Partitioning variance in immune traits in a zooplankton host—Fungal parasite system

Abstract

Host immune traits arise from both genetic and environmental sources of variation. When immune traits have a strong genetic basis, the presence and severity of disease in a population may influence the distribution of those traits. Our study addressed how two immune‐related traits (gut penetrability and the hemocyte response) are shaped by genetic and environmental sources of variation, and how the presence of a virulent disease altered the relative frequency of these traits in natural populations. Daphnia dentifera hosts were sampled from five Indiana lakes between June and December 2017 before and during epidemics of their fungal pathogen, Metschnikowia bicuspidata . Collected Daphnia were experimentally exposed to Metschnikowia and assayed for their gut penetrability, hemocyte response, and multi‐locus genotype. Mixed‐effects models were constructed to partition variance in immune traits between genetic and environmental sources. We then isolated the genetic sources to produce genotype‐specific estimates of immune traits for each multi‐locus genotype. Finally, we assessed the relative frequency and dynamics of genotypes during epidemics and asked whether genotypes with more robust immune responses increased in frequency during epidemics. Although genotype was an important source of variation for both gut penetrability and the hemocyte response, environmental factors (e.g., resource availability, Metschnikowia prevalence, and co‐infection) still explained a large portion of observed variation, suggesting a high degree of flexibility in Daphnia immune traits. Additionally, no significant associations were detected between a genotype's immune traits and its frequency in a population. Our study highlights the power of variance partitioning in understanding the factors driving variation in Daphnia traits and motivates further research on immunological flexibility and the ecological drivers of immune variation.

Phylogeny, morphology, virulence, ecology, and host range ofOrdospora pajunii(Ordosporidae), a microsporidian symbiont ofDaphniaspp

“…Moreover, MicG was capable of infecting D. longispina , the host from which O. pajunii was described (49), which is not present in North America. Overall, our data indicates that MicG and O. pajunii are the same species, hence the microsporidium previously called “MicG” (4, 16, 48) should be considered Ordospora pajunii .…”

“…Moreover, MicG was capable of infecting D. longispina, the host from which O. pajunii was described (49), which is not present in North America. Overall, our data indicates that MicG and O. pajunii are the same species, hence the microsporidium previously called "MicG" (4,16,48) should be considered Ordospora pajunii. However, there are also some phenotypic differences between MicG and the European O. pajunii, including that spores of both O. colligata and O. pajunii isolated from Europe form chains (49), whereas MicG spores do not; this is particularly notable, as the formation of chains has been proposed as diagnostic for the genus Ordospora (49).…”

“…Given that Daphnia are the dominant grazers in many lakes, that microsporidian transmission stages are very abundant in lakes (90), and the very high prevalence of infection in some populations (Figure 5), we propose that O. pajunii outbreaks might have important ecosystem-level consequences. Our genetic analyses of the microsporidian symbiont of Daphnia formerly referred to as "MicG" (4,16,48) revealed that it belongs to Ordospora pajunii (49). It is found in North American lakes as well as coastal ponds in Finland, which suggests a very broad geographical distribution.…”

“…Prior ecological studies on this microsporidian have shown a clear impact on its host and on other symbionts competing for the same host (4,16), as well as shifts along a mutualismparasitism continuum; this all suggests promise as an interesting study system, especially since the host is already a model organism (17,18,52). In the present study, we characterized the phylogeny, morphology, virulence, ecology, and host range of this symbiont, with a goal of understanding the impact of this pathogen on the host and facilitating future work on this system.…”

“…However, while the host is a well-established model, with a wealth of information about its genetics, physiology and ecology (10,11), little is known about the microsporidian. Indeed, in the prior studies showing context dependent outcomes (6,9), it was referred to as "MicG" (because it is a microsporidian that attacks the gut), without a formal scientific name.…”