- Thermal adaptation in butterflies: patterns, significance and mechanisms (2008)
- Temperature is one of the most important ecological factors affecting biological organization directly and indirectly on nearly all spatial and temporal scales. As in nature organisms are often faced with variation in mean temperatures as well as in temperature extremes, they have to adapt plastically and/or genetically to their respective environmental conditions or will otherwise risk extinction. Using the Copper butterfly Lycaena tityrus as model organism, this study focuses on the patterns, significance and mechanisms of thermal adaptation in ectotherms on three main issues: (1) the mechanistic basis of the temperature-size rule (TSR), (2) altitudinal patterns potentially related to thermal performance and (3) the genetic background of such variation. Following the TSR (being bigger at colder rearing temperatures) in L. tityrus is mainly caused by two different components: a behavioural and a physiological one. During the prolonged development at colder temperatures, larvae showed an increased food intake, a lower assimilation, but a higher efficiency in converting the ingested food into body matter (chapter 5). Sexual differences in body mass, however, were caused by another mechanism. The males’ higher growth rates are evidently combined by a higher daily food consumption, while total food consumption and assimilation was higher in females. And, in contrast to temperature-induced variation in body size, sexes did not differ in the efficiency of converting ingested food into body matter. In addition to such phenotypic patterns, a contribution of directional selection on traits related to fitness is inferred from clinal variation in such traits, and analyzing such variation has consequently become a key element in investigating adaptive evolution. In L. tityrus, altitudinal variation in life-history traits, temperature-stress resistance and flight performance (chapter 6.1), but also in the expression of heat-shock proteins (chapter 6.2), is present. While longer developmental times in high-altitude populations can be explained by a change in voltinism, reduced heat resistance and plasticity in the expression of heat-shock proteins, but increased cold resistance and flight duration across a range of ambient temperatures demonstrate local adaptations to regional climates. Furthermore, by rearing butterflies in both studies at different temperatures, environmentally-induced plasticity is demonstrated to be as important as genetic factors in mediating adaptive responses. Consequently both sources of variation need to be considered when trying to predict responses to short- (such as particularly hot or cold days / nights) or long-term temperature variation (such as global warming). Finally, this thesis also deals with answering the genetic background of such altitudinal variation. Butterflies from L. tityrus populations varying in altitude are clearly separated into an alpine (high-altitude) and a non-alpine (low-altitude) cluster (chapter 7.1). This geographic differentiation is primarily caused by variation at one single locus, the PGI locus, with one homozygote genotype, PGI-2-2, dominating in all alpine populations, while low-altitude populations show much more heterogeneous distributions with many heterozygotes. Interestingly, the genotype dominating in high-altitude populations (PGI 2-2) exhibited the shortest chill-coma recovery times compared to all other genotypes, and also shows intermediate to long development times, thus showing characters typical of high-altitude populations (chapter 7.2). These findings support the notion that the PGI locus is involved in thermal adaptation in L. tityrus and possibly other arthropods.