Discussion
In this study, we characterized the gut bacterial microbiota of five
lizard species from Portugal (the native Podarcis virescens ,P. bocagei and P. lusitanicus, and the introduced P.
siculus and Teira dugesii ) using a metabarcoding approach. Our
results showed that locality was the main predictor of microbial
diversity, significantly influencing microbiota composition and
structure. Moreover, host sex and size also had an effect, albeit more
discrete, in gut bacterial communities. All lizards shared the same most
abundant bacteria phyla with results being in accordance with what has
been found in other studies in lizards [e.g. 43, 44].
The two habitats in which lizards were captured are very different, with
lizards from Lisbon living in an urbanized and artificial habitat, with
greater environmental disturbance, compared to lizards from Moledo,
which live in a semi-natural habitat. Plausibly, differences in habitat
may lead to differences in the composition and diversity of the gut
microbiome [45]. We detected higher microbiota diversity in the more
urbanized environment which could be explained by the higher variety of
diet items, which may also include human food waste. Additionally,
environmental microbiota, which could be horizontally transferred to
lizards, may also be more diverse in urban habitat than in semi-natural
ones [17]. We hypothesize that habitat disturbance, the co-existence
with humans and urban animals (such as cats, dogs and rats), may
influence dietary behaviour and contribute to a higher bacterial load in
the environment, which can then be acquired by lizards. These results
agree with those on the Australian water dragon, where the gut
microbiome of lizards in urban areas was more diverse compared to those
residing in semi-natural habitats [17].
The proportion of some of the most abundant bacterial phyla and genera
found in our study differed between lizard species at each locality.
Additionally, we found that the species and sex also had an effect in
the abundance of some of the gut microbiota components in P.virescens and P. siculus . The influence of host taxonomy
in gut microbiota, which is a proxy not only for host genetics but also
its general ecology, has been reported in many animals [46, 47]
including reptiles [18]. The influence of sex in the abundance of
major bacterial groups, has also been reported in striped Plateau
lizards (Sceloporus virgatus Smith, 1938), with sex-specific
cloaca microbiomes being related to different hormone levels [48].
In our study, the influence of sex was small when compared to the effect
of habitat and host taxonomy. Nonetheless, the influence it exerts could
also be linked to differences in the ecology of the two sexes.
Importantly, the difference in size between female and male lizards may
lead to slight differences in feeding behavior, and consequently on the
gut microbial diversity.
Interestingly, we found a positive correlation between lizard lenght and
alpha bacterial diversity in males from P. siculus . This lizard
is larger than the other studied species and is also a very successful
invasive species [27, 28]. Indeed, males of P. siculus can be
more aggressive than males of the native Podarcis species
[49], and also more exploratory, bolder, and better at exploiting
food resources when compared to the native P. virescens in our
study location [28, 29]. These behaviours can be associated with the
displacement of P. virescens from gardens now inhabited byP. siculus [50] and can also be leading to a wider ecological
and trophic niche, and consequentially to a higher microbiome diversity
in P. siculus .
Finally, our analysis of potential bacterial transmission between the
lizards living in sympatry, indicates a balanced transmission between
species at Moledo and unbalanced transmission between species in Lisbon,
with the invasive P. siculus estimated to receive a higher
proportion of bacteria from the native P. virescens . These
differences in estimates of bacterial transmission may reflect different
non-exclusive aspects of the ecology of the hosts, such as the
similarity in dietary niches and habitat occupancy (i.e., horizontal
bacterial transmission from the environment). Differences in the
microbiome of P. siculus and P. virescens could be related
to an increased habitat occupancy and successful adaptation to the
environment by the invasive species, which facilitated the acquisition
of a higher quantity of local microbiota upon its arrival. These results
could also be reflecting an increased ability to exploit a variety of
food resources, or most likely a combination of both. The populations ofP. siculus and P. virescens live in sympatry, occupying
roughly the same area, but rarely in syntopy, although sightings of
these two species within 50 m of each other have been recorded [50,
pers. obs.]. On the other hand, both Podarcis species from
Moledo are considered syntopic and may have greater overlap and
similarity in their habitat occupancy. Moreover, is very likely they
consume the same or very similar prey items [24], and also encounter
each other more frequently. All these factors may explain the much more
balanced transmission we found between these two species.
The present study contributes to the existing knowledge on the effects
of environmental and host factors on the dynamics of the gut microbiome
of lizards. Our results also set the stage for future research exploring
the influence of other factors on the microbiome, particularly diet, as
well as the use of sympatric Podarcis lizards as models to test
the effects of behaviour on lizard microbial composition.