Distribution of dominant bacterial taxa
Microbiomes of the tested species/lineages of the lice are dominated by several bacterial taxa with a complex distribution pattern. For two of these bacteria, Legionella polyplacis from Polyplax and Neisseriaceae from Hoplopleura lice, their symbiotic nature could be clearly demonstrated based on the complete genome characteristics and distribution within the host. For other bacteria, partial 16S rRNA gene amplicons provide two (not entirely independent) kinds of information: taxonomical assignment and GC content. Besides Neisseriaceae, two more OTUs, identified as dominant taxa (see methods for definition) and taxonomically assigned to Blochmania and Arsenophonusgenera, were associated with Hoplopleura samples (Figure 2). In addition to the sequence characteristic (GC content of 43.9% and 49.3%), the distribution of these OTUs among the two Hoplopleuraspecies and H. acanthopus populations points to their symbiotic nature.
For Polyplax serrata a comprehensive population-wide amplicon screening revealed (besides L. polyplacis ) 9 dominant taxa assigned to the genus or family level (Figure 3). The distribution of OTUs with a low GC content, i.e. Buchnera (45.1% GC) andArsenophonus 2 (49.4% GC), reflects the genealogy of the host and thus indicates putative obligate coevolving symbionts. For the other taxa, the taxonomic assignment and GC content >50% (with the exception of Cloacibacterium ) indicates that the bacteria may represent environmental contamination or very early symbiotic associations, e.g. Neisseriaceae taxon and Arsenophonus 1. However, it is important to note that the taxonomical assignment of the OTUs are based on a short sequence and should be interpreted as approximate affiliations rather than precise phylogenetic position, particularly compared to highly derived genomes like Buchnera andBlochmannia .