Former postdoc Syuan-Jyun Sun has been writing up projects from his time in the lab, and had four papers come out recently!
First, Sun led an experiment that looked at how temperature modifies trait-mediated infection outcomes in the Daphnia-fungal parasite system that we focus on. This paper was recently published in the ‘Infectious disease ecology and evolution in a changing world’ theme issue of Philosophical Transactions, B that was compiled and edited by Kayla King, Matt Hall, and Justyna Wolinska. In this study, we found that Daphnia reared at warmer temperatures had more robust physical barriers to infection but decreased cellular immune responses during the initial infection process. Infected hosts at warmer temperatures also suffered greater reductions in fecundity and lifespan. Perhaps most interestingly, the relationship between a key trait—gut epithelium thickness, a physical barrier—and the likelihood of terminal infection reversed at warmer temperatures. These results highlight the complex ways that temperatures can modulate host–parasite interactions and show that different defense components can have qualitatively different responses to warmer temperatures. Postdoc Marcin Dziuba and undergrad Riley Jaye are coauthors on this study. The paper can be found here and the data and code from the study can be found here.
Sun followed up with two related studies, one looking at transgenerational effects on parasite fitness, the other looking at transgenerational impacts on host fitness. The study on the impact of transgenerational plasticity on parasite fitness was published in Parasitology (link to paper, link to data & code). In this experiment, we once again exposed Daphnia dentifera to its naturally co-occurring fungal parasite Metschnikowia bicuspidata, rearing the parasite at cooler (20°C) or warmer (24°C) temperatures and then, factorially, using those spores to infect at 20 and 24°C. We found that infections by parasites reared at warmer past temperatures produced more mature spores, but only when the current infections were at cooler temperatures. Moreover, the percentage of mature spores was impacted by both rearing and current temperatures, and was highest for infections with spores reared in a warmer environment that infected hosts in a cooler environment. In contrast, virulence was influenced only by current temperatures. These results highlight the potential for plasticity in parasite traits — something that we think is fascinating and wish more folks worked on! Postdocs Marcin Dziuba and Kris McIntire and undergrad Riley Jaye are coauthors on this paper.
Graphical abstract from Sun et al. Parasitology.
The follow up experiment that focused on transgenerational effects on host fitness recently appeared in Ecology and Evolution (link to paper, link to data & code). In this study, we tested the effects of biotic and abiotic environmental changes on within- and transgenerational plasticity in the Daphnia–Metschnikowia system. By exposing parents and their offspring consecutively to the single and combined effects of elevated temperature and parasite infection, we showed that transgenerational plasticity induced by temperature and parasite stress influenced host fecundity and lifespan; offsprings of mothers who were exposed to one of the stressors were better able to tolerate elevated temperature, compared with the offspring of mothers who were exposed to neither or both stressors. Yet, the negative effects caused by parasite infection were much stronger, and this greater reduction in host fitness was not mitigated by transgenerational plasticity. We also showed that elevated temperature led to a lower average immune response, and that the relationship between immune response and lifetime fecundity reversed under elevated temperature: the daughters of exposed mothers showed decreased fecundity with increased hemocyte production at ambient temperature but the opposite relationship at elevated temperature. Postdoc Marcin Dziuba and undergrad Riley Jaye are coauthors on this study.
Finally, Sun has a paper in press at Functional Ecology that explores the ways in which host and parasite functional morphology influence the outcomes of host-parasite interactions. This work uses the Metschnikowia system again, but in this case isolated Metsch from both Daphnia dentifera and Ceriodaphnia dubia, building on earlier work by Clara Shaw. We studied how host gut traits, parasite spore size and host immune responses influenced the infection process. We collected parasite spores from two host species, the larger Daphnia dentifera and the smaller Ceriodaphnia dubia, and exposed both host species to spores sourced from each host. The ability of a spore to embed in the host gut and to penetrate into the body cavity was influenced by the host species that was exposed to the parasite (‘exposure host species’) and the species from which the spores were sourced (‘source host species’). Spores sourced from D. dentifera were better able to attack both hosts, but were especially good at attacking D. dentifera. These differences likely resulted from morphological differences, with a striking correspondence between the diameter of host guts and the size of the parasite spores. Immune responses were also influenced by both exposure and source host. Interestingly, only 13.5% of hosts that had at least one parasite spore penetrate ended up with terminal infections; all but one of these infections occurred in D. dentifera hosts exposed to D. dentifera-sourced spores. Lab technician Siobhan Calhoun is a coauthor on this study; the paper can be found here and the data and code can be found here.
Photo (by Syuan-Jyun Sun): The much larger Daphnia dentifera and smaller Ceriodaphnia dubia. Fungal spores isolated from each host are strikingly similar in length to the width of the gut of the species from which they were isolated.