 2.0, Image Cropped

The Crux

When it comes to understanding how parasites and pathogens spread, immune defenses may be an especially important factor. The immune system is the gatekeeper for parasites and pathogens (I’ll just use the term “pathogen” from here on out). Whether you are exposed to influenza, a parasitic worm, or a tick-borne bacterium, your immune response will determine the outcome of infection — either you will become infected (which benefits the pathogen’s reproduction) or you will not (which is a barrier to the pathogen’s reproduction). So now, picture a whole population of individuals. A room full of individuals with poor immune responses should result in more infections (and more transmission) than a room full of individuals with strong and robust immune defenses. By shaping the fate of pathogens, host immune defenses can shape transmission.

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Some (Don’t) Like it Hot

Do latitudinal and bioclimatic gradients drive parasitism in Odonata? (2021) da Silva et al., International Journal for Parasitology. https://doi.org/10.1016/j.ijpara.2020.11.008

Image Credit: Adam Hasik, image cropped

The Crux

If there is one thing that people know about me and my research it’s that I love parasites. They’re everywhere, and more than half of all animals are parasites. They also make ecosystems more stable and link organisms within food webs to one another. For example, some parasites connect prey animals and their predators by making it easier for the predator to find and/or eat the prey. Though they can be found all over the world, there are a variety of environmental factors that make it more likely for a parasite to be found in a given environment. Today’s study focuses on one particular hypothesis related to the effects of the environment, the latitudinal diversity gradient (LDG, see Did You Know).

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Incorporating Parasites Into Community Ecology

I’ve said it before and I’ll say it again until I retire*: parasitism is THE most interesting (and arguably the most successful) life history strategy on the planet. Parasites are present in every ecosystem on the planet, and it is incredibly unlikely that any study system or ecological community is parasite-free. So why don’t we talk about them more?

As a disease ecologist, my work focuses on parasites and their place in the natural world, so I think about these organisms a lot. My PhD was centered on incorporating parasites into food webs to understand how they affect species interactions (and how species interactions in turn affect them). Failing to consider parasites can lead scientists to miss important aspects of an ecosystem and draw false conclusions.

Yet most ecological studies – even those which look at entire communities – fail to consider parasites and their effects on other organisms. I can’t blame them, parasite ecology can be difficult to get your head around. So today, I want to try and give ecologists everywhere some tips on incorporating parasites into their work.

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Is the Enemy of My Enemy My Friend?

Natural enemies have inconsistent impacts on the coexistence of competing species (2021) Terry et al., Journal of Animal Ecology. http://doi.org/10.1111/1365-2656.135434

Image Credit: Alandmanson, CC BY 4.0

The Crux

In nature, organisms are often competing with other organisms for food, mates, or even just for a place to call home. This competition comes in two forms: interspecific competition (meaning competition between two different species) and intraspecific competion (meaning competition within the same species). These two forms of competition play into the phenomenon known as mutual invasibility (see Did You Know), which is a necessary component of coexistence. If two organisms coexist, one species will not outcompete the other and drive it extinct, and thus the two species will coexist over time.

Because competition plays such a strong role in species coexistence, any factor that affects competition between two species has the potential to also affect coexistence. Today’s authors wanted to ask how an antagonistic species interaction (specifically, interactions with a parasitoid) affected coexistence in rainforest flies.

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Cause and Effect

Temporally consistent species differences in parasite infection but no evidence for rapid parasite-mediated speciation in Lake Victoria cichlid fish (2020) Gobbin et al., Journal of Evolutionary Biology. https://doi.org/10.1111/jeb.13615

Image Credit: Kevin Bauman, CC BY 1.0

The Crux

Ecological speciation (see Did You Know?) can be driven by both abiotic (non-living) and biotic (living) factors. The biotic factors that tend to be studied in regards to ecological speciation are antagonistic in nature, such as competition for resources or interactions with predators. However, parasitism is another antagonistic species interaction that is ubiquitous in nature, and therefore might be expected to contribute to ecological speciation via its effects on host-parasite coevolutionary dynamics.

Though a number of studies have investigated the effects of parasites on ecological speciation, little is known about the role of parasites in adaptive radiations, which are bursts of speciation from a single ancestor to many descendent species that then adapt to fill new ecological niches. In other words, an ancestor will be adapted to a specific environment/food types, but its descendants adapt to live in different environments/eat different food. One of the best examples of an adaptive radiation are the Africa lake cichlids, which are the focus of today’s study. The authors wanted to understand if parasites may have contributed to/caused the adaptive radiation seen in African lake cichlids.

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The More the Merrier

Viral zoonotic risk is homogenous among taxonomic orders of mammalian and avian reservoir hosts (2020) Mollentze & Streicker, PNAS. https://doi.org/10.1073/pnas.1919176117

Image Credit: Tom Spinker, CC BY-NC-ND 2.0

The Crux

Diseases that jump from other animals to humans, or zoonotic diseases (see Did You Know?) have become something that all of us are now very familiar with. COVID-19 is one such disease, and the impact it has had on the world as a whole is all the evidence that anyone could ever need for understanding why it is important to know where these diseases come from. Classically, specific groups of animals have been thought to act as reservoirs for the viruses that cause these diseases. Take rabies, for example. This is the disease that results in rabid animals, but you may not know that bats act as a reservoir for rabies, meaning that the rabies virus survives within bat populations and can be spread by them.

This is known as the “special reservoir hypothesis”, and it posits that there are certain traits associated with these reservoir species and/or their ecology that make them more likely to act as reservoirs for these viruses. In contrast, it could be that all animal species are equally likely to act as a reservoir for zoonotic viruses, and the risk of virus transmission is instead due to how many host species are within a given group of animal hosts. All this means is that you expect to find more diverse groups of animals hosting a more diverse group of viruses. This is known as the “reservoir richness hypothesis”.

In order to better manage zoonotic disease emergence and even predict where it is likely to occur in the future, it is important to understand if there are indeed special reservoirs among animal hosts, or if disease emergence is instead a consequence of host species richness. Today’s authors utilized data on zoonotic viruses and host species to understand this relationship.

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Changing With the Times

High parasite diversity accelerates host adaptation and diversification (2018) Betts et al., Science. https://doi:10.1126/science.aam9974

Image Credit: Dr. Graham Beards, CC BY-SA 3.0

The Crux

Host-parasite relationships are often thought of or depicted in a pairwise structure. That is, one host is attacked by one parasite, without an acknowledgement or consideration of how complex the relationship can be. For example, hosts are often attacked by more than one type of parasite, and the parasites themselves have to compete with one another for resources from the host. Because parasites are costly for a host, the hosts benefit from evolving resistance to the parasites. It follows that the more parasites a host is attacked by, the higher the benefit of evolving resistance, so we’d expect to see more resistance in hosts that are attacked more often. This should then result in differential evolutionary rates among hosts, which would then result in greater evolutionary divergence (see Did You Know?)

To test this idea, the authors of today’s study used a bacterium (Pseudomonas aeruginosa) and five lytic viral parasites (hereafter bacteriophages). These bacteriophages reproduce within host cells until they eventually cause the host to burst, killing the host (think of the chestburster in Alien, but a LOT of them). Because their reproduction results in the death of the host, lytic parasites impose a very strong selection pressure on hosts, making this a perfect host-parasite system to test the above prediction.

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Everywhere I’m Local

The cost of travel: how dispersal ability limits local adaptation in host–parasite interactions (2020) Johnson et al., Journal of Evolutionary Biology. https://doi:10.1111/jeb.13754

Image Credit: Francis Eartherington, CC BY-NC 2.0, Image Cropped

The Crux

There are countless parasites in nature, and many of them tend to have relatively short life-cycles. For example, ticks live for about two years, while may of their hosts (us included) live for much longer. Because there is such a disparity in lifespan, parasites are predicted to have a greater evolutionary potential than their hosts. In other words, parasites should evolve faster than their hosts, which theoretically means that parasites should be more fit on local hosts than they would be on non-local hosts, as they would have had more time to adapt (i.e., local adaption, see Did You Know?).

Despite these predictions, the evidence from experimental studies of parasite local adaptation is mixed at best. Some studies show the adaptation to local hosts we’d expect, but some studies don’t. One reason for the lack of consistent evidence is that parasite dispersal between habitats can limit the ability of parasites to adapt. To help explain that I’ll use a comparison to cooking. If you are cooking a dish and you want to make it spicier you add in more spice. But imagine that when you add in that spice, you are also adding a lot of cream. The dish could be spicy, because you are adding spice, but the cream is diluting the spice and masking any potential heat. That is what parasite dispersal does to local adaptation: parasites within a given habitat (the dish) may have the ability to adapt to their hosts (become spicier), but because parasites from other habitats (the cream) are coming into their habitat and diluting those adaptations it masks any overall adaptation to the host (never gets spicy). Today’s authors therefore wanted to test how parasite dispersal affected local adaptation to hosts.

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The Enemy of My Competitor is My Friend

Image Credit: Andreas Kay, CC BY-NC-SA 2.0, Image Cropped

Specifc parasites indirectly influence niche occupation of non‑hosts community members (2018) Fernandes Cardoso et al., Oecologia, https://doi.org/10.1007/s00442-018-4163-x

The Crux

One of the oldest questions in community ecology is why do some species seem to co-occur with one another, while others don’t? Two hypotheses have been put forward to explain why this happens: environmental filtering and niche partitioning. Environmental filtering is when some abiotic feature of a given environment – such as the temperature or oxygen levels – prohibits some species from ever living in the same location as another. A very broad (and overly simplistic) example of this is that you would never see a shark living in the same habitat as a lion, because the shark needs to live in the ocean and the terrestrial Savannah of Africa where lions are found “filter” the sharks out. Niche partitioning, on the other hand, involves species adapting to specialize on a given part of the environment, thus lessening competition for a niche by dividing it up. You can see this with some of Darwin’s Finches, which adapted differently-sized beaks to feed on differently-sized seeds. They all still eat seeds, but they are not eating the same seeds. 

Interactions with other organisms, either direct or indirect, can also influence which species co-occur. If one species can out-compete another, they likely won’t be able to co-occur because the better competitor will take most of the resources, forcing the other out. This can all change, however, if a third organism affects the competitive ability of the superior competitor, allowing the inferior competitor to persist despite its lesser ability.

Today’s authors used two spider species to study community assembly and how it may be affected by a fungal parasite. Chrysso intervales (hereafter inland spiders) builds webs further away from rivers, while Helvibis longicauda builds webs close to the river (hereafter river spiders). Interestingly, only the river spiders are infected with the fungal parasite, thus they investigated how interactions between the two spiders may be mediated by this fungal parasite. Read more