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.

Did You Know: Ecological speciation

Ecological speciation is the process by which differences in the local ecology drive differences in evolution and speciation. African lake cichlids (the focus of today’s study) are one of the best-known examples of ecological speciation in the scientific literature, with a multitude of species adapting to varying habitats and food types. Interestingly enough, there are also a number of African lake cichlids that haven’t undergone ecological speciation, despite inhabiting the a number of differing environments that would provide them with the perfect opportunity to do so.

What They Did

For parasites to play a role in an adaptive radiation, three criteria must be met. First, selection by parasites must differ within/between host populations, meaning that different host populations should have differing amounts and/or species of parasites. Second, infection by parasites must impose a cost on the host, as this is what drives selection on the host to adapt and overcome the cost. Finally, differences in parasite-mediated selection among populations must remain consistent over time. For example, if population A has a lot of parasites at time point 1, then it must also have a lot of parasites at time point 2.

Today’s authors evaluated criteria one and three by collecting hundreds of individual fish from 18 cichlid species and surveying the parasite community at two different time points, once in 2010 and once in 2014. Previous work has shown that parasites impose a cost on these fish, thus there is no need to evaluate criteria two. If fish from different populations differed in the number of parasites and parasite species they had, then this would fulfill criteria one for a role of parasites in adaptive radiations. Additionally, if these differences among host populations remain the same for both time points, then criteria three is fulfilled.

What They Found

The number of parasites differed among host populations, and these differences were consistent for both time points. However, the host species did not differ in how many parasite species they had. This means that host population A may have had 1000 parasites, and host population B may have had 100 parasites, but both host populations were infected by parasite species A. Because all of the radiating species had the same parasites, they were unlikely to have caused the adaptive radiation seen in these cichlids.

Though it may look calm, beneath the surface of Lake Victoria an intense and never-ending evolutionary battle rages between the various cichlid species and their multifarious parasites (Image credit: Linda de Volder, CC BY-NC-ND 2.0).

Problems?

Studies like the one we’re looking at today take a lot of work, but sometimes they are constrained by logistic or temporal constraints that may limit sampling. I do not know if that happened with today’s study, but the authors had a limited number of samples from some host-parasite associations. I doubt that these limited numbers affected the results, but it’s always better to have more samples. I would also be interested in knowing if these consistent temporal differences would remain on longer timescales?.

So What?

Because of their ubiquitous presence and the costs they impose on hosts, parasites make great candidates as potential drivers of adaptive radiations in nature. Though this study did not find evidence to support this theory, it is a crucial step in the right direction for allowing us to better understand the origin of species in nature.

Dr. Adam Hasik is an evolutionary ecologist interested in the ecological and evolutionary dynamics of host-parasite interactions. You can read more about his research and his work for Ecology for the Masses here, see his personal website here, or follow him on Twitter here.

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