Biology subjects

Dharmarajan, G.

Publications and source records attributed to Dharmarajan, G..

5 recordsLinked to original sources

bioRxiv2026

Host and environmental factors differentially affect patterns of diversity in specialist and generalist parasites

AimElucidating the mechanisms shaping parasite diversity patterns is critical because parasites encompass about 40% of known species, and are crucial for ecosystem structure and function. In free-living species, diversity patterns in the Anthropocene are shaped by niche-breadth because specialists (narrow niche-breadth taxa) are more sensitive to environmental disturbance compared to generalists (broad niche-breadth taxa). Like free-living species, parasites too can be categorized as specialists or generalists according to their niche-breadth (i.e., diversity of hosts they can infect). However, unlike free-living species, the effects of niche-breadth on parasite diversity patterns remain unclear. Here, we used haemosporidian parasites as a model system to identify factors affecting parasite diversity patterns, and test if these patterns differ between specialist (Haemoproteus) and generalist (Plasmodium) parasites. LocationSouthern India TaxonHaemoproteus spp. and Plasmodium spp. (Haemosporida) MethodsBlood samples from wild birds were screened using molecular tools to identify haemosporidian parasite lineages. Statistical analyses, including random forest models and generalized dissimilarity models, were utilized to evaluate how environmental and host factors drive spatial patterns of parasite and {beta} diversity. ResultsOur results reveal that phylogenetic diversity is primarily shaped by host-related variables in the specialist parasites, but by numerous host- and environment-related factors in the generalists. In keeping with ecological theory, the specialist parasites showed higher diversity and lower evenness compared to the generalists. Additionally, while {beta} diversity of the specialist parasites was primarily driven by spatial differences in richness (e.g., taxon nestedness) rather than replacement (e.g., taxon turnover), the opposite pattern was found in the generalist. Main conclusionThe differential patterns and drivers of diversity in specialist vs. generalist parasites demonstrates why specialists parasites are good indicators of ecosystem health and elucidates the mechanism by which anthropogenic disturbance increases the risk of emerging infectious diseases which are primarily caused by generalist parasites.

ecology↗

bioRxiv2025

Host niche breadth differentially modulates the effects of anthropogenic disturbance across generalist and specialist parasites

Anthropogenic disturbances in natural habitats increase the risk of emerging infectious diseases in free-ranging host communities. Hence, understanding the processes driving such patterns is critical towards One Health. While anthropogenic disturbance is known to promote habitat-generalist host species through biotic homogenisation, generalist parasites (wider host breadth) also respond positively to the disturbed habitats. We hypothesise that generalist parasites are more likely to infect generalist hosts. We tested this hypothesis in a sky island system where generalist (Plasmodium) and specialist (Haemoproteus) haemosporidian parasites infect a range of bird hosts in disturbed and natural forest patches. We used a natural experiment framework to control for climatic differences (similar elevation) and habitat quality (same habitat type, but varying disturbance matrix). We collected 1106 samples from the field and examined the genus-level parasite prevalence and the host specificity of individual parasite lineages. Our results suggest that the generalist (Plasmodium) parasites are more prevalent in generalist birds, irrespective of disturbance. However, the specialist (Haemoproteus) parasites were more prevalent in specialist birds in natural forests than in disturbed forests. Among the individual parasite lineages, we found the host specificity to be associated with the degree of habitat specialisation of their host species. Our results provide evidence for the tendency of generalist parasites to infect generalist host species - a potential mechanism for a higher risk of emerging infectious diseases in human-dominated regions. We emphasise the role of host ecology in understanding the impact of anthropogenic disturbance on parasite prevalence in free-ranging host communities.

ecology↗

bioRxiv2021

Solving the Sample Size Problem for Resource Selection Analysis

O_LISample size sufficiency is a critical consideration for conducting Resource-Selection Analyses (RSAs) from GPS-based animal telemetry. Cited thresholds for sufficiency include a number of captured animals M [≥] 30 and as many relocations per animal N as possible. These thresholds render many RSA-based studies misleading if large sample sizes were truly insufficient, or unpublishable if small sample sizes were sufficient but failed to meet reviewer expectations. C_LIO_LIWe provide the first comprehensive solution for RSA sample size by deriving closed-form mathematical expressions for the number of animals M and the number of relocations per animal N required for model outputs to a given degree of precision. The sample sizes needed depend on just 2 biologically meaningful quantities: habitat selection strength and a novel measure of landscape complexity, which we define rigorously. The mathematical expressions are calculable for any environmental dataset at any spatial scale and are applicable to any study involving resource selection (including sessile organisms). We validate our analytical solutions using globally relevant empirical data including 5,678,623 GPS locations from 511 animals from 10 species (omnivores, carnivores, and herbivores living in boreal, temperate, and tropical forests, montane woodlands, swamps, and arctic tundra). C_LIO_LIOur analytic expressions show that the required M and N must decline with increasing selection strength and increasing landscape complexity, and this decline is insensitive to the definition of availability used in the analysis. Our results contradict conventional wisdom by demonstrating that the most biologically relevant effects on the utilization distribution (i.e. those landscape conditions with the greatest absolute magnitude of resource selection) can often be estimated with far fewer data than is commonly assumed. C_LIO_LIWe identify several critical steps in implementing these equations, including (i) a priori selection of expected model coefficients, and (ii) sampling intensity for background (absence/pseudo-absence) data within a given definition of availability. We show that random sampling of background data violates the underlying mathematics of RSA, leading to incorrect values for necessary M and N and potentially incorrect RSA model outputs. We argue that these equations should be a mandatory component for all future RSA studies. C_LI

ecology↗

bioRxiv2018

Determining the link between alpha-gal-containing antigens in North American ticks and red meat allergy

Development of specific IgE antibodies to the oligosaccharide galactose--1, 3-galactose (-gal) following tick bites has been shown to be the source of red meat allergy. In this study, we investigated the presence of -gal in four tick species: the lone-star tick (Amblyomma americanum), the Gulf-Coast tick (Amblyomma maculatum), the American dog tick (Dermacentor variabilis), and the black-legged tick (Ixodes scapularis) by using a combination of immunoproteome, carbohydrate analysis, and basophil activation approaches. Using anti--gal antibodies, -gal was identified in the salivary glands of both Am. americanum and Ix. scapularis, while Am. maculatum and De. variabilis appeared to lack the sugar. PNGase F treatment confirmed the deglycosylation of N-linked -gal-containing proteins in tick salivary glands. Immunolocalization of -gal moieties to the salivary secretory vesicles of the salivary acini also confirmed the secretory nature of -gal-containing antigens in ticks. Am. americanum ticks were fed human blood (lacks -gal) using an artificial membrane feeding system to determine the source of -gal. N-linked glycan analysis revealed that Am. americanum and Ix. scapularis have -gal in their saliva and salivary glands, but Am. maculatum contains no detectable quantity. Salivary samples from Am. americanum and Ix. scapularis stimulated activation of basophils primed with plasma from -gal allergic subjects. Together, our data support the idea that bites from certain tick species may specifically create a risk for the development of -gal-specific IgE and hypersensitivity reactions in humans. Alpha-Gal syndrome challenges the current food allergy paradigm and broadens opportunities for future research.

immunology↗

bioRxiv2018

Variation in tolerance to parasites in natural Asian tiger mosquito populations and its effect on vectorial capacity

The vectorial capacity of mosquitoes depends upon the magnitude of reduction of parasite load upon infection through resistance mechanisms (e.g., immune-mediated killing) and the ability of mosquitoes to offset infection-mediated costs through tolerance mechanisms (e.g., tissue repair). Here we use a common-garden experimental framework to measure variation in resistance and tolerance to dog heartworm (Dirofilaria immitis) between natural Aedes albopictus mosquito populations representing areas of low and high transmission intensity. Our data revealed that survival to the extrinsic incubation period, the earliest time point at which infective L3 larvae develop, significantly differed between populations (ranging from 10-60%) when mosquitoes infected with D. immitis at both the low (15 microfilaria/l blood) and high (30 microfilaria/l blood) infection dose (Dose: {chi}2 = 191.473; P < 0.001; Population: {chi}2 = 24.485; P = 0.001; Dose x Population: {chi}2 = 35.566; P = 0.001). Contrary to expectations, we found that mosquito populations with highest resistance (i.e., greatest reduction in parasite load) also exhibited highest mortality upon infection (F1,12 = 6.781, P = 0.023; Dose: F1,12 = 6.747; P = 0.023; Mortality x Dose: F1,12 = 0.111, P = 0.744). Expressing the effect of the number of killed (NKILLED) and live (NLIVE) parasite on survival of mosquitoes from the different population, we document a significant inter-population variation in the survival cost of additional parasite (i.e., tolerance to infection (NLIVE x Population: {chi}2 = 22.845; P = 0.002; NKILLED x Population: {chi}2 = 31.959; P = < 0.001; NLIVE x NKILLED x Population: {chi}2 = 22.266; P = 0.002), in conjunction with negative relationship between tolerance and resistance (Resistance: F1,12 = 11.870, P = 0.005; Dose: F1,12 = 16.0170, P = 0.002; Resistance x Dose: F1,12 = 9.699, P =0.009). Importantly, populations from areas with high transmission intensity (as measured by parasite prevalence in dogs) showed elevated tolerance (Prevalence: F1,12 = 9.5, P = 0.012; Prevalence2: F1,12 = 4.353, P = 0.064; Dose: F1,12 = 38.855, P = <0.001), and these populations were also associated with increased vectorial capacity (Tolerance: F1,12 = 8.175, P = 0.014; Dose: F1,12 = 0.005, P = 0.946; Tolerance x Dose: F1,12 = 0.920, P = 0.356). Consequently, our data indicate that spatial variation in disease transmission intensity is linked to the evolution of tolerance in natural mosquito populations, which in turn can feedback to impact disease risk.

ecology↗