Genomic characteristics of the Neisseria-related symbionts suggest
an intermediate stage of the evolution
We propose that the Neisseria -related symbionts are in an early
or intermediate state of symbiosis evolution. We base this view on
comparisons with other known symbionts. Full genomes are currently
available for four lineages of lice-associated symbionts. Three are
highly reduced and display strong compositional shift towards AT,
typical for many P-symbionts (Table 1). The fourth possesses a large
genome and is supposedly a recent acquisition (Boyd et al., 2016). The
genomes of the two Neisseria -related symbionts can thus be
compared to various lice P-symbionts and to “free living” members of
Neisseriales. Both “Neisseria ”-H and “Neisseria ”-P
display significantly weaker genome degeneration than the highly reducedRiesia , Puchtella, and Legionella (Boyd et al.,
2017; Rihova et al., 2017). They have higher GC content, considerably
larger genomes (approximately three times higher number of genes), and
consequently more complete metabolic pathways (SupplementaryData1). On
the other hand, their genomes are recognizably reduced and the GC
content decreased when compared to their relatives (e.g. the genusNeisseria , but also the bee symbiont Snodgrassella ) or to
the presumably young Sodalis -like symbiont from the louseProechinophthirus fluctus (Boyd et al., 2016). The comparison of
selected metabolic pathways shows that the Neisseria -related
symbionts retain considerably greater numbers of genes in various
categories than the reduced genomes of L. polyplacis and R.
pediculicola (SupplementaryData1). This difference is particularly
strong in the Recombination and repair category (as one example)
but also in several amino acid biosynthesis pathways. An interesting
example of differences between the two Neisseria -related
symbionts is the complete histidine pathway in the
“Neisseria- P” symbiont, entirely lost in the more reduced
“Neisseria- H” (as well as in L. polyplacis and R.
pediculicola ). Another conspicuous difference between theNeisseria -related symbionts and the other two louse symbionts
regards their capacity to build the cell walls. Similar to the L.
polyplacis and R. pediculicola , the Neisseria -related
symbionts retain the path for peptidoglycan synthesis, but unlike them,
they also possess the genes for penicillin binding protein class A and a
complete pathway for lipid A, required for synthesis of
lipopolysaccharide. In contrast, both Neisseria -related symbionts
seem to lack the rod-shape coding genes. Finally, the genomes of bothNeisseria -related symbionts retain various genes connected to DNA
exchange and/or transport, such as mobile elements, type IV pili, and
secretion systems (SupplementaryData1). If, as suggested by many of the
described genomic features, “Neisseria” -H is a young symbiont
in an early state of evolution, we could assume that it only recently
replaced a more ancient P-symbiont, which was fulfilling the nutritional
role prior to the acquisition of the Neisseria -related symbiont.
As discussed below, such a putative P-symbiont was indeed detected in
the majority of the H. acanthopus microbiomes. Also, the FISH
survey indicated that the bacteriocytes might be inhabited by two
different bacteria. However, this interpretation should be taken with
caution since the non-overlapping signal may reflect different
properties of the used chromophores (Figure 4).