Abstract
Social spiders have remarkably low species-wide genetic diversities, potentially increasing the relative importance of microbial symbionts for host fitness. Here we explore the bacterial microbiomes of three species of social Stegodyphus (S. dumicola, S. mimosarum, and S. sarasinorum), within and between populations, using 16S rRNA gene amplicon sequencing. The microbiomes of the three spider species were distinct but shared similarities in membership and structure. This included low overall diversity (Shannon index 0.5–1.7), strong dominance of single symbionts in individual spiders (McNaughton’s dominance index 0.68–0.93), and a core microbiome (>50% prevalence) consisting of 5–7 specific symbionts. The most abundant and prevalent symbionts were classified as Chlamydiales, Borrelia, and Mycoplasma, all representing novel, presumably Stegodyphus-specific lineages. Borrelia- and Mycoplasma-like symbionts were localized by fluorescence in situ hybridization (FISH) in the spider midgut. The microbiomes of individual spiders were highly similar within nests but often very different between nests from the same population, with only the microbiome of S. sarasinorum consistently reflecting host population structure. The weak population pattern in microbiome composition renders microbiome-facilitated local adaptation unlikely. However, the retention of specific symbionts across populations and species may indicate a recurrent acquisition from environmental vectors or an essential symbiotic contribution to spider phenotype.
Highlights
Microbiomes are ubiquitous in the animal world and can provide their hosts with beneficial and sometimes essential functions like energy, nutrition, or protection against pathogens (McFall-Ngai et al, 2013)
The majority of all Amplicon Sequence Variants (ASVs) (83%) occurred in only 1–2 samples and at relative abundances
The McNaughton’s dominance index (DMN, the sum of the two most abundant ASVs, Figure 2C) shows that dominant ASVs are more common in S. dumicola than in S. mimosarum and S. sarasinorum, which matches the much lower alpha diversity in S. dumicola
Summary
Microbiomes are ubiquitous in the animal world and can provide their hosts with beneficial and sometimes essential functions like energy, nutrition, or protection against pathogens (McFall-Ngai et al, 2013). The composition and distribution of microbiomes among host individuals and populations may provide insights into the potential of the microbiome to facilitate adaptive responses to the local environment. A significant role in local host adaptation predicts populationspecific patterns with host individuals from the same population carrying more similar microbiomes compared to host individuals from different populations. Random forces such as low transmission fidelity and drift-like processes caused by host population dynamics may cause a less structured pattern of microbiome distribution within and across populations. Symbiont-mediated adaptations have been hypothesized to be especially important in species with low genetic diversity (Mueller et al, 2019), since the possibility of evolutionary responses based on standing genetic variation of such hosts is reduced (Bell, 2013; Charlesworth et al, 2017; Ørsted et al, 2019)
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