Current Biology Vol 18 No 18R876

Biomechanical Acclimation: FlyingCold

Why are animals reared at colder temperatures larger? A new study shows thatfruit flies reared at lower temperatures are better able to fly in the cold.

Axonal Domains: Role for ParanodalJunction in Node of Ranvier Assembly

A new study shows that communication between axons and glia at theparanodal junction can orchestrate the formation of the node of Ranvier.

David A. Lyons* and William S. Talbot

In the vertebrate nervous system,myelin facilitates the rapid conductionof neural impulses. Consecutivesegments of myelin along the length

of an axon are separated by shortunmyelinated domains, called nodesof Ranvier, which contain a highconcentration of voltage-gated sodiumchannels that propagate the neuronalimpulse. Each node of Ranvier is

Joel G. Kingsolverand Tyson L. Hedrick

The temperature–size rule describesone of the most common patterns ofphenotypic plasticity in nature: in mostspecies, individuals reared at lowertemperatures have increased adultbody sizes [1]. A variety of adaptive andnon-adaptive hypotheses for thetemperature-size rule have beenproposed, but a general explanationremains elusive [2,3]. Bergmann’s ruledescribes a distinct but relatedempirical pattern found in many animaltaxa: populations or species that occurin colder environments have evolvedrelatively larger adult sizes [4]. ManyDrosphila follow both of these rules[5,6]. Why are flies reared at coldertemperatures larger? Why do fliesliving in colder environmentsevolve larger size?

The clue to addressing thesequestions for flies may lie in theallometric scaling of different aspectsof size. In populations of Drosophilasubobscura on three continents, lowerdevelopmental temperatures generatelarge increases in wing length and wingarea, but more modest changes inbody mass; as a result, flies reared atlower temperatures have substantiallyreduced wing loading — that is, bodymass/wing area ratio [5]. Gilchristand Huey [5] suggested that thisplastic response in morphology isbiomechanically adaptive: reducedwing loading could facilitate flight atcolder temperatures where themechanical power output of flightmuscles is reduced.

A recent study by Frazier et al. [7]provides an experimental test of thishypothesis in Drosophilamelanogaster. As in earlier studies,the authors found that Drosophilamelanogaster reared at lowertemperatures had a larger wing arearelative to their body size, reducingthe amount of mass that must besupported by a given unit of wing.Furthermore, wing length alsoincreased relative to body mass, evenafter accounting for the increase in total

wing area. This suggests that thesecond moment of area of the wings[8], a measure of wing shape and thebest morphological predictor of slowflight capability, also increasedrelative to body mass.

Many factors other than wing areaand length contribute to flightperformance, however, so positiveallometric scaling of these wingparameters is not proof of actualcapability. Frazier et al. [7] tested flightperformance directly by eliciting takeoffs from flies reared over a rangeof temperatures. Virtually all flieswere able to take off at warmerenvironmental temperatures (18�C),but only flies reared at the coldesttemperature in the study (15�C) wereable to take off when the environmentaltemperature was reduced to 14�C.Thus, not only do flies reared at coolertemperatures have the biomechanicalequipment for efficient low speedflight, they exhibit improvedflight performance.

The study [7] indicates thatdevelopment plasticity to cold rearingtemperatures may be beneficial to fliesby increasing flight performance at coldtemperatures. A full demonstration ofthe beneficial plasticity hypothesis,however, would require evidence thatflies reared at high temperatures haveincreased performance at hightemperatures [9,10]. Why do flies rearedat higher temperatures have relatively

smaller wings? Intriguingly, flies rearedat lower temperatures also had a lowerwingbeat frequency, a factor whichshould reduce forward flight speed andother aspects of flight performance.Whether this possible tradeoff providesthebasis forbeneficialplasticity in thisorother insects will require further study.

References1. Atkinson, D. (1994). Temperature and organism

size-a biological law for ectotherms? Adv. Ecol.Res. 3, 1–58.

2. Angilletta, M.J., and Dunham, A.E. (2003). Thetemperature-size rule in ectotherms: Simpleevolutionary explanations may not be general.Am. Nat. 162, 333–342.

3. Walters, R., and Hassall, M. (2006). Thetemperature-size rule in ectotherms: maya general explanation exist after all?Am. Nat. 167.

4. Blanckenhorn, W.U., and Demont, M. (2004).Bergmann and converse Bergmann latitudinalclines in arthropods: Two ends of a continuum?Int. Comp. Biol. 44, 413–424.

5. Gilchrist, G.W., and Huey, R.B. (2004). Plasticand genetic variation in wing loading asa function of temperature within and amongparallel clines in Drosophila subobscura. Int.Comp. Biol. 44, 461–470.

6. Partridge, L., and Coyne, J.A. (1997).Bergmann’s rule in ectotherms: is it adaptive?Evolution 51, 623–635.

7. Frazier, M.R., Harrison, J.F., Kirkton, S.D., andRoberts, S.P. (2008). Cold rearing improvescold-flight performance in Drosophila viachanges in wing morphology. J. Exp. Biol. 211,2116–2122.

8. Ellington, C.P. (1984). The aerodynamics ofhovering insect flight. II. Morphologicalparameters. Phil. Trans. R. Soc. B 305, 41–78.

9. Huey, R.B., Berrigan, D., Gilchrist, G.W., andHerron, J.C. (1999). Testing the adaptivesignificance of acclimation: A strong interenceapproach. Am. Zool. 39, 323–336.

10. Kingsolver, J.G., and Huey, R.B. (1998).Selection and evolution of morphological andphysiological plasticity in thermally varyingenvironments. Am. Zool. 38, 545–560.

Department of Biology, University of NorthCarolina, Chapel Hill, North Carolina 27599,USA.E-mail: jgking@bio.unc.edu, thedrick@bio.unc.edu

DOI: 10.1016/j.cub.2008.07.036