Biology subjects

Rajarajan, A.

Publications and source records attributed to Rajarajan, A..

4 recordsLinked to original sources

bioRxiv2026

Host outnumbered: microbiomes and a fungal parasite interact to shape host genotype-dependent survival

Microbiomes are key determinants of host health, yet empirical evidence demonstrating their influence on host-parasite interactions is limited. We conducted a proof-of-principle experiment using the water flea Daphnia magna and its virulent yeast para-site Australozyma monospora (formerly Metschnikowia bicuspidata) to test if micro-biome composition alters infection outcomes. Axenic hosts were inoculated with a control microbiome (homogenates of laboratory-cultured Daphnia reared in natural freshwater) or a single bacterial strain (Rhizobium sp.), and exposed to the parasite. Host survival differed markedly between microbiome treatments and depended on parasite exposure. Prior to parasite exposure, hosts of one genotype exhibited high juvenile mortality when inoculated with the control microbiome (79%), compared to Rhizobium (19%) (the other genotype: 48% vs. 50%). Parasite exposure reduced survival, but the extent varied with microbiome composition: survival of hosts with a control microbiome (averaged across genotypes) declined from 66% to 0%; survival of those inoculated with Rhizobium sp. declined from 35% to 10%. In contrast, micro-biome composition did not influence parasite infectivity or net reproduction. Our results indicate that microbiome effects on host survival are genotype- and life-stage-dependent, differ between parasite-free and parasite-exposed conditions and may impact host-parasite dynamics primarily by influencing host viability rather than para-site performance.

ecology↗

bioRxiv2025

Warming increases richness and shapes assemblages of eukaryotic parasitic plankton

BackgroundAnthropogenic activities have led to a global rise in water temperatures, prompting increased interest in how warming affects infectious disease ecology. While most studies have focused on individual host-parasite systems, there is a gap in understanding the impact of warming on multi-host, multi-parasite assemblages in natural ecosystems. To address this gap, we investigated freshwater eukaryotic parasite communities in ten natural lakes near Konin, Poland: five artificially heated and five non-heated "control" lakes. Since 1958, the heated lakes have experienced a mean annual temperature increase of 2 {degrees}C due to hot water discharge from two adjacent power plants. We collected seasonal environmental DNA (eDNA) samples from surface waters over a two-year period and applied targeted metabarcoding to compare the richness and distribution of eukaryotic parasites across lake types, with a focus on protists and fungi. ResultsUsing literature searches and sequence metadata from GenBank, we identified putative parasites which included Alveolates, Stramenopiles, basal Fungi and Ichthyosporeans, as well as their associated hosts. Heated lakes harboured distinct parasite assemblages, with higher richness of chytrids and aphelids, suggesting thermal preferences among certain freshwater microeukaryotic parasites. Other groups exhibited clear seasonal trends, with richness of oomycetes peaking in spring and summer, and that of Cryptomycota in winter and autumn. A general linear model revealed a marginally positive correlation between chytrid parasite richness and richness of their green algal, diatom, and dinoflagellate hosts. Post-hoc analyses indicated that heated lakes exhibited greater seasonal variation in chytrid parasite richness and a stronger correlation between host and parasite richness than control lakes. ConclusionThese findings demonstrate that warming can induce strong shifts in the richness and assemblages of freshwater microeukaryotic parasites. Using chytrids as a focal group, we additionally demonstrate that warming may amplify seasonal variation in parasite richness and strengthen host-parasite richness relationships.

ecology↗

bioRxiv2024

Zooplankton bacterial communities are influenced by infection status, environmental conditions and diet quantity across natural epidemics

Zooplankton-associated microbiomes play an important role for host health and contribute to ecosystem processes such as nutrient cycling. Yet, few studies have assessed how environmental gradients and biotic interactions, including parasitism and diet, may shape the microbiome composition of wild zooplankton. Here, we analysed the microbiomes of water fleas from the Daphnia longispina species complex using 16S rRNA gene sequencing and a long-term field dataset spanning six sampling events over 13 years. Sampling coincided with outbreaks of the virulent eukaryotic gut parasite Caullerya mesnili. Additionally, we explored how microbiome structure varied in relation to water parameters, phytoplankton density (i.e. Daphnia diet), and zooplankton density and community structure. Daphnia microbiomes displayed strong temporal variation, and comparatively small differences based on host infection status. Microbiome beta diversity correlated with phytoplankton density but not with its community composition, including green algae, protists and cyanobacteria. Environmental conditions, including temperature, dissolved oxygen and cyanobacterial abundance - previously found to drive Caullerya epidemics - were also associated with distinct microbiome structures. Importantly, microbiome beta diversity co-varied with infection prevalence, suggesting a link between microbiome shifts, epidemic size, and environmental conditions driving large epidemics. Dominant bacterial taxa correlated with Daphnia density, whereas the phylogenetic composition of rare taxa was associated with total zooplankton density. These findings demonstrate the dynamic nature of Daphnia microbiomes and suggest potential mechanisms by which they may mediate disease dynamics, particularly through associations with diet quantity, temperature, and host population density.

ecology↗

bioRxiv2022

Infection by a eukaryotic gut parasite in wild Daphnia sp. associates with a distinct bacterial community

Host-associated bacterial communities can play an important role in host fitness and resistance to diseases. Yet, few studies have investigated tripartite interaction between a host, parasite and host-associated bacterial communities in natural settings. Here, we use 16S amplicon sequencing to compare gut- and body-bacterial communities of wild water fleas belonging to the Daphnia longispina complex, between uninfected hosts and those infected with the common and virulent eukaryotic gut parasite Caullerya mesnili (Family: Ichthyosporea). We report community-level changes in host-associated bacteria with the presence of the parasite infection; namely decreased alpha diversity and increased beta diversity at the site of infection, i.e. host gut (but not host body). We also report decreased abundance of bacterial taxa proposed elsewhere to be beneficial for the host, and an appearance of taxa specifically associated with infected hosts. Our study highlights the host-microbiota-infection link in a natural system and raises questions about the role of host-associated microbiota in natural disease epidemics as well as the functional roles of bacteria specifically associated with infected hosts.

ecology↗