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Acclimation Responses of Fruit Flies to Environmental Warming
Nyamukondiwa, C. and Terblanche, J.S. 2010. Within-generation variation of critical thermal limits in adult Mediterranean and Natal fruit flies Ceratitis capitata and Ceratitis rosa: thermal history affects short-term responses to temperature. Physiological Entomology 35: 255-264.

The authors write that "acclimation, acclimatization or hardening responses all describe different forms of phenotypic plasticity (i.e. the ability of an organism to respond to environmental stimuli with a change in phenotype)," as described by West-Eberhard (2003) and Whitman (2009); and they further state that "phenotypic plasticity may alter the performance of an organism through compensatory modifications of physiological function and tolerance as a result of changes in environmental conditions," suggesting that this multifaceted phenomenon will likely play a major role in determining future geographic distributions of insects in a warming world.

What was done
Nyamukondiwa and Terblanche explored this phenomenon as it operates in two species of fruit fly (Ceratitis rosa and C. capitata). Using a full-factorial design, as they describe it, one-week-long acclimation responses of each species' critical minimum and maximum temperature (CTmin and CTmax) to exposure to temperatures of 20, 25 and 30°C were investigated, as well as their interactions with short-term sub-lethal temperature exposures to the same conditions as arrived at via different rates of warming.

What was learned
The two South African scientists report that "generally, increasing the acclimation temperature significantly increased CTmax, whereas decreasing the acclimation temperature significantly lowered CTmin." In addition, they found that "slower ramping rates significantly increase CTmax in both C. rosa and C. capitata," suggesting that "more time during heating (i.e. a slower heating rate) provides the flies with an opportunity to develop some heat protection, and therefore suggests that C. capitata, and possibly also C. rosa, might have short-term phenotypic plasticity of high temperature tolerance," which they describe as being "similar to the rapid heat hardening or the heat shock responses in Drosophila (Loeschcke and Hoffmann, 2006; Johnson et al., 2009) and other insect species (Huang et al., 2007)."

What it means
In light of their several observations, Nyamukondiwa and Terblanche conclude that fruit flies "are capable of adjusting their thermal tolerance within a single generation at both weekly and hourly time scales," noting that "high temperature acclimation improves high temperature tolerance, in keeping with much of the literature on thermal acclimation (Whitman, 2009)." They also state that "both C. rosa and C. capitata have the capacity to adjust their thermal tolerance over short timescales in the wild," while further stating that this phenomenon will "probably allow both species to track changes in ambient temperature and survive sudden extremes of temperature that might otherwise be potentially lethal," additionally citing Chown and Nicolson (2004) in this regard.

And this phenomenon is not restricted to flies. The two researchers indicate, for example, that in several insect species "survival of lethal temperatures or critical thermal limits to activity can be significantly improved by prior exposure to sub-lethal temperatures," citing the work of Lee et al. (1987), Kelty and Lee (2001), Shreve et al. (2004) and Powell and Bale (2006), while further indicating that this phenomenon is a major mechanism used by insects to cope with temperature variation at both daily (Kelty and Lee, 2001; Kelty, 2007; Overgaard and Sorensen, 2008) and seasonal (Powell and Bale, 2006; Hoffmann et al., 2005; Terblanche et al., 2006) time scales.

Chown, S.L. and Nicolson, S.W. 2004. Insect Physiological Ecology: Mechanisms and Patterns. Oxford University Press, United Kingdom.

Hoffmann, A.A., Shirriffs, J. and Scott, M. 2005. Relative importance of plastic versus genetic factors in adaptive differentiation: geographic variation for stress resistance in Drosophila melanogaster from eastern Australia. Functional Ecology 19: 222-227.

Huang, L.H., Chen, B. and Kang, L. 2007. Impacts of mild temperature hardening on thermo tolerance, fecundity and Hsp gene expression in Liriomyza huidobrensis. Journal of Insect Physiology 53: 1199-1205.

Johnson, T.K., Cockerell, F.E., Carrington, L.B., Rako, L., Hoffmann, A.A. and McKechnie, S.W. 2009. The capacity of Drosophila to heat harden associates with low rates of heat-shocked protein synthesis. Journal of Thermal Biology 34: 327-331.

Kelty, J.D. 2007. Rapid cold-hardening of Drosophila melanogaster in a field setting. Physiological Entomology 32: 343-350.

Kelty, J.D. and Lee Jr., R.E. 2001. Rapid cold-hardening of Drosophila melanogaster (Diptera: Drosophilidae) during ecologically based thermoperiodic cycles. Journal of Experimental Biology 204: 1659-1666.

Lee, R.E., Chen, C.P. and Denlinger, D.L. 1987. A rapid cold-hardening process in insects. Science 238: 1415-1417.

Loeschcke, V. and Hoffmann, A.A. 2006. Consequences of heat hardening on a field fitness component in Drosophila depend on environmental temperature. American Naturalist 169: 175-183.

Overgaard, J. and Sorensen, J.G. 2008. Rapid thermal adaptation during field temperature variations in Drosophila melanogaster. Cryobiology 56: 159-162.

Powell, S.J. and Bale, J.S. 2006. Effect of long-term and rapid cold hardening on the cold torpor temperature of an aphid. Physiological Entomology 31: 348-352.

Shreve, S.M., Kelty, J.D. and Lee, R.E. 2004. Preservation of reproductive behaviors during modest cooling: rapid cold-hardening fine-tunes organismal response. Journal of Experimental Biology 207: 1797-1802.

Terblanche, J.S., Klok, C.J., Krafsur, E.S. and Chown, S.L. 2006. Phenotypic plasticity and geographic variations in thermal tolerance and water loss of tsetse Glossina pallidipes (Diptera: Glossinidae): implications for distribution modeling. American Journal of Tropical Medicine and Hygiene 74: 786-794.

West-Eberhard, M.J. 2003. Developmental Plasticity and Evolution. Oxford University Press, New York, New York, USA.

Whitman, D.W. 2009. Acclimation. In: Whitman, D.W. and Ananthakrishnan, T.N. (Eds.) Phenotypic Plasticity of Insects. Mechanisms and Consequences. Science Publishers, Enfield, New Hampshire, USA, pp. 675-739.

Reviewed 19 January 2011