Biology subjects

Agashe, D.

Publications and source records attributed to Agashe, D..

9 recordsLinked to original sources

bioRxiv2018

Host dietary specialization and neutral assembly shape gut bacterial communities of wild dragonflies

Host-associated gut microbial communities can have large impacts on host ecology and evolution, and are typically shaped by host taxonomy and diet. Different host species often harbor distinct microbial communities, potentially because (1) host dietary specialization determines microbial colonization, (2) host-specific selection acts on diet-acquired microbiota, and (3) a combination of both processes. While the first possibility involves passive community structuring, the other two may arise from a functional association and should produce stable microbial communities. However, these alternatives have rarely been tested in wild host populations. We used 16S rRNA amplicon sequencing to characterize the gut bacterial communities of six dragonfly species collected across multiple seasons and locations. We found that variation in bacterial community composition was predominantly explained by sampling season and location, and secondarily by host species. To distinguish the role of host dietary specialization and host-imposed selection, we used insect-specific primers to identify prey in the gut contents of three focal dragonfly species. We found that these dragonflies - considered to be generalist predators - consumed distinct prey, with seasonal diet variation. Together, the patterns of host dietary specialization and spatial and temporal variation suggest a strong role of passive processes in shaping the gut bacterial community. Indeed, the abundance and distribution of ~76% of the bacterial community members were consistent with neutral community assembly. Our results contradict the pervasive expectation that host-imposed selection shapes gut microbial communities, and highlight the importance of joint analyses of variation in host diet and gut microbial communities of natural host populations.

evolutionary biology↗

bioRxiv2018

Restoring ancestral microbiome aids beetle adaptation to new diets

Eukaryotic hosts often depend on microbes that enhance their fitness, and such relationships may be relatively easily maintained in a stable environment. What is the fate of these associations under rapid environmental change? For instance, if the host switches to a new diet and/or encounters a different microbial community, how does the host-microbiome relationship change? Are the changes adaptive, and how rapidly do they occur? We addressed these questions with the red flour beetle Tribolium castaneum, a generalist insect pest that both consumes and lives in stored grain flour. We found that beetle fitness is enhanced by flour-acquired microbes in the ancestral habitat (wheat flour), but not in novel suboptimal environments (e.g. corn flour) that have a different resident microbial community. Beetles that disperse to new habitats thus have low fitness and a dramatically altered microbiome. Enriching novel habitats with ancestral (wheat-derived) microbes increased beetle fitness, suggesting a viable adaptive strategy. Indeed, within a few generations of laboratory adaptation to two distinct novel habitats, we found that beetle populations gradually restored their ancestral microbiome. Importantly, evolved populations showed a microbe-dependent increase in fecundity and survival on the new diet. We suggest that such repeated, rapid restoration of host-microbe associations may allow generalists to successfully colonize new habitats and escape extinction despite sudden environmental changes.

evolutionary biology↗

bioRxiv2018

Antagonistic pleiotropy is unexpectedly rare in new mutations

Pleiotropic effects of mutations may underlie diverse biological phenomena such as ageing and specialization. In particular, antagonistic pleiotropy (\"AP\": when a mutation has opposite fitness effects in different environments) generates tradeoffs, which may constrain adaptation. Models of adaptation typically assume that AP is common - especially among large-effect mutations - and that pleiotropic effect sizes are positively correlated. The rare empirical tests of these assumptions have largely focused on beneficial mutations observed under strong selection, whereas most mutations are actually deleterious or neutral, and are removed by selection. We quantified the incidence, nature and effect size of pleiotropy for carbon utilization across 80 single mutations in Escherichia coli that arose under mutation accumulation (i.e. weak selection). Although ~46% of the mutations were pleiotropic, only 11% showed AP, which is lower than expected given the distributions of fitness effects for each resource. In some environments, AP was more common in large-effect mutations (but not synergistic pleiotropy, SP); whereas pleiotropic effect sizes were positively correlated for SP (but not AP). Thus, AP is generally rare, is not consistently enriched in large-effect mutations, and often involves weakly deleterious antagonistic effects. Our unbiased quantification of mutational effects therefore suggests that antagonistic pleiotropy is unlikely to cause maladaptive tradeoffs.

evolutionary biology↗

bioRxiv2018

The induction of larval resource preference in heterogeneous habitats

Animals often have to evaluate and choose between multiple food sources in their habitat, and these potentially complex decisions can have a large impact on their fitness. Among other factors, previous experience with an alternative resource can significantly increase subsequent preference for the resource (\"induction of preference\"). Such induction of resource preference is particularly relevant in spatially or temporally heterogeneous habitats. Although most mobile species - especially generalists - probably frequently encounter habitat heterogeneity, the impact of preference induction on individual behaviour and fitness in heterogeneous habitats is poorly understood. We analysed larval preference induction in wheat-adapted generalist red flour beetles (Tribolium castaneum) under three types of habitat heterogeneity. We first analysed the induction of larval preference for novel resources (other cereal flours) under temporal heterogeneity, exposing larvae to new resources during development. We found that larvae preferred a new resource if they experienced it recently, but that the magnitude of induction varied across resources. Interestingly, we also observed specific induction for a homogenous mix of wheat and a novel resource, with larvae preferring the mix over either pure resource. To analyse induction under spatial heterogeneity, we placed beetle eggs in one of two alternative resource patches and tested the preference of emerged larvae. Unexpectedly, hatching into a novel resource did not always induce preference. Finally, we found that induction of preference for new resources could be maladaptive for larval development. Together, our work demonstrates that experience-based plasticity of larval resource choice may strongly impact larval preference and fitness in heterogeneous habitats.

ecology↗

bioRxiv2018

Pathogen susceptibility and fitness costs explain variation in immune priming across natural populations of flour beetles

In many insects, individuals primed with low doses of pathogens live longer after being exposed to the same pathogen later in life. Yet, our understanding of the evolutionary and ecological history of priming of immune response in natural insect populations is limited. Previous work demonstrated population-, sex- and- stage specific variation in the survival benefit of priming response in flour beetles (Tribolium castaneum) infected with their natural pathogen Bacillus thuringiensis. However, the evolutionary forces responsible for this natural variation remained unclear. Here, we tested whether the strength of the priming response (measured as the survival benefit after priming and subsequent infection relative to unprimed controls) was associated with multiple fitness parameters across 10 flour beetle populations. Our results suggest two major selective pressures that may explain the observed inter-population variation in priming: (A) Basal pathogen susceptibility - populations that were more susceptible to infection produced a stronger priming response, and (B) Reproductive success - populations where primed females produced more offspring had lower survival benefit, suggesting a trade-off between priming response and reproduction. Our work is the first empirical demonstration of multiple selective pressures that may govern the adaptive evolution of immune priming in the wild. We hope that this motivates further experiments to establish the role of pathogen-imposed selection and fitness costs in the evolution of priming in natural insect populations.

evolutionary biology↗

bioRxiv2018

The Evolutionary History and Impact of Bacterial tRNA Modifications

Along with tRNAs, enzymes that modify anticodon bases are a key aspect of translation across the tree of life. tRNA modifications extend wobble pairing, allowing specific (\"target\") tRNAs to recognize multiple codons and cover for other (\"non-target\") tRNAs, often improving translation efficiency and accuracy. However, the detailed evolutionary history and impact of tRNA modifying enzymes has not been analyzed. Using ancestral reconstruction of five tRNA modifications across 1093 bacteria, we show that most modifications were ancestral to eubacteria, but were repeatedly lost in many lineages. Most modification losses coincided with evolutionary shifts in non-target tRNAs, often driven by increased bias in genomic GC and associated codon use, or by genome reduction. In turn, the loss of tRNA modifications stabilized otherwise highly dynamic tRNA gene repertoires. Our work thus traces the complex history of bacterial tRNA modifications, providing the first clear evidence for their role in the evolution of bacterial translation.

evolutionary biology↗

bioRxiv2017

DIETARY AND DEVELOPMENTAL SHIFTS IN BUTTERFLY-ASSOCIATED BACTERIAL COMMUNITIES

Bacterial communities associated with insects can substantially influence host ecology, evolution and behavior. Host diet is a key factor that shapes bacterial communities, but the impact of dietary transitions across insect development is poorly understood. We analyzed bacterial communities of 12 butterfly species across different development stages, using 16S rDNA amplicon sequencing. Butterfly larvae typically consume leaves of a single host plant, whereas adults are more generalist nectar feeders. Thus, we expected bacterial communities to vary substantially across butterfly development. Surprisingly, very few species showed significant developmental transitions in bacterial communities, suggesting weak impacts of dietary transitions across butterfly development. On the other hand, bacterial communities were strongly influenced by butterfly species identity and dietary variation across species. Larvae of most butterfly species largely mirrored bacterial community composition of their diets, suggesting passive acquisition rather than active selection. Overall, our results suggest that although butterflies harbor distinct microbiomes across taxonomic groups and dietary guilds, the dramatic dietary shifts that occur during development do not impose strong selection to maintain distinct bacterial communities.

ecology↗

bioRxiv2017

Experimental Evolution Of Insect Immune Memory Vs. Pathogen Resistance

Under strong pathogen pressure, insects often evolve resistance to infection. Many insects are also protected via immune memory ( immune priming), whereby sub-lethal exposure to a pathogen enhances survival after secondary infection. To understand the evolution and consequences of these immune responses, we imposed strong pathogen selection on flour beetles (Tribolium castaneum), infecting them with Bacillus thuringiensis (Bt) for 11 generations. Populations injected first with heat-killed and then live Bt each generation evolved high basal resistance against multiple Bt strains. In contrast, all populations injected only with a high dose of live Bt evolved less effective but strain-specific priming response. Control populations injected with heat-killed Bt did not evolve priming; and in the ancestor, priming was effective only against a low Bt dose. Thus, pathogens can select for rapid modulation of insect priming ability, leading to divergent immune strategies (generalized resistance vs. specific immune priming) with distinct mechanisms and adaptive benefits.

evolutionary biology↗

bioRxiv2017

FEMALE CHEMICAL WARFARE DRIVES FITNESS EFFECTS OF GROUP SEX RATIO

In animals, skewed sex ratios can affect individual fitness either via sexual interactions (e.g. intersexual conflict or intrasexual mate competition) or non-sexual interactions (e.g. sex-specific resource competition). Because most analyses of sex ratio focus on sexual interactions, the relative importance of these mechanisms remains unclear. We addressed this problem using the flour beetle Tribolium castaneum, where male-biased sex ratios increase female fitness relative to unbiased or female-biased groups. Although flour beetles show both sexual and non-sexual (resource) competition, we found that sexual interactions did not explain female fitness. Instead, female fecundity was dramatically reduced even after a brief exposure to flour conditioned by other females. Earlier studies suggested that quinones (secreted toxins) might mediate density-dependent population growth in flour beetles. We identified ethyl- and methyl-benzoquinone (EBQ and MBQ) as the primary components of adult stink glands that regulate female fecundity. In female-biased groups (i.e. at high female density), females upregulated quinones and suppressed each others reproduction. In male-biased groups, low female density lead to low quinone levels, allowing higher fecundity. Thus, quinones serve both as indicators and mediators of female competition, resulting in the observed fitness decline in female-biased groups. Our results underscore the importance of non-sexual interference competition that may often underlie the fitness consequences of skewed sex ratios.

evolutionary biology↗