No signal of geographic home range on Wolbachia strain sharing
As a first step in the host shift process, Wolbachia need to
physically reach the recipient host species, which requires direct or
indirect species interactions. Therefore, it is expected that one should
observe host shifting among species with an overlap in geographic
distribution. However, our model indicates that the geographic home
range of scale insect species has no significant contribution toWolbachia sharing (Figure 3). This finding may firstly relate to
the age of infection. Estimation of the Wolbachia infection age
and consequently the intervals of host-shift events is controversial,
with vastly different estimates across different case studies being
reported (from a few thousand (Turelli et al. 2018; Cooper et al. 2019)
to nine million years (Bailly-Bechet et al. 2017), see also Sanaei et
al. (2021a)). If the changes in the host geographic distribution occur
faster than Wolbachia host shift events, the current geographic
distribution may not be able to explain host-shift events (and thus, we
would need to reconstruct the historical home range). In addition, the
geographic distribution of a given species is not necessarily
representative of the realised niche of that species, including
ecological connectivity (Pulliam 2000; Kearney 2006; Peterson & Soberón
2012). Therefore, two species may have the same geographic distribution
but have no direct or even indirect physical interactions (e.g., via
sharing foods or other resources). In that case, host ecological niche
may be a better tool to explain Wolbachia host-shifting. However,
ecological niches are technically harder to measure, especially when
trying to account for ecological interactions.
Role of scale insect associate species in Wolbachia host
shifts
Another possible reason why host
geographic distance effect has such a weak effect on Wolbachiasharing is host shifting via ecological vectors. Such vectors can carry
on the infection, either temporarily or permanently, and transmit it to
a recipient species at a distant geographic location from the donor
species. From prey-predator (Johanowicz & Hoy 1996; Le Clec’h et al.
2013) to host-parasitoid (Vavre et al. 1999; Kageyama et al. 2010; Tzuri
et al. 2020) and trophallaxis interactions (Ramalho & Moreau 2020),
there are several direct and indirect ecological pathways which can be
routes of Wolbachia transfer. Intimacy of direct physical
interaction between ants and scale insects may provide a route of
microbial exchange, as seen in other hemipteran groups (Pringle &
Moreau 2017; Ivens et al. 2018).
Moreover, Gruwell et al., (2007)
found plausible routes of Cardinium horizontal transmission
between armoured scale insects and their associate parasitoids.Wolbachia may also utilize these ecological routes to spread
within scale insect communities.
In a previous study, a positive correlation between Wolbachiainfection in scale insects and their associates indicated that ants may
play a role in host-shifting (Sanaei et al. 2021b). Here, we found that
only one out of five case of infected ant-scale insect pairs shares the
same Wolbachia strain. While we do not have enough statistical
power to test which route of transfer is the most common and in which
directions these transfers take place, our data support the hypothesis
that the associates tested in the current study play a role in host
shifting. Although positive correlations were not previously observed
between infection of scale insects and their associates (Sanaei et al.
2021b), here we observed sharing similar Wolbachia strains
between pairs of scale insect and not only ants but also wasps, and
flies (File S3). In addition, infection by the super-spreader stain
“w Sph1” of several species of scale insects, ants, and wasps is
another source of evidence for a substantial contribution of associate
species in Wolbachia host-shifting in scale insects.
This study provided the first insight into Wolbachia strain
diversity in scale insects, revealed a high portion of co-infected
samples and detected w Sph1 as one the most common strain ofWolbachia in scale insects. We also found that the host
phylogenetic distance effect plays a critical role in host shifting in
scale insects. In future studies, the methodology suggested by this
study could be applied to a larger data set to detect the factors
influencing host-shifting on the global
perspective.