Discussion
In both La Lopé and Rabai, our study found that mosquito larval breeding
sites in the forest and villages (including peridomestic and domestic
sites) had different physical and biological characteristics, though
this between-habitat contrast varies among variables. Notably, bacterial
community composition showed clear and consistent difference between
habitats in both localities. Despite this environmental difference,
behavioral investigations suggested that Ae. aegypti in the
forest readily accepted artificial containers as oviposition and larval
breeding sites. Aedes aegypti colonies derived from the forest
and villages also showed similar weak oviposition preferences in the
lab. These results are consistent with the hypothesis that Ae.
aegypti are generalists in larval breeding site choice. This hypothesis
was also supported by the indistinguishable conditions between Ae.
aegypti present and absent larval breeding sites within each habitat,
suggesting that the mosquitoes were likely not selective and can accept
a wide range of larval habitats. Lastly, oviposition choices in the
laboratory were highly heterogeneous, consistent with a lack of strong
preference.
Being versatile in larval habitat allows Ae. aegypti to take
advantage of novel artificial containers when natural breeding sites are
scarce. This has been proposed as a key driver for this mosquito to move
into domestic habitats in the first place (Brown et al., 2014; Powell et
al., 2018; Rose et al., 2020). Consistent with this hypothesis, movement
between habitats was suggested by genetic studies showing little genetic
differentiation between forest and village Ae. aegyptipopulations in La Lopé and Rabai (Kotsakiozi et al., 2018; Paupy et al.,
2014; Rose et al., 2020; Xia et al., 2020). It should be noted that this
genetic similarity in Rabai, as well as the lack of behavioral
difference observed in this study and Rose et al. 2020, contrasts to
studies in the 1970s and 2009 where Rabai forest and village Ae.
aegypti showed significant genetic differences, feeding preference
difference, and ovipositional difference (Brown et al., 2011; McBride et
al., 2014; Petersen, 1977; Tabachnick et al., 1979). These strong
differences found before 2017 resulted from the introduction of
non-African Aaa to Rabai villages (Brown et al., 2011;
Gloria‐Soria et al., 2016). The exotic Aaa population was no
longer found during our fieldwork in 2017, and the village mosquito
population was likely originated from the local forest (i.e.,Aaf ).
Once the mosquito established themselves in the novel habitat (likely
moving from forest to domestic habitat), ecological divergence could
take place. However, this study was unable to detect evidence of
consistent ovipositional divergence by the laboratory oviposition
experiments. One possibility is that the habitat shifts in La Lopé and
Rabai happened recently and that the extensive connectivity in the local
scale between habitats may hinder phenotypic divergence. Between more
distant localities when gene flow is less frequent, there may be more
differences between mosquitoes from different habitats or places with
different human population density, as found for host preference (Rose
et al., 2020). A third possibility is that ecological divergence may
happen in the immature stages, e.g., egg, larvae, and pupae (Saul et
al., 1980). This study did not examine larval performance, but future
investigations comparing eggs hatching and larval development in
different water conditions that mimic either forest or village larval
breeding sites could be insightful in this regard. For example,
microbial density was significantly lower in Rabai domestic containers,
which might pose selection pressure on larval starvation resistance
(Barrera & Medialdea, 1996; Souza et al., 2019), leading to higher
resistance in the domestic population.
In addition to these plausible ecological and evolutionary
considerations, we cannot rule out the possibility that our laboratory
oviposition experiments lacked the power to detect oviposition
preference or differences between colonies, although the two-choice or
multi-choice assays have been used widely to investigate Ae.
aegypti oviposition preference (but see Singer (2004) for more
discussion on measuring preference). Colonies may have also lost
distinctive traits due to adaptation to laboratory conditions (Hoffmann
& Ross, 2018). Moreover, the design of using five females per cage
instead of one female might introduce some unknown complexity, for
instance, interference between individuals (Allan & Kline, 1998).
Lastly, the contrast of oviposition choices might not be of a magnitude
detectable by female Ae. aegypti . However, the choices used in
this study were informed by characteristics of natural oviposition
sites, and therefore should be ecologically relevant for the mosquitoes.
A recent study using the same Ae. aegypti colonies did find
between-habitat ovipositional difference towards more extreme but
unnatural conditions (Xia, 2021). The complexities regarding Ae.
aegypti oviposition and experimental design warrant future studies to
examine more environmental conditions or combinations of multiple
variables, applying multiple preference measurements, and use younger
colonies in a more natural setting (e.g. conducting choices assays in
the field with mosquitoes collected from larval breeding sites).
Besides adding to our understanding of the domestication history ofAe. aegypti , this study also provided the first detailed physical
and biological characterization of Ae. aegypti larval breeding
sites, at least in Africa. Dickson et al. (2017) described the bacterial
community composition in larval breeding sites in
La Lopé and found a strong
difference between habitats, echoed in our study. Yee et al. (2012)
found consistent differences between tree holes and tires in the U.S.
although Ae. aegypti were not present in most containers. While
our work provides useful baseline information for future studies onAe. aegypti ecology and behavior, we acknowledge that some
caveats still exist in our field sampling, so the results should be
interpreted with caution. For example, the absence of Ae. aegyptiin a larval breeding site did not necessarily reflect avoidance by the
female nor that it is inhospitable for the larvae, especially as we
could not inspect the existence of unhatched eggs. However, this does
not affect our speculation that Ae. aegypti are not selective
about larval breeding sites, as the Ae. aegypti present and
absent sites had similar range of variation. We also only characterized
larval breeding sites in a narrow temporal window during the rainy
season. Future studies examining larval breeding sites throughout the
year would be particularly relevant to the recent work suggesting the
importance of seasonality in driving the domestication of Ae.
aegypti (Rose et al., 2020). Furthermore, the sample sizes in our field
study were relatively small. Field studies with larger sample sizes and
more balanced sampling between different habitats and larval breeding
site groups could further validate this study’s results. We also grouped
tree holes and rock pools as “natural” containers due to the
limitation of sample sizes, yet previous studies have implied that they
could be two distinct larval habitats (Soghigian et al., 2017). However,
our preliminary analysis suggested that grouping or separating them did
not affect the main findings from the field data. We also need to
acknowledge that the chemical profiles of larval breeding sites in Rabai
reported in this study were probably not complete and therefore calls
for future studies with improved sample collection and analysis
techniques. Lastly, in addition to the condition of each individual
larval breeding sites, the local context could also be important, e.g.,
vegetation around the sites (Rey & O’Connell, 2014).
In summary, this study suggested that Ae. aegypti in Africa were
likely generalists in their larval habitat choice, which allowed them to
readily accept artificial containers as larval breeding sites and
potentially facilitated their introduction into domestic habitats. Being
flexible in oviposition and larval breeding site choices could benefitAe. aegypti by spreading the risk during reproduction and reduce
larval competition. This is consistent with the observations that this
mosquito has a bet-hedging ‘skip oviposition’ behavior (i.e., lay small
batches of eggs in multiple containers) (Colton et al., 2003; Starrfelt
& Kokko, 2012). However, outside of Africa, Ae. aegypti are
closely associated with human communities and use almost exclusively
artificial containers for larval breeding sites (Day, 2016; Swan et al.,
2018; Vezzani, 2007; Yee, 2008), raising the interesting question of
when and how this specialization on artificial containers evolved. A few
recent studies suggested that human specialization may happen somewhere
in West Africa, such as Sahel or Angola (Crawford et al., 2017; Powell
et al., 2018; Rose et al., 2020). On the other hand, the
human-specialized non-African Aaa could also move back to
ancestral breeding sites, for instance, in the Caribbean (Chadee et al.,
1998). It would be interesting to examine such processes and test
whether the mosquitoes resumed generalist in larval breeding site choice
during this process.