Experimental setup
We performed a cross-factorial experiment in which parental flies (P)
were raised in constant and variable thermal environments, and their
offspring were split and maintained in either the parental environment
or the opposite (Figure 1). To obtain this experimental design, we use
more than 200 inseminated D. melanogaster collected in central
Chile (33°26′S; 70°39′W at 500 m above sea level) during 2016 in a
nearly 500 m2 habitat. After collection, twenty groups
were established with approximately ten females each. Groups were reared
in controlled conditions at a constant ambient temperature
Ta = 24 °C and a Light:Dark 12:12 photoperiod. Flies
were maintained for three generations in 250 mL glass vials with Burdick
culture medium (Burdick, 1955). Third generation adult males and virgin
females from this stock were randomly assigned to two thermal treatments
that differ in the variance of temperature. Thermal treatments were 28 ±
0 ° C and 28 ± 4 ° C, a constant (C) and variable (V) thermal
environment, respectively, and crossed under these conditions.
Acclimation temperatures were chosen based on the well-known limits of
fruit fly eggs viability (Hoffmann, 2010) (eggs-to adult viability is
80% at 28ºC and 0 to 5% at 32ºC, for details see (Hoffmann, 2010;
Cavieres et al. , 2018)). In the variable thermal environment,
during the day, temperature started to increase linearly at 7:00,
reached the maximum at 11:00, then stayed constant, and began to
decrease at 19:00 and reached 24°C at 23:00h, the heating/cooling rate
between the minimum and maximum temperatures was 0.03°C
min−1 (Figure 1).
The offspring, which corresponds to the parental generation P in our
breeding setup, was maintained from eggs to adult in each thermal
treatment. Subsequently, adult males and virgin females from both
treatments were evenly divided into two breeding groups and transferred
to constant and variable thermal conditions. Their offspring correspond
to the F1 in our breeding setup, resulting in a factorial experiment
with two P (C and V for constant and variable, respectively) and four F1
groups (CC, CV, VV and VC, which reflect both the parental and offspring
thermal environment). As detailed below, we estimated for all P and F1
groups the lower and upper thermal critical limits
(CTmin and CTmax ,
respectively), net reproductive rate (R 0) and
generation time (Tg ).