Distribution and origin of the symbionts in the lice microbiomes
Since the process of genome degradation starts once the bacterium
becomes an obligate vertically-transmitted symbiont, we should expect,
at least during the initial phase of the symbiogenesis, a correlation
between the degree of genome degradation and the duration of
host-symbiont coevolution (Moran, 1996). For example, among the
symbionts of sucking lice, the high degree of genome degradation ofR. pediculicola and L. polyplacis indicates a relatively
long and intimate association with the host. In correspondence with this
presumption, the Riesia lineage has been found in several louse
species of two different genera, Pediculus and Phthirus,and L. polyplacis is hosted at least by two louse species,P. serrata and P. spinulosa (Hypsa & Krizek, 2007).
Moreover, our extensive amplicon screening shows that L.
polyplacis is consistently present in a broad geographic and
phylogenetic sample of P. serrata as a dominant bacterium (Figure
3). When compared to these two well documented examples of established
P-symbionts, the Neisseria -related symbionts, with an
intermediate degree of the genome degeneration, show more restricted and
patchy distribution (Figure 3). Only in H. acanthopus were they
consistently present (the Neisseriaceae OTU) as the most dominant
bacterium (with the exception of two specimens from one population;
Figure 2), but were not found in the two examined specimens of the
related species (H. edentula) . The overall variability of the
microbiomes was apparently correlated with the lice genetic background:
in Bulgarian samples the Neisseriaceae OTU was the only present
bacterium, in other populations of H. acanthopus it was
accompanied by an unknown bacterium which the blast search affiliated
with Blochmania , and in the two samples of H. edentula the
only present OTU corresponded to the genus Arsenophonus. SinceP. serrata is known to harbor the typical obligate P-symbiontLegionella polyplacis (Rihova et al., 2017), we screened this
louse more extensively across several populations and genetic lineages.
The results confirmed a ubiquitous presence of L. polyplacis ,
which in most cases was the most abundant OTU, and only occasional
co-occurrence of the Neisseriaceae OTU (Figure 3). The split of L.
polyplacis into two different OTUs correlated to the host’s phylogeny,
reflects evolutionary changes during the evolution of the symbiont in
distant host lineages, but certainly does not suggest the presence of
two independent symbiotic lineages (in fact phylogenetic and genomic
analyses confirm that Polyplax -Legionella co-evolution
crosses the host species boundaries and the same symbiont is also
present in the related louse species P. spinulosa ; (Hypsa &
Krizek, 2007). Three additional OTUs in P. serrata microbiomes
show affinity to known insect symbionts, the Buchnera OTU and twoArsenophonus OTUs. Based on their genetic divergence and the
differences in their GC content, the two Arsenophonus OTUs seem
to represent two different lineages.
In respect to the general concept of symbiont acquisition, loss, and
replacement within insects, and the high dynamism of louse microbiomes,
two OTUs are of particular interest. Both the Buchnera OTU fromP. serrata and the Blochmania OTU from H.
acanthopus seem to represent strongly derived symbiotic genomes (Figure
2 & 3), which blast-assigned taxonomy reflects the low GC content
rather than real phylogenetic relationships. Since our metagenomic data
did not yield any reliable information on either of these bacteria, it
is difficult to hypothesize about their phylogenetic origin and function
in the host. However, strong genome reduction, deduced from the GC
content of the 16S rRNA gene amplicon, suggests that they may represent
the scattered remains of ancient symbionts, now retreating from the
host’s population and replaced with more recent acquisitions.
Interestingly, the FISH analysis shows that apart from the
“Neisseria -H” symbiont, the bacteriocytes of H.
acanthopus harbor another bacterium (Figure 4). Since the metagenomic
assembly did not contain any other bacterial contigs, we were not able
to identify the origin of this second symbiotic bacterium.
The diversity of microbiomes and the rapid process of symbionts
acquisitions/replacements make sucking lice an interesting model for
studying the conditions and processes in early stage of symbiogenesis.
Considering the distribution patterns and the low degree of genome
modifications in the two Neisseria- related symbionts, it is
unlikely that their occurrence in two different lice lineages is due to
a common symbiotic origin in the Hoplopleura -Polyplaxancestor. The most parsimonious explanation is thus an independent
origin of the symbiosis in each louse genus. This poses an interesting
question on the source of these symbionts and the mechanisms underlying
their acquisition and symbiogenesis. Co-occurrences of closely related
symbiotic bacteria in related insect hosts are usually consequences of
either co-speciation or a tendency of specific bacterial lineages to
frequently establish symbiosis with specific insect hosts (e.g.Arsenophonus , Wolbachia ). However, neither of these
explanations can be applied to the lice-Neisseria association.
Members of the family Neisseriaceae are only rarely found in symbiotic
association with insects. The only well documented case of obligate
symbiosis is the genus Snodgrassella found in several species of
bees and bumblebees (Kwong & Moran, 2013). Based on the 16S rRNA gene
phylogeny, the closest relative of the louse-associated Neisseriais an uncultured bacterium described from a flea Oropsylla
hirsuta (Jones, McCormick, & Martin, 2008), for which no other
information is currently available. It is interesting to note that,
similar to Legionella polyplacis , the Neisseria -like
symbionts originate from a bacterial lineage which is rarely found in
insects and is a well-known vertebrate pathogen. Phylogenetic
correlation between the vertebrate pathogens and symbionts of
blood-feeding arthropods was previously reported in ticks (Ahantarig,
Trinachartvanit, Baimai, & Grubhoffer, 2013; Felsheim, Kurtti, &
Munderloh, 2009; Guizzo et al., 2017; Niebylski, Peacock, Fischer,
Porcella, & Schwan, 1997; Noda, Munderloh, & Kurtti, 1997). For one of
the tick symbionts, Francisella -like bacterium, the origin from
mammalian pathogen was recently suggested by Gerhart, Moses and Raghavan
(2016). Based on the data available for the few louse genera examined so
far, their microbiomes contain both vertebrate pathogens and typical
insect-associated bacteria (e.g. Arsenophonus , Sodalis ). A
more detailed high throughput screening of different groups of sucking
lice is now needed to assess significance of these two ecological groups
of bacteria as a source of nutritional symbionts, and to study the
processes during the early symbiogenesis.