Abstract
The hindgut protists of wood-feeding termites are usually colonized by prokaryotic symbionts. Many of the hurdles that have prevented a better understanding of these symbionts arise from variation among protist and termite host species and the inability to maintain prominent community members in culture. These issues have made it difficult to study the fidelity, acquisition, and differences in colonization of protists by bacterial symbionts. In this study, we use high throughput amplicon sequencing of the V4 region of 16S rRNA genes to determine the composition of bacterial communities associated with single protist cells of six protist species, from the genera Pyrsonympha, Dinenympha, and Trichonympha that are present in the hindgut of the termite Reticulitermes flavipes. By analyzing amplicon sequence variants (ASVs), the diversity and distribution of protist-associated bacteria was compared within and across these six different protist species. ASV analysis showed that, in general, each protist genus associated with a distinct community of bacterial symbionts which were conserved across different termite colonies. However, some ASVs corresponding to ectosymbionts (Spirochaetes) were shared between different Dinenympha species and to a lesser extent with Pyrsonympha and Trichonympha hosts. This suggested that certain bacterial symbionts may be cosmopolitan to some degree and perhaps acquired by horizontal transmission. Using a fluorescence-based cell assay, we could observe the horizontal acquisition of surface-bound bacteria. This acquisition was shown to be time-dependent, involve active processes, and was non-random with respect to binding locations on some protists.
Highlights
The lower termite R. flavipes harbors symbionts from the three domains of life, all of which make significant contributions to the digestion of lignocellulose and provisioning of essential nutrients
We found that the associations between R. flavipes hindgut protists and their symbiotic bacteria exhibited specificity in host range and community structures
We found that Dinenympha species, as a group, associated with a more diverse community of bacteria compared to the larger protist species P. vertens and T. agilis (Fig 2B and S1 File)
Summary
The lower termite R. flavipes harbors symbionts from the three domains of life, all of which make significant contributions to the digestion of lignocellulose and provisioning of essential nutrients. Perhaps all, of these protists are colonized by both endo- and ectosymbionts belonging to various bacterial taxa [2] These protist-associated bacteria often exhibit complex community structures and occupy different sites on and within their unicellular hosts [2]. Previous studies have shown that Oxymonadida protists in Reticulitermes speratus are cocolonized with Treponema ectosymbionts from two distinct phylogenetic clusters (Termite Treponema clusters I and II) [3,4] as well as a member of the Bacteroidales [5] These three ectosymbiotic lineages attach by one cell pole to the plasma membrane of their host [6] and exhibit intermixed colonization [5]. We show that the horizontal acquisition of these bacteria required active biological processes and the symbionts exhibited preferential spatial binding to their host cells in some cases
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