Discussion
The results of the combined genomic and amplicon analyses which we present here, illustrate a positive relationship between the dynamics of louse microbiomes and the observed high diversity of symbiotic bacteria in lice (Allen et al., 2016; Boyd et al., 2016; Boyd et al., 2014; Fukatsu et al., 2009; Hypsa & Krizek, 2007). The twoNeisseria -related symbionts, which are the primary focus of this study, represent a novel lineage, extending the known phylogenetic span of louse symbionts. Considering their genomic characteristics together with their distribution across the lice populations, we hypothesize that they are bacteria in an early/intermediate stage of evolution towards obligate symbiosis, which established their symbiosis independently within two different genera of lice. Consistent with this view, FISH analysis highlighted their localization within putative bacteriocytes (Figure 4 & SupplementaryInformation/SupplementaryFigure4), and amplicon screening showed that they are part of a rich microbiomes diversity (Figures 2 & 3), in which some of the other dominant taxa are closely related to the other known obligate symbionts in insects (the most degenerated ones may even be disappearing ancient P-symbionts). This complex picture suggests dynamic turnover of the symbiotic bacteria, their frequent acquisitions, losses and replacements even within local louse populations and young phylogenetic lineages.