vol. 171, no. 6 the american naturalist june 2008 Genotypic and Environmental Effects on Flight Activity and Oviposition in the Glanville Fritillary Butterfly Marjo Saastamoinen * and Ilkka Hanski † Department of Biological and Environmental Sciences, University of Helsinki, P.O. Box 65 (Viikinkaari 1), Helsinki FI-00014, Finland Submitted July 9, 2007; Accepted January 4, 2008; Electronically published April 15, 2008 abstract: Adverse environmental conditions constrain active flight and thereby limit reproduction in most insects. Butterflies have evolved various adaptations in order to thermoregulate, allowing females to search for nectar and to oviposit under unfavorable ther- mal conditions. We studied experimentally and with observational data the effect of low ambient temperatures experienced in the morn- ing on the timing of oviposition and clutch size in the Glanville fritillary butterfly (Melitaea cinxia). Comparisons were made between individuals with different forms of the gene Pgi, encoding the gly- colytic enzyme phosphoglucose isomerase, since naturally segregating variation at Pgi is known to be correlated with flight metabolic rate, flight performance, and fecundity. Experiencing low temperature in the morning delayed the initiation of oviposition and decreased clutch size. We used a thermal image camera to measure the thoracic surface temperature of butterflies immediately after voluntary flight. Single nucleotide polymorphism at Pgi was associated with thoracic temperature at low ambient temperatures. This has consequences for reproduction because females that are able to fly at lower ambient temperatures generally initiate oviposition earlier in the afternoon, when the environmental conditions are most favorable and the av- erage egg clutch size is generally largest. These results suggest that variation in physiological and molecular capacity to sustain active flight at low ambient temperature has significant fitness-related con- sequences in insects. Keywords: body temperature, clutch size, flight performance, phos- phoglucose isomerase, single nucleotide polymorphism (SNP). Temperature is a key environmental factor that affects de- velopment, growth, and survival of individuals and the * E-mail: marjo.saastamoinen@helsinki.fi. † E-mail: ilkka.hanski@helsinki.fi. Am. Nat. 2008. Vol. 171, pp. 701–712.  2008 by The University of Chicago. 0003-0147/2008/17106-42712$15.00. All rights reserved. DOI: 10.1086/587531 dynamics of populations (Sinclair et al. 2003) especially in ectotherms, which lack effective intrinsic mechanisms to control body temperature. In terrestrial ectothermic ver- tebrates such as reptiles, behavioral thermoregulation is causally related to reproduction and patterns of life-history evolution (reviewed in Shine 2005). In butterflies and many other insects, active flight is essential for foraging, mate location, and oviposition; hence, time available for flight may constrain the ability of an individual to utilize resources and to reproduce (Kingsolver 1983). Insect flight is energetically very expensive (Suarez 2000) and requires the maintenance of high muscle temperature, around 30°– 35°C in many butterflies (Watt 1973; Dennis 1993). To attain high body temperature, butterflies often use solar radiation (Shreeve 1992), and many species have evolved various behavioral, physiological, and morphological ad- aptations in order to thermoregulate (Van Dyck 2003). Thermoregulation is of great importance in temperate spe- cies in particular because they need to increase their body temperature well above the ambient air temperature to initiate flight. At the same time, small butterflies cool rap- idly by convection during flight, especially in low ambient temperatures (Gilchrist 1990). Comprehensive studies on Colias butterflies (reviewed in Watt 1992, 2003) and more recently on the Glanville fritillary (Melitaea cinxia; Haag et al. 2005; Saastamoinen 2007a) have demonstrated a significant correlation be- tween allelic variation in the glycolytic enzyme phos- phoglucose isomerase (PGI) and variation in flight met- abolic rate, flight performance, and activity. In Colias, individuals with different PGI genotypes (allozymes) differ in their metabolic capacity, which leads to differences in flight performance (Watt et al. 1983). Both female fe- cundity and male mating success are directly influenced by flight performance and thereby also by the PGI ge- notype (Watt 1992). Female Colias with a particular PGI genotype (3/4 heterozygotes) are able to fly at lower am- bient temperatures, which increases time available for flight (Kingsolver 1983) and gives these females more time for oviposition (Watt 1992), increasing their reproductive performance. For a comparable reason, in male Colias but-