Selection against photoperiodic reproductive diapause in Drosophila littoralis

Selection for reproduction under short photoperiods changes diapause-associated traits and induces widespread genomic divergence

The incidence of reproductive diapause is a critical aspect of life history in overwintering insects from temperate regions. Much has been learned about the timing, physiology and genetics of diapause in a range of insects, but how the multiple changes involved in this and other photoperiodically regulated traits are interrelated is not well understood. We performed quasinatural selection on reproduction under short photoperiods in a northern fly species, Drosophila montana, to trace the effects of photoperiodic selection on traits regulated by the photoperiodic timer and / or by a circadian clock system. Selection changed several traits associated with reproductive diapause, including the critical day length for diapause (CDL), the frequency of diapausing females under photoperiods that deviate from daily 24 h cycles and cold tolerance, towards the phenotypes typical of lower latitudes. However, selection had no effect on the period of free-running locomotor activity rhythm regulated by the circadian clock in fly brain. At a genomic level, selection induced extensive divergence between the selection and control line replicates in 16 gene clusters involved in signal transduction, membrane properties, immunologlobulins and development. These changes resembled ones detected between latitudinally divergent D. montana populations in the wild and involved SNP divergence associated with several genes linked with diapause induction. Overall, our study shows that photoperiodic selection for reproduction under short photoperiods affects diapause-associated traits without disrupting the central clock network generating circadian rhythms in fly locomor activity.

Northern Drosophila montana flies show variation both within and between cline populations in the critical day length evoking reproductive diapause

Reproductive diapause, and its correct timing, plays an important role in the life cycle of many insect species living in a seasonally varying environment at high latitudes. In the present paper we have documented 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 1.6h 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 3h. The steepness of the PPRCs correlated with the age of the line and this trait showed no clear latitudinal cline. Further 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 variation 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.

Effect of temperature on the duration of sensitive period and on the number of photoperiodic cycles required for the induction of reproductive diapause in Drosophila montana

Correct timing of the induction of photoperiodic reproductive diapause has been found to play an important role in the life cycle of several northern insect species. However, even when the environmental conditions are favourable for diapause, the switch to diapause can only take place when the females are in a proper developmental and physiological stage, referred to as the sensitive period (SP) for diapause. We have previously shown that in a northern fly species, Drosophila montana, the developmental pathway of the ovaries (direct maturation vs. diapause) is determined by photoperiodic cues that the females receive after eclosion. Here, we have studied the effects of temperature on the duration of the sensitive period, and on the number of short day cycles that the females have to experience before half of them will enter diapause (RDN=required day number). Ovarian development rate of the females was first traced under long and short day conditions in 16 and 19°C, and then reciprocal transfers were done between the diapause-inducing short day conditions, and the vitellogenesis-inducing long day conditions to determine the females' SP and RDN. Close to 100% of the females of all study strains entered reproductive diapause under short day conditions in 16°C, and the same occurred also in 19°C in strains from the more northern univoltine population. The sensitive period for diapause induction was affected by temperature, as it was shorter in higher temperature (circa 8 days in 16°C and 4-5 days in 19°C), and was restricted by the faster development rate of the ovaries. D. montana females had to experience approximately three short day cycles during the sensitive period, before half of them entered diapause, which also explains the decrease in the number of diapausing females at higher temperatures. This system clearly differs from that of the more southern Drosophila species, e.g. D. melanogaster, where the females' developmental pathway is determined already during the first day after eclosion.

Photoperiodic regulation of life-history traits before and after eclosion: egg-to-adult development time, juvenile body mass and reproductive diapause in Drosophila montana

Photoperiod is the main environmental cue used by northern insects to predict the forthcoming seasonal changes and to adjust their life-history traits to fit these changes. We studied the effects of photoperiod on egg-to-adult development time, juvenile body mass and female reproductive diapause in two northern Drosophila montana populations with different patterns of voltinism. The most interesting findings were consistent between the populations: (1) when maintained before eclosion in short day conditions, representing early autumn, the flies developed faster and were lighter than when maintained in long day conditions, representing early summer, (2) photoperiodic time measurement is apparently reset after eclosion, adjusting the flies' development according to post eclosion conditions, (3) the sensitive period for diapause induction took place after eclosion and (4) there was no direct connection between females' egg-to-adult development time and their reproductive state at adulthood, which suggests that these traits can be determined by photoperiodic cues through different time measurement systems. Independence of photoperiodic regulation of life-history traits before and after eclosion enable D. montana flies to respond to changing photoperiods on a short time scale and match their life-history traits according to seasons.

Adaptation to a seasonally varying environment: a strong latitudinal cline in reproductive diapause combined with high gene flow in Drosophila montana

Adaptation to seasonal changes in the northern hemisphere includes an ability to predict the forthcoming cold season from gradual changes in environmental cues early enough to prepare for the harsh winter conditions. The magnitude and speed of changes in these cues vary between the latitudes, which induces strong selection pressures for local adaptation.

We studied adaptation to seasonal changes in Drosophila montana, a northern maltfly, by defining the photoperiodic conditions leading to adult reproductive diapause along a latitudinal cline in Finland and by measuring genetic differentiation and the amount of gene flow between the sampling sites with microsatellites. Our data revealed a clear correlation between the latitude and the critical day length (CDL), in which half of the females of different cline populations enter photoperiodic reproductive diapause. There was no sign of limited gene flow between the cline populations, even though these populations showed isolation by distance. Our results show that local adaptation may occur even in the presence of high gene flow, when selection for locally adaptive life-history traits is strong. A wide range of variation in the CDLs of the fly strains within and between the cline populations may be partly due to gene flow and partly due to the opposing selection pressures for fly reproduction and overwinter survival. This variation in the timing of diapause will enhance populations’ survival over the years that differ in the severity of the winter and in the length of the warm period and may also help them respond to long-term changes in environmental conditions.

Insect photoperiodism and circadian clocks: models and mechanisms

Photoperiodic clocks allow organisms to predict the coming season. In insects, the seasonal adaptive response mainly takes the form of diapause. The extensively studied photoperiodic clock in insects was primarily characterized by a "black-box" approach, resulting in numerous cybernetic models. This is in contrast with the circadian clock, which has been dissected pragmatically at the molecular level, particularly in Drosophila. Unfortunately, Drosophila melanogaster, the favorite model organism for circadian studies, does not demonstrate a pronounced seasonal response, and consequently molecular analysis has not progressed in this area. In the current article, the authors explore different ways in which identified molecular components of the circadian pacemaker may play a role in photoperiodism. Future progress in understanding the Drosophila circadian pacemaker, particularly as further output components are identified, may provide a direct link between the clock and photoperiodism. In addition, with improved molecular tools, it is now possible to turn to other insects that have a more dramatic photoperiodic response.

Diapause in Drosophila melanogaster females: a genetic analysis

Female Drosophila melanogaster exhibit ovarian diapause at low temperatures and short day lengths. We found that D. melanogaster isofemale lines from Windsor (Ontario, Canada) had a significantly higher percentage of females in diapause than did those from Cartersville (Georgia, U.S.A.). To investigate the heredity of this trait, we performed a 16-reciprocal cross analysis using two extreme isofemale lines called W and C. We found that diapause in D. melanogaster is inherited as a simple autosomal recessive trait with the C response (less flies in diapause) completely dominant to the W one. Maternal and cytoplasmic factors did not affect differences in diapause in these lines. The result of our genetic analysis of diapause in D. melanogaster opens may avenues for the genetic dissection of this ecologically relevant trait.

Genetic correlation between circadian eclosion rhythm and photoperiodic diapause in Drosophila littoralis

Populations of Drosophila littoralis are known to be latitudinally highly variable in photoperiodic adult diapause and pupal eclosion rhythm. Phenotypic correlations between the two time-measuring systems among the strains from different latitudes are, however, weak. In the present study, two differing strains were crossed reciprocally in order to search for causal (genetic) correlations between the two traits in the strains. Segregation in the F2 generations showed that variation in each trait was based on a few variable loci only. In the F2, flies having different eclosion times also differed in their diapause. This association was not complete and could have been due to genetic linkage between the traits. For that reason, the hybrid generations were raised for eight generations more to allow recombination between the traits. In F8, selection against diapause was started in the lines by raising them in a light-dark cycle of 15:9, where only females of the southern type reproduce. After eight selected generations, the lines were studied for the traits. Diapause was completely of the southern type, and the eclosion rhythm had also changed in parallel. The change in the phase of the free-running rhythm was not complete. From the present experiment, and from earlier knowledge of the geographical variation in D. littoralis, I conclude that the same pacemaker that is seen in the eclosion rhythm could also participate in daylength measurement for diapause. However, there are also noncorrelated variable parts in the measuring systems of both traits, which may mask the correlated variation.