Spatial and environmental influences on the assembly of silk microbiomes in a social spider

In nesting animals, the built environment can play an important role in host-associated microbiome assembly. However, the sources and processes structuring the resulting microbiome remain underexplored. In the social spider Stegodyphus dumicola, philopatric sisters collectively build and maintain a silken nest, capture prey, and exhibit alloparental care. We used S. dumicola as a test system to assess the source and spatial/environmental processes structuring the nest microbial communities. We collected paired silk and soil samples along two orthogonal transects in southern Africa. Bacterial and fungal communities were extracted using high-throughput sequencing of 16S-rRNA and ITS barcoding genes and assessed using the SourceTracker tool for R, distance-based regression of soil and silk dissimilarity, and variation partitioning. Silk bacteria were partially derived from soil bacteria, but there is no apparent difference in the contribution of local vs. non-local soils: nest microbial communities are no more similar to microbes in the local soil beneath them than soil found hundreds of kilometers away. In contrast, silk fungi receive few taxa from the soil community but show a stronger relationship with local over non-local soils. Silk bacterial communities are weakly structured by spatial and environmental processes, (i.e. dispersal and abiotic filtering) suggesting low dispersal limitation and a stronger influence of ecological drift. Silk fungal communities, in contrast, indicated stronger associations with spatial and environmental processes. The large contribution of extrinsic sources to the nest microbiome suggests high immigration potential for opportunistic or harmful microbes that may be partly responsible for eventual colony collapse.