Introduction
Species respond differently to anthropogenic habitats, such as villages
and urban areas (McKinney, 2008; Otto, 2018; Szulkin et al., 2020).
While generally these ecological changes have negative consequences, a
handful of species have successfully exploited these novel human
environments (Johnson & Munshi-South, 2017). Among them, one of the
most notorious examples is the yellow fever mosquito, Aedes
aegypti , a major vector of several arboviral diseases that cause
millions of infections each year, including yellow fever, dengue,
chikungunya, and Zika (Aubry et al., 2018; World Health Organization,
2014). The mosquitoes’ high efficacy at transmitting diseases stems
partly from their adaptation to human-made domestic habitats, assuring
close contact with humans (Carvalho & Moreira, 2017; Fontenille &
Powell, 2020).
The recent evolutionary history of Ae. aegypti is strongly linked
to human activities (Powell et al., 2018; Powell & Tabachnick, 2013).Ae. aegypti originated on southwestern Indian Ocean islands and
moved to continental Africa around 85,000 years ago before spreading
across the continent in tropical forests (Soghigian et al., 2020).
Probably five to ten thousand years ago, this species invaded human
settlements and likely evolved domestic adaptations (Crawford et al.,
2017; Kotsakiozi et al., 2018). This domestication prepared them for
later spread to the rest of the world a few hundred years ago, likely
from West Africa and facilitated by human movements (Brown et al., 2014;
Gloria‐Soria et al., 2016; Powell & Tabachnick, 2013). Extant
mosquitoes in and out of Africa show a clear genetic distinction
(Gloria‐Soria et al., 2016, but see exceptions in Kotsakiozi et al.,
2018 and Rose et al., 2020), which roughly matches the two classical
subspecies: Ae. aegypti formosus (Aaf) and Ae.
aegypti aegypti (Aaa) , respectively. Complexities exist in this
subspecies definition (Powell & Tabachnick, 2013), but in this paper,
we refer to them simply based on their geographic range (in or out of
Africa). Non-African Aaa is mostly human specialized and lives
only in domestic settings such as urban area (McBride, 2016; Powell &
Tabachnick, 2013), except for a few forest-living populations in the
Caribbean and Argentina (Chadee et al., 1998; Mangudo et al., 2015). On
the other hand, African Aaf inhabits both forest and domestic
habitats (Kotsakiozi et al., 2018; Paupy et al., 2014; Sylla et al.,
2009), with the latter likely representing an intermediate step towards
true human specialization outside Africa (i.e., Aaa ).
Despite a well-characterized evolutionary path of Ae. aegyptideduced from genetic data, how this species initially moved into
domestic habitats in Africa is not fully understood. In addition, most
mosquito species (over 3,500 species) have not colonized domestic
habitats, including many African mosquitoes that share the same forest
habitats as Ae. aegypti (Clements, 1999). This raises the
question of why Ae. aegypti , among only a few mosquito species,
was able to invade domestic habitats (Carvalho & Moreira, 2017;
Fontenille & Powell, 2020). Addressing these questions could help us
further understand the unique evolutionary history of this
epidemiologically important species, provide insights into mosquito
control, and possibly predict other emerging disease vectors.
For Ae. aegypti , the transition from the ancestral forest habitat
to human settlement involved two major behavioral changes: a preference
for humans as a blood source (McBride, 2016) and using human-made
containers for larval breeding (Day, 2016). Ancestral forest-livingAaf in Africa is a feeding generalist and bites wildlife for
blood, while Aaa out of Africa specializes in biting humans
(Powell et al., 2018). Recent studies have demonstrated the variations
of blood-feeding preference across Africa in different habitats and
between the two subspecies (McBride, 2016; McBride et al., 2014; Rose et
al., 2020). They also identified dry season intensity and human
population density as two main ecological drivers for the evolution of
feeding preference for humans in Africa.
In comparison, larval breeding sites are relatively understudied,
especially in Africa. Aedes aegypti lay eggs at the edge of small
water containers, i.e., oviposition sites that becomes larval breeding
sites (Christophers, 1960). Non-African Aaa uses various
artificial containers, consistent with being a human specialist (Day,
2016; Swan et al., 2018; Vezzani, 2007; Yee, 2008). In Africa,Aaf in the forest and domestic habitats utilize different larval
breeding sites: the former uses natural containers like rainwater-filled
tree holes and rock pools (Lounibos, 1981), while the latter resemblingAaa , relies mostly on artificial containers, such as plastic
buckets, tires, and clay pots (Leahy et al., 1978; McBride et al., 2014;
Petersen, 1977). Some artificial containers hold human-stored water
year-round and could provide valuable or even the only available larval
habitats during the dry season when natural containers dry out.
Therefore, it is hypothesized that seeking human water storage for
oviposition during the dry season likely drove Ae. aegypti to
enter domestic habitats, leading to the evolution of feeding preference
for humans (Petersen, 1977; Powell et al., 2018; Rose et al., 2020).
Despite the presumed key role of larval breeding habitats in the
domestic adaptation of Ae. aegypti , few studies have
characterized natural versus artificial larval breeding sites in Africa
(Dickson et al., 2017).
If a substantial difference exists between natural and artificial
containers, it could pose challenges to both female oviposition and
larvae development. Oviposition preference and larvae performance are
likely associated (Wong et al., 2012) but not always aligned
(Albeny-Simoes et al., 2014; Refsnider & Janzen, 2010). Given the large
variety of larval habitats, it is possible that ancestral Ae.
aegypti were generalist egg-layers, and the larvae can tolerate a wide
range of conditions, which allows them to take advantage of artificial
containers in harsh environments (Powell et al., 2018; Rose et al.,
2020). Prolonged breeding in distinct containers could then lead to
ecological divergence (Gimonneau et al., 2010; Schluter, 2000; Shafer &
Wolf, 2013), resulting in adaptations and specializations to each
container type. Such behavioral differentiation, in turn, could
facilitate population segregation (Ayala et al., 2011; Nosil et al.,
2009; Servedio et al., 2011). Indeed, a few studies have implied that
oviposition divergence may have emerged between Aaa andAaf (Leahy et al., 1978; Petersen, 1977). Larvae of the two
subspecies also showed higher fitness in containers representing their
respective preferred habitats (Saul et al., 1980). However, previous
work only compared the two subspecies, i.e., two ends of the
domestication history of Ae. aegypti . Whether divergence in
oviposition or larval performance already exist within AfricanAaf living in forest versus domestic habitats remains largely
unknown. Examining this potential incipient divergence could provide
valuable insights on when, where, and how Ae. aegypti adapted to
domestic habitats.
In this study, we characterized the environmental conditions ofAe. aegypti larval breeding sites in forest and domestic habitats
and examined whether oviposition divergence has evolved. We focused on
two locations in Africa, La Lopé in Gabon and Rabai in Kenya. Mosquitoes
in both locations are Aaf based on their morphology and broad
genetic pattern (Kotsakiozi et al., 2018; Xia et al., 2020), but can be
found in forest and village environments in close proximity. In each
location, forest and village populations showed little genetic
differentiation (Xia et al., 2020), suggesting local habitat expansion
instead of external introduction. Therefore, Ae. aegypti in these
two locations possibly exemplify the initial colonization step of
domestic habitats. We first compare the physical characteristics,
competition and predation, bacterial profiles, and chemical volatiles of
natural and artificial containers used as larval breeding sites between
habitats (forest and village). Many of these environmental variables
have been shown to affect Ae. aegypti oviposition (Afify &
Galizia, 2015; Harrington et al., 2008; Ponnusamy et al., 2008; Reiskind
& Zarrabi, 2012; Zahiri & Rau, 1998). Therefore, we also investigated
the oviposition choices of forest and domestic Aaf towards some
variables that showed the greatest differences between natural and
artificial containers. This allowed us to examine whether the mosquitoes
in different habitats remained oviposition generalist or have developed
behavioral specialization.