Can artificially selected phenotypes influence
a component of field fitness? Thermal selection
and fly performance under thermal extremes
Torsten Nygaard Kristensen1,2,3,*, Volker Loeschcke2and Ary A. Hoffmann1
1Center for Environmental Stress and Adaptation Research, Departments of Genetics and Zoology,
The University of Melbourne, Melbourne, Victoria 3010, Australia
2Aarhus Centre for Environmental Stress Research, Department of Ecology and Genetics, University of Aarhus,
Ny Munkegade, Building 1540, 8000 Aarhus C, Denmark
3Department of Genetics and Biotechnology, Danish Institute of Agricultural Sciences, 8830 Tjele, Denmark
Artificially selected lines are widely used to investigate the genetic basis of quantitative traits and make
inferences about evolutionary trajectories.Yet, the relevance of selected traits to field fitness is rarely tested.
Here, we assess the relevance of thermal stress resistance artificially selected in the laboratory to one
component offield fitness by investigating the likelihood of adult Drosophila melanogaster reaching food bait
under different temperatures. Lines resistant to heat reached the bait more often than controls under hot
and cold conditions, but less often at intermediate temperatures, suggesting a fitness cost of increased heat
resistance but not at temperature extremes. Cold-resistant lines were more common at baits than controls
under cold as well as hot field conditions, and there was no cost at intermediate temperatures. One of the
replicate heat-resistant lines was caught less often than the others under hot conditions. Direct and
correlated patterns of responses in laboratory tests did not fully predict the low performance of the heat
selected lines at intermediate temperatures, nor the high performance of the cold selected lines under hot
conditions. Therefore, lines selected artificially not only behaved partly as expected based on laboratory
assays but also evolved patterns only evident in the field releases.
Keywords: field releases; genotype!environment interactions; resource location;
thermal stress resistance; selection experiments
In experimental physiology and evolutionary biology,
artificial and natural laboratory selection experiments are
often used to study evolutionary processes. They provide
an opportunity to observe evolution as it occurs and can
contribute to anunderstanding
mechanisms and specific genes under selection (Gibbs
1999; Harshman & Hoffmann 2000; Brakefield 2003;
Conner 2003). Furthermore, direct and correlated
responses can be studied and yield information on the
pleiotropic basis of evolutionary constraints resulting from
tradeoffs between traits (Rose et al. 1992) and insights
about the genetic variance and covariance structure within
andbetweentraitscanbeobtained(Brakefield2003;Sgro ` &
Selection experiments on stress resistance traits have
received much attention recently, partly because they can
indicate processes that might be involved in adaptation to
climate change (reviewed in Hoffmann et al. 2003).
Drosophila has often been the organism of choice in such
investigations because (i) it is possible to maintain
replicate lines including unselected control lines and (ii)
selective conditions can be easily defined in the laboratory.
In addition, the genomic information available for several
Drosophila species helps to link variation at different levels
of biological organization and assists in detecting specific
genes involved in phenotypic shifts.
Yet, despite the popularity of laboratory selection
experiments, they have limitations for making physiologi-
cal and evolutionary inferences (Gibbs 1999; Harshman &
Hoffmann 2000). Similar selection regimes have often
produced different patterns of direct and correlated
responses, and results can be highly dependent on the
genetic background and the environmental conditions
under which selection is performed (Rose et al. 1992;
Chippindale et al. 1994; Tower 1996; Harshman et al.
1999; Partridge et al. 1999; Bubliy & Loeschcke 2005).
The importance of genotype!environment interactions
means that results from one laboratory might not be
extrapolated to another one or to field conditions
(Harshman & Hoffmann 2000). Given that field con-
ditions are more complex than laboratory environments,
selection experiments might yield only limited insights
into adaptive processes in the field (Santos et al. 2005).
One way of linking laboratory experiments to adaptive
processes in the field is to examine the performance of
lines generated by laboratory selection under field
conditions. If selected phenotypes influence fitness in
nature, there should be predictable fitness benefits and
costs under field conditions. Location of field resources
has previously been adopted to investigate physiological
condition in Drosophila (Turelli & Hoffmann 1988) and
Proc. R. Soc. B (2007) 274, 771–778
Published online 19 December 2006
Electronic supplementary material is available at http://dx.doi.org/10.
1098/rspb.2006.0247 or via http://www.journals.royalsoc.ac.uk.
*Author for correspondence (firstname.lastname@example.org).
Received 11 November 2006
Accepted 24 November 2006
This journal is q 2006 The Royal Society
expression and variation in heat stress resistance traits.
Funct. Ecol. 15, 289–296. (doi:10.1046/j.1365-2435.
Sørensen, J. G., Kristensen, T. N. & Loeschcke, V. 2003 The
evolutionary and ecological role of heat shock proteins.
Ecol. Lett. 6, 1025–1037. (doi:10.1046/j.1461-0248.2003.
Taylor, M. & Feyereisen, R. 1996 Molecular biology and
evolution of resistance to toxicants. Mol. Biol. Evol. 13,
Thomson, L. J., Robinson, M. & Hoffmann, A. A. 2001 Field
and laboratory evidence for acclimation without costs in
an egg parasitoid. Funct. Ecol. 15, 217–221. (doi:10.1046/
Tower, J. 1996 Aging mechanisms in fruit flies. Bioassays 18,
Turelli, M. & Hoffmann, A. A. 1988 Effects of starvation and
experience on the response of Drosophila to alternative
resources. Oecologia 77, 497–505. (doi:10.1007/BF003
West, S. A., Flanagan, K. E. & Godfray, H. C. J. 1996 The
relationship between parasitoid size and fitness in the field,
a study of Achrysocharoides zwoelferi (Hymenoptera: Eulo-
phidae). J. Anim. Ecol. 65, 631–639. (doi:10.2307/5742)
778 T.N. Kristensen et al.
Thermal selection and fly performance
Proc. R. Soc. B (2007)