Several insect species in temperate regions develop and reproduce during the summer, but become dormant as the winter approaches to avoid unfavourable conditions for development and reproduction. This dormancy is usually linked with a period of developmental arrest, i.e. diapause, at the embryonic, larval, pupal or adult stage. Adult reproductive diapause, where oogenesis and vitellogenesis are arrested, has been studied in monarch butterflies  and in several grasshopper  and Drosophila species [e.g. ; see also ]. In Drosophila species, this kind of diapause has been found to offer resource allocation trade-offs: the females entering reproductive diapause early in the summer will not produce progeny during the ongoing season, but they have a higher stress resistance, age more slowly  and have better chances to survive over the winter period and produce progeny during the next summer than the non-diapausing females. In most organisms the onset of reproductive diapause is largely determined by the length of the light period (photoperiodic diapause), which is not surprising, as seasonal changes in the day length in different latitudes are more predictable and consistent than changes in temperature or humidity . Photoperiodic diapause also allows the females to anticipate the coming cold season in advance, which is very important for organisms with a long generation time. From an evolutionary point of view, reproductive diapause is an important biological process which involves many life-history parameters important for survival and reproductive fitness at both individual and population levels.
The discovery of an adult photoperiodic reproductive diapause in Drosophila melanogaster [6, 7] opened a possibility to study the role of the circadian clock genes in photoperiodic time measurement and diapause, as well as interactions between genes affecting photoperiodism and hormonal and physiological changes involved in diapause. The finding also gave a chance to search for genes affecting fitness-related traits linked with diapause, e.g. life span, age-specific mortality, fecundity, resistance to cold and starvation stress, lipid content, development time, egg-to adult viability, accessory gland synthesis and mating behaviour [8–10]. D. melanogaster females are known to enter adult reproductive diapause in response to short days and low temperatures in Europe  and in North America [10, 12]. However, Emerson et al.,  showed lately that in two natural populations representing latitudinal extremes in eastern North America temperature is the main determinant of the diapause. Even though the reproductive diapause in D. melanogaster seems to be a relatively weak response to photoperiod and is only observed at temperatures below 14°C [7, 14], the genes found to be linked with sexual maturation and diapause in this species are good candidate genes for diapause studies in more northern Drosophila species with stronger, longer-lasting photoperiodic diapause responses.
Our study species, D. montana, belongs to the D. virilis group, which originated in continental Asia. It has expanded its distribution around the northern hemisphere from 30°N to 70°N in latitude and from 0 to 3000 meters in altitude, adapting to various kinds of environmental conditions [15, 16]. The females of this species have the ability to survive through the harsh winter conditions in photoperiodic adult reproductive diapause . They have adapted to environmental conditions in northern Scandinavia with a mean temperature of less than 0°C for up to six winter months, a minimum temperature reaching occasionally -30°C or even below. D. montana flies predict the forthcoming cold season mainly by changes in the day length even though also the temperature seems to have some effect on the percentage of diapausing females .
We had two aims in this study. The first aim was to create a custom-made microarray for D. montana with candidate genes known to play a role in traits linked with reproductive diapause in D. melanogaster. The second aim was to determine which of the candidate genes show expression changes, and in which direction, in comparisons among diapausing (i.e. those in reproductive diapause), reproducing and young D. montana females. Our candidate gene microarray proved to be a practical and cost-effective way to use information on gene function obtained largely through mutagenesis in a genetic model species and to study whether the same genes play a role in adaptation to different environmental conditions in an ecologically interesting non-model organism. Out of the 101 genes assayed, 24 showed differences in their expression patterns in comparisons among D. montana females at different reproductive modes.