Northern Drosophila montana flies show variation both within and between cline populations in the critical day length evoking reproductive diapause Pekka Lankinen a , Venera I. Tyukmaeva b,c,⇑ , Anneli Hoikkala b a Department of Biology, University of Oulu, PL 3000, 90014 Oulu, Finland b Department of Biological and Environmental Science, University of Jyväskylä, Survontie 9, P.O. Box 35, FIN-40014 Jyväskylä, Finland c School of Biology, Dyers Brae, University of St. Andrews, St. Andrews, Fife KY16 9TH, UK article info Article history: Received 14 March 2013 Received in revised form 10 May 2013 Accepted 13 May 2013 Available online 20 May 2013 Keywords: Adaptation Life cycle Cline Photoperiodic response Variation abstract Reproductive diapause, and its correct timing, plays an important role in the life cycle of many insect spe- cies living in a seasonally varying environment at high latitudes. In the present paper we have docu- mented variation in the critical day length (CDL) for adult reproductive diapause and the steepness of photoperiodic response curves (PPRCs) in seven clinal populations of Drosophila montana in Finland between the latitudes 61 and 67°N, paying special attention to variation in these traits within and between cline populations. The isofemale lines representing these populations showed a sharp transition from 0% to 100% in females’ diapause incidence in the shortening day lengths, indicated by steep PPRCs. The mean CDL showed a clear latitudinal cline decreasing by 1.6 h from North to South regardless of the age of the lines, variation within the populations (i.e. among lines) in this trait being up to 3 h. The steep- ness of the PPRCs correlated with the age of the line and this trait showed no clear latitudinal cline. Fur- ther studies on a large number of lines from one D. montana population confirmed that while maintaining the flies in diapause preventing conditions in the laboratory has no effect on CDL, older lines had steeper PPRCs. High variation in CDL within and between D. montana cline populations is likely to be heritable and provide a good potential for the evolution of photoperiodic responses. Information on genetic vari- ation in life-history traits, such as diapause, is of utmost importance for predicting the ability of insects to survive in seasonally changing environmental conditions and to respond to long term changes in the length of the growing period e.g. by postponing the timing of diapause towards shorter day length and later calendar date. Ó 2013 Elsevier Ltd. All rights reserved. 1. Introduction Insect species have evolved various kinds of strategies, includ- ing dormancy and migration, to survive in seasonally varying envi- ronments. Diapause, a form of insect dormancy, is usually expressed as a state of low metabolic activity that is associated with a set of physiological and behavioural changes facilitating resistance to harsh environmental conditions (Tauber et al., 1986). In late summer when the day length (and/or temperature) has decreased below the critical point, newly emerged females en- ter adult reproductive diapause by arresting their oogenesis at a pre-vitellogenic stage. Together with other physiological changes this increases the females’ chances to survive over the cold period and produce progeny during the next summer. Entering reproduc- tive diapause is metabolically expensive and thus its correct timing plays a crucial role in survival and progeny production. The fe- males that develop ovaries too late during the growing season will not survive over the cold period and they may also produce prog- eny that fail to reach adulthood and/or are not able to collect en- ergy reserves before the winter. On the other hand, the energy reserves expended during diapause have a profound effect on fe- males’ post-diapause fitness (Hahn and Denlinger, 2007) and thus the females that enter reproductive diapause too early may be less successful in producing progeny after overwintering. Adult reproductive diapause can be triggered by a variety of environmental cues, such as photoperiod, temperature, humidity and nutrition (Saunders, 2002). These cues are commonly called ‘token stimuli’, as they do not represent unfavourable conditions themselves, but are only seasonally correlated with them (Tauber 0022-1910/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.jinsphys.2013.05.006 Abbreviations: CDL, critical day length; GLM, generalized linear model; LD, light:dark; PPRC, photoperiodic response curve. ⇑ Corresponding author at: School of Biology, Dyers Brae, University of St. Andrews, St. Andrews, Fife KY16 9TH, UK. Tel.: +44 1334 463372. E-mail addresses: pekka.lankinen@oulu.fi (P. Lankinen), vtyukmaeva@gmail.com (V.I. Tyukmaeva), anneli.hoikkala@jyu.fi (A. Hoikkala). Journal of Insect Physiology 59 (2013) 745–751 Contents lists available at SciVerse ScienceDirect Journal of Insect Physiology journal homepage: www.elsevier.com/locate/jinsphys