INTRODUCTION:
In oviparous vertebrates, maternal provisioning of nutrients and
signaling molecules is critical for proper development and can be a
major determinant of offspring traits (Groothuis et al., 2005; Radder et
al., 2007; Van Dyke and Griffith, 2018). Complex biological and
ecological factors, including maternal diet (Royle et al., 2003; Warner
and Lovern, 2014), stress (McCormick, 1998; Saino et al., 2005), and age
(Beamonte‐Barrientos et al., 2010; Urvik et al., 2018) can influence the
quantity and quality of resources provisioned to embryos (Moore et al.,
2019; Mousseau and Fox, 1998). In addition, components of the
developmental environment can influence how maternal resources are
utilized by developing embryos (Brown et al., 2011; Du and Shine, 2022,
2008; Mueller et al., 2015; Shine and Brown, 2002). For example, egg
mass is a primary determinant of hatchling mass (Deeming and Birchard,
2007), but incubation temperature has been shown to modify the
efficiency by which maternal resources are converted into somatic tissue
(Bock et al., 2021; Marshall et al., 2020; Pettersen et al., 2019) and
how those resources are allocated to different phenotypes (Flatt, 2001;
Telemeco et al., 2010). However, despite the importance of maternal
provisioning and incubation temperature in shaping hatchling phenotypes,
the extent to which these factors contribute to trait variation across
populations is not well resolved (but see (Bodensteiner et al., 2019;
Orizaola and Laurila, 2016, 2009; Richter‐Boix et al., 2015)).
Species with broad geographic ranges are likely under selective
pressures to maximize fitness under population-specific ecological
conditions, which can include altering embryonic responses to maternal
resources and environmental variables (Conover and Schultz, 1995;
Kawecki and Ebert, 2004; Merilä et al., 2000; Orizaola and Laurila,
2009). For instance, in oviparous reptiles, latitudinal differences in
the influence of temperature on incubation duration have been shown to
occur across populations (Du et al., 2010a; Pettersen, 2020). Whereas
cooler incubation temperatures slow development, populations from
northern latitudes often show counter-gradient variation in incubation
duration, developing faster than populations from more southern
latitudes at the same temperatures (Pettersen, 2020). Similarly, high
altitude populations of wall lizards (Podacris uralis ) have been
shown to allocate more maternal resources towards somatic tissue
relative to low altitude populations when raised at a common temperature
(Pettersen et al., 2023). However, the extent to which populations vary
in how maternal provisioning and incubation temperature shape additional
fitness-related traits in taxonomically diverse species remains unclear.
Such information is critical to understand how responses to
developmental conditions contribute to adaptive evolutionary change.
In the present study, we test whether embryonic responses to maternal
provisioning and incubation temperature show interpopulation variation
by examining several fitness-related traits in a large reptile, the
American alligator (Alligator mississippiensis ). Like many
turtles and some lizards, crocodilians display temperature-dependent sex
determination (TSD), in which thermal signals experienced during a
discreet developmental window determine sex, along with additional
phenotypic traits (Allsteadt and Lang, 1995; Bock et al., 2021; Kohno et
al., 2014; McCoy et al., 2016): incubations at warmer, male-promoting
temperatures (MPT) result in larger hatchlings with greater residual
yolk reserves than those incubated at female-promoting temperatures
(FPT). Recent reports demonstrate that temperature sensitive traits,
including body mass index (BMI) and snout-vent length (SVL), correlate
with juvenile survival in this species (Bock et al. 2023, preprint;
Johnson et al. 2023), which may contribute to the adaptive significance
of TSD (Schwanz et al. 2016; Bock et al. 2023, preprint). This life
history strategy, combined with the unique taxonomic position of
crocodilians relative to other extant vertebrates, makes alligators
particularly informative in deciphering how variation in response to the
developmental environment contributes to trait diversity across
populations. The alligator’s range extends from southern Florida to
northeastern North Carolina, providing ample opportunity for local
adaptation of phenotypic responses to maternal resources and incubation
temperature. Yet, the extent to which embryonic response to these
components of the developmental environment vary across populations is
currently unknown. Here, we use a common garden incubation and grow out
design to resolve the relative influence of egg mass (a proxy for
maternal provisioning) and incubation temperature on morphological and
metabolic traits across two northern and two southern populations
spanning a large latitudinal portion of the alligator’s geographic
range.