For symbionts, transmission is dispersal. When free-living species (e.g., lions, aphids, and ducks) disperse, we expect them to have dispersal strategies that have been favored by natural selection; they should leave habitats where fitness prospects are low and preferentially disperse to habitats where fitness prospects are high, as long as the fitness benefits outweigh the costs. Logically, symbionts should also move from low quality to high quality habitats, where the “habitats” are hosts or locations on the host. However, we almost always assume that symbiont transmission/dispersal is a random process with a fixed rate. That is, we assume that host quality or site quality on the host doesn’t matter. But guess what? IT DOES MATTER. And you can read all about it in my favorite symbiont ecology paper from 2015! I’ll summarize it for you here:
The branchiobdellidan-crayfish symbiosis is one of my favorite symbiont-host systems, so I’ve blogged about it several times previously (e.g., here and here). In contrast to the Chaetogaster-snail system that I talked about last week, it’s relatively easy to measure branchiobdellidan fitness, because the branchiobdellidans lay cocoons on their crayfish hosts. Adult branchiobdellidans stay nearby and tend their little cocoon gardens (adorable!), so it’s easy to quantify each worm’s reproductive output.
In a field survey of branchiobdellidans on crayfish, Skelton et al. (2015) found that branchiobdellidan reproduction depended on crayfish size and the microhabitat on the host; some microhabitats favored branchiobdellidan reproduction, while cocoons were never found in other microhabitats. Also, there was a limit to the number of worms found in any given microhabitat, where some microhabitats on the crayfish could support more worms than others. And here’s something even more awesome: branchiobdellidans weren’t found in the suboptimal microhabitats unless the better microhabitats were already full. Ideal free distribution, anyone? SO. COOL.
But it gets better. Using the field survey data, Skelton et al. (2015) built a symbiont fitness-based dispersal model that incorporated crayfish size and microhabitat occupancy and quality, where there was some fitness threshold below which worms would disperse from donor to receiver crayfish. Then they ran a lab experiment where they put donor crayfish (with worms) in tanks with receiver crayfish (without worms), and counted how many worms dispersed and where on the hosts the worms ended up. Skelton et al. (2015) didn’t know what the worm fitness threshold was, so they used a model fitting procedure to figure out which threshold produced the best fits to the experimental data. The resulting fitness-based dispersal model could predict whether worms would disperse with 95% accuracy. 95% ACCURACY!! And 67% of the time, the model predicted the exact number of worms that dispersed. In contrast, the model that assumed a fixed rate of dispersal – with no influence of host size or microhabitat occupancy – couldn’t predict dispersal any better than a null model. When’s the last time an ecologist predicted something with 95% accuracy?!
So, symbiont dispersal not only depends on symbiont fitness prospects, but knowing which factors influence symbiont fitness can allow us to predict symbiont transmission/dispersal with incredible accuracy – much better than if we assumed a fixed rate. This has huge implications for the way that we model symbiont transmission! Go check out the paper. It’s beautiful.
Skelton, J., R.P. Creed, and B.L. Brown. 2015. A symbiont’s dispersal strategy: condition-dependent dispersal underlies predictable variation in direct transmission among hosts. Proceedings of the Royal Society B 282: 20152081.