DOI: 10.5061/dryad.59zw3r2cr
title: Temperature adaptation and its impact on the shape of performance curves in Drosophila populations

description: Understanding how species adapt to different temperatures is crucial to
 predict their response to global warming, and thermal performance curves
 (TPCs) have been employed recurrently to study this topic. Nevertheless,
 fundamental questions regarding how thermodynamic constraints and
 evolution interact to shape TPCs in lineages inhabiting different
 environments remain unanswered. Here, we study Drosophila
 simulans along a latitudinal gradient spanning 3,000 km to
 test opposing hypotheses based on thermodynamic constraints
 (‘hotter-is-better’) versus biochemical adaptation
 (‘jack-of-all-temperatures’) as primary determinants of TPCs variation
 across populations. We compare thermal responses in metabolic
 rate and the egg-to-adult survival as descriptors of organismal
 performance and fitness, respectively, and show that different descriptors
 of TPCs vary in tandem with mean environmental temperatures, providing
 strong support to hotter-is-better. Thermodynamic constraints
 also resulted in a strong negative association between maximum performance
 and thermal breadth. Lastly, we show that descriptors of TPCs for
 metabolism and egg-to-adult survival are highly
 correlated, providing evidence of coadaptation and that curves for
 egg-to-adult survival are systematically narrower and displaced
 towards lower temperatures. Taken together, results support the pervasive
 role of thermodynamics constraining thermal responses
 in Drosophila populations along a latitudinal gradient,
 that are only partly compensated by evolutionary adaptation. 
description: Animal collection and
 maintenance The study collected <em>D.
 simulans</em> adult flies from eight locations in Chile, spanning
 almost 3,000 km from North to South, during summer 2018 and 2019. The
 flies were caught using nets over banana or decomposing fruit traps and
 brought to the laboratory for taxonomic identification. Separate
 laboratory lines were established from each location and maintained in
 plexiglass cages under controlled conditions for at least three
 generations before experiments. Two replicate subcultures per line were
 maintained in separate cases, and populations were pooled every generation
 to minimize genetic drift. Maternal and grand maternal effects were
 removed by maintaining individuals under common garden conditions for at
 least three generations prior to experiments.
 Metabolic
 rate The study measured the standard metabolic rate
 (SMR or VCO2) in adult <em>D. simulans</em> flies using
 CO<sub>2</sub> production through a flow-through respirometry
 design. Individual female and male flies were measured in custom-made
 glass metabolic chambers at temperatures ranging from 12 to 37 °C. A total
 of 1,321 flies were measured, and data were extracted using LabAnalyst
 software. SMR was estimated for each 5 min period as the 30s average when
 VCO<sub>2</sub> was stable.
 Overall egg-to-adult
 survival New food plates were placed in maintenance cages
 to allow <em>D. simulans</em> flies to oviposit for 24 hours.
 Eggs were then collected and 20 eggs from each plate were transferred to
 vials with fresh culture medium at temperatures ranging from 12 to 32°C.
 Hatching was monitored every 24 hours until the last egg hatches or for up
 to 60 days. Egg-to-adult survival rate was assessed as the ratio of
 viability to development time, serving as a measure of fitness. A total of
 1,119 vials were used for the experiment, with 20 replicates per
 temperature and population. Statistical analysis
 Descriptor parameters of the
 TPCs, including optimal temperature (T<sub>opt</sub>),
 critical thermal maxima (CT<sub>max</sub>), thermal breadth
 (T<sub>breadth</sub>), and maximum performance
 (P<sub>max</sub>), were extracted from nonlinear regressions.
 A principal component analysis was used to collapse TPC parameters into a
 single response variable, which was then tested with multiple regressions
 to determine the effects of diurnal mean temperatures and thermal range.
 The degree of correlation between
 P<sub>max</sub>, T<sub>opt</sub>,
 T<sub>breadth</sub>, and CT<sub>max</sub> was
 assessed for each trait, as well as the correlation between
 P<sub>max</sub> and T<sub>opt</sub> and
 T<sub>breadth</sub> separately for metabolic rate and
 egg-to-adult survival. The assumptions of normality and homoscedasticity
 of residuals were validated visually, and all statistical analyses were
 performed using R software.
description: R-software is required to open and analyse the data files.