Pekka Lankinen

Geographical variation in circadian eclosion rhythm and photoperiodic adult diapause inDrosophila littoralis

SummaryThe time measuring system ofDrosophila littoralis strains originating between 40–70° N was found to be highly variable and latitude dependent. The critical daylength for photoperiodic adult diapause varied from 12 h or no diapause response in the south to 20 h in north. The median timing of pupal eclosion rhythm varied correspondingly from 21 h to 12 h from lights off in LD 3∶21, and the period of free-running rhythm of eclosion from 24 h to 19 h. The phase of the free-running rhythm was also variable, and correlated with the phase of the entrained rhythm. Latitudinal variation in the entrained rhythm of eclosion and in diapause is adaptive, leading to eclosion early in the morning and to overwintering at the adult stage. In some strains with a late phase of eclosion, strong transient cycles were seen following the transition from LL to DD. A total damping of the free-running eclosion rhythm within 2–7 days was common to all strains. This damping was more pronounced in the northern strains. The phase and period of eclosion rhythms were statistically independent. Diapause was not correlated with any parameters of the eclosion rhythm in the analysis. Diapause may still be influenced by the period of the eclosion rhythm, even though its minor contribution may be masked by a more variable, eclosion rhythm independent system in the determination of diapause.

Independence of Genetic Geographical Variation between Photoperiodic Diapause, Circadian Eclosion Rhythm, and Thr-Gly Repeat Region of the Period Gene in Drosophila littoralis

Drosophila littoralis is a latitudinally widespread European species of the Drosophila virilis group. The species has ample genetic variation in photoperiodism (adult diapause) and circadian rhythmicity (pupal eclosion rhythm), with adaptive latitudinal clines in both of them. The possible common genetic basis between the variability of photoperiodism and circadian rhythms was studied by a long-term crossing experiment. A northern strain (65 °N) having long critical day length (CDL = 19.9 h) for diapause, early phase of the entrained rhythm in LD 3:21 (ψLD3:21 = 12.3 h), and short period (τ= 18.8 h) of the free-running rhythm for the eclosion rhythm was crossed with a southern strain (42 °N) having short CDL (12.4 h), late eclosion phase (ψLD3:21 = 20.2 h), and long period (τ= 22.8 h). After 54 generations, including free recombination, artificial selection, and genetic drift, a novel strain resulted, having even more “southern” diapause and more “northern” eclosion rhythm characteristics than found in any of the geographical strains. The observed complete separation of eclosion rhythm characteristics from photoperiodism is a new finding in D. littoralis; in earlier studies followed for 16 generations, the changes had been mostly parallel. Evidently, the genes controlling the variability of the eclosion rhythm and photoperiodism in D. littoralis are different but closely linked. To test for the possible gene loci underlying the observed geographical variability, the period gene was studied in 10 strains covering all the known clock variability in D. littoralis. The authors sequenced the most suspected Thr-Gly region, which is known to take part in the adaptive clock variability in Drosophila melanogaster. No coding differences were found in the strains, showing that this region is not included in the adaptive clock variability in D. littoralis.

Weak circadian eclosion rhythmicity in Chymomyza costata (Diptera: Drosophilidae), and its independence of diapause type

Abstract Pupal eclosion rhythms were studied in seven strains of C. costata originating between latitudes 43°N in Japan to 69°N in Finland. The strains represented the following types of diapause: larval obligatory, larval photoperiodic (critical daylengths from 14 h in the south to 21 h in the north), and no photoperiodic diapause. The eclosion rhythmicity of all strains was weak in all diel photoperiods. The free-running rhythms of eclosion were also of low amplitude. Three other species of Chymomyza were studied for comparison: Chymomyza distincta and C. fuscimana with a larval diapause and C. caudatula with an adult diapause. Only C. caudatula had the rhythmic eclosion that drosophilids have in general. We conclude that the exceptionally weak eclosion rhythmicity in C. costata is an adaptation for eclosion at any permissive time of day around midsummer. The weak rhythmicity of eclosion in C. costata would thus the phenotypic plasticity, an adaptation to the exceptional phenological timing of the species. There was no parallel variation between circadian eclosion rhythms and photoperiodism.

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.

Natural Selection and the α-GPDH Locus in Drosophilidae

The approximate amount of variability at the gene level has now been known for about ten years. This knowledge is mainly based on the technique of gel electrophoresis of proteins and of enzymes in particular. It has been difficult, however, to demonstrate an unequivocal correlation between the physiological function of an enzyme and its elec-trophoretically detectable variants. Once a physiological difference has been established, it may be assumed to have an influence on the adaptive norm of an individual. There are, however, numerous difficulties with this approach. E.g. the substitution of a single amino acid for another may be thought to have an infinitesimally small effect on the total function of the enzyme molecule and the fitness of its bearer.