DOI: 10.5061/dryad.n2z34tmvs
title: Thermal tolerance and preference are both consistent with the clinal distribution of house fly proto-Y chromosomes

description: Selection pressures can vary within localized areas and across massive
 geographical scales. Temperature is one of the best studied ecologically
 variable abiotic factors that can affect selection pressures across
 multiple spatial scales. Organisms rely on physiological (thermal
 tolerance) and behavioral (thermal preference) mechanisms to
 thermoregulate in response to environmental temperature. In addition,
 spatial heterogeneity in temperatures can select for local adaptation in
 thermal tolerance, thermal preference, or both. However, the concordance
 between thermal tolerance and preference across genotypes and sexes within
 species and across populations is greatly understudied. The house fly,
 Musca domestica, is a well-suited system to examine how genotype and
 environment interact to affect thermal tolerance and preference. Across
 multiple continents, house fly males from higher latitudes tend to carry
 the male-determining gene on the Y chromosome, whereas those from lower
 latitudes usually have the male-determiner on the third chromosome. We
 tested whether these two male-determining chromosomes differentially
 affect thermal tolerance and preference as predicted by their geographical
 distributions. We identify effects of genotype and developmental
 temperature on male thermal tolerance and preference that are concordant
 with the natural distributions of the chromosomes, suggesting that
 temperature variation across the species range contributes to the
 maintenance of the polymorphism. In contrast, female thermal preference is
 bimodal and largely independent of congener male genotypes. These sexually
 dimorphic thermal preferences suggest that temperature-dependent mating
 dynamics within populations could further affect the distribution of the
 two chromosomes. Together, the differences in thermal tolerance and
 preference across sexes and male genotypes suggest that different
 selection pressures may affect the frequencies of the male-determining
 chromosomes across different spatial scales.
description: We performed our experiments using five nearly isogenic house fly
 strains, three with IIIM males and two with YM males. We reared each
 strain at 18°C, 22°C, and 29°C for two generations in order to evaluate
 how thermal acclimation affects thermal tolerance and thermal preference.
 Flies from each developmental temperature were assayed at equivalent
 physiological ages estimated by accumulated degree days. For our heat and
 cold tolerance assays, we used flies 22–50 total degree days after
 eclosion. For thermal preference assays, we used flies 96–115 total degree
 days after eclosion.   <b>Thermal tolerance</b>
 We measured heat and cold tolerance in individual male and female
 flies. To measure heat tolerance, lightly anaesthetized individual flies
 were transferred to a 1.5 ml centrifuge tube that was sealed with fabric.
 We placed the 1.5 ml tube in a heat block set to 53°C. The time at which a
 fly fell to the bottom of the tube and could not make its way back to the
 top of the tube was considered the knockdown time. To measure cold
 tolerance, lightly anaesthetized flies were transferred to a fabric-sealed
 20 ml glass vial individually, and the vials were placed in a 4°C
 refrigerator with a transparent door. Knockdown occured when a fly fell on
 its back to the bottom of the vial. We gently tapped the assay vial every
 2–3 minutes to ensure flies were active. We also compared heat and cold
 tolerance between males raised at 22°C and 29°C, using the same approaches
 as described above.  
 <b>Thermal preference</b> We
 measured thermal preference estimated from the position of individual
 flies along a 17–37°C thermal gradient. For each individual fly, we report
 mean thermal preference (Tpref) as the average position during a 10 minute
 assay window (measured once per minute). We also report thermal breadth,
 Tbreadth  as the coefficient of variation of individual-level Tpref during
 the assay window. Tbreadth provides an estimate of how individuals utilize
 thermal space within their environment 
description: <strong>Cold tolerance</strong> A)
 These are raw data for the cold tolerance experiments carried out between
 flies raised at 18C and 29C.
 Cold_tolerance_dataset.csv Note: The missing
 batches are because data were discarded upon discovery of contaminated
 strains that affected proto-Y chromosome genotypes. B)
 These are raw data for the cold tolerance experiments carried out between
 flies raised at 22C and 29C.
 Cold_tolerance_dataset2.csv  
 <strong>Heat tolerance</strong> A)
 These are raw data for the heat tolerance experiments carried out between
 flies raised at 18C and 29C.
 Heat_tolerance_dataset.csv Note: The missing
 batches are because data were discarded upon discovery of contaminated
 strains that affected proto-Y chromosome genotypes.  B)
 These are raw data for the heat tolerance experiments carried out between
 flies raised at 22C and 29C
 Heat_tolerance_dataset2.csv  
 <strong>Thermal preference</strong>
 These are raw data for the thermal preference
 experiments.
 Temperature_preference_dataset.csv