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We are living in a system that underlies permanent environmental changes due to the rotation of our planet. These changes are rhythmic with the most prominent one having a period of about 24 hours, but also shorter and longer rhythms characterize our environment. To cope with the ever-changing environmental conditions, it is thought to be beneficial if an organism can track and anticipate these changes. The so called endogenous clocks enable this and might provide a fitness advantage. To investigate and unravel the mechanism of endogenous clocks Chronobiologists have used different model organisms. In this thesis Drosophila melanogaster was used as model organism with its about 150 clock neurons representing the main endogenous clock of the fly in the central brain.
The molecular mechanisms and the interlocked feedback loops with the main circadian key players like period, timeless, clock or cycle are under investigation since the 1970s and are characterized quite well so far. But the impact of a functional endogenous clock in combination with diverse factors and the resulting fitness advantages were analysed in only a few studies and remains for the most part unknown. Therefore the aim of this thesis was to unravel the impact of Drosophila melanogaster`s endogenous clock on the fitness of the fly. To achieve this goal different factors – like day length, humidity and food composition – were analyzed in wild type CS and three different period mutants, namely perL, perS and per01, that carry a point mutation altering or abolishing the free-running period of the fruit fly as well as a second arrhythmic strain, clkAR.
In competition assay experiments wild type and clock mutant flies competed for up to 63 generations under a normal 24 hour rhythm with 12 hours light/day and 12 hours darkness/night (LD12:12) or T-cycles with 19 or 29 hours, according to the mutants free-running period, or constant light (LL) in case of the arrhythmic mutant as well as under natural-like outdoor conditions in two consecutive years. Overall the wild type CS strain was outcompeting the clock mutant strains independent of the environmental conditions. As the perL fly strain elongated their free-running period, the competition experiments were repeated with naturally cantonized new fly strains. With these experiments it could be shown that the genetic background of the fly strains – which are kept for decades in the lab, with backcrosses every few years – is very important and influences the fitness of flies. But also the day length impacts the fitness of the flies, enabling them to persist in higher percentage in a population under competition. Further factors that might influence the survival in a competing population were investigated, like e.g. mating preferences and locomotor activity of homo- and heterozygous females or sperm number of males transferred per mating. But these factors can still not explain the results in total and play no or only minor roles and show the complexity of the whole system with still unknown characteristics.
Furthermore populations of flies were recorded to see if the flies exhibit a common locomotor activity pattern or not and indeed a population activity pattern could be recorded for the first time and social contact as a Zeitgeber could be verified for Drosophila melanogaster.
In addition humidity and its impact on the flies´ fitness as well as a potential Zeitgeber was examined in this thesis. The flies experienced different relative humidities for eclosion and wing expansion and humidity cycle phase shifting experiments were performed to address these two different questions of fitness impact and potential Zeitgeber. The fruit fly usually ecloses in the morning hours when the relative humidity is quite high and the general assumption was that they do so to prevent desiccation. The results of this thesis were quite clear and demonstrate that the relative humidity has no great effect on the fitness of the flies according to successful eclosion or wing expansion and that temperature might be the more important factor. In the humidity cycle phase shifting experiments it could be revealed that relative humidity cannot act as a Zeitgeber for Drosophila melanogaster, but it influences and therefore masks the activity of flies by allowing or surpressing activity at specific relative humidity values.
As final experiments the lifespan of wild type and clock mutant flies was investigated under different day length and with different food qualities to unravel the impact of these factors on the fitness and therefore survival of the flies on the long run. As expected the flies with nutrient-poor minimum medium died earlier than on the nutrient-rich maximum medium, but a small effect of day length could also be seen with flies living slightly longer when they experience environmental day length conditions resembling their free-running period. The experiments also showed a fitness advantage of the wild type fly strain against the clock mutant strains for long term, but not short term (about the first 2-3 weeks).
As a conclusion it can be said that genetic variation is important to be able to adapt to changing environmental conditions and to optimize fitness and therefore survival. Having a functional endogenous clock with a free-running period of about 24 hours provides fitness advantages for the fruit fly, at least under competition. The whole system is very complex and many factors – known and unknown ones – play a role in this system by interacting on different levels, e.g. physiology, metabolism and/or behavior.
The monarch butterfly (Danaus plexippus) performs one of the most astonishing behaviors in the animal kingdom: every fall millions of these butterflies leave their breeding grounds in North Amerika and migrate more than 4.000 km southwards until they reach their overwintering habitat in Central Mexico. To maintain their migratory direction over this enormous distance, the butterflies use a time-compensated sun compass. Beside this, skylight polarization, the Earth’s magnetic field and specific mountain ranges seem to guide the butterflies as well the south. In contrast to this fascinating orientation ability, the behavior of the butterflies in their non-migratory state received less attention. Although they do not travel long distances, they still need to orient themselves to find food, mating partners or get away from competitors. The aim of the present doctoral thesis was to investigate use of visual cues for orientation in migrating as well as non-migrating monarch butterflies. For this, field experiments investigating the migration of the butterflies in Texas (USA) were combined with experiments testing the orientation performance of non-migratory butterflies in Germany.
In the first project, I recorded the heading directions of tethered butterflies during their annual fall migration. In an outdoor flight simulator, the butterflies maintained a southwards direction as long as they had a view of the sun’s position. Relocating the position of the sun by 180° using a mirror, revealed that the sun is the animals’ main orientation reference. Furthermore, I demonstrated that when the sun is blocked and a green light stimulus (simulated sun) is introduced, the animals interpreted this stimulus as the ‘real’ sun. However, this cue was not sufficient to set the migratory direction when simulated as the only visual cue in indoor experiments. When I presented the butterflies a linear polarization pattern additionally to the simulated sun, the animals headed in the correct southerly direction showing that multiple skylight cues are required to guide the butterflies during their migration.
In the second project, I, furthermore, demonstrated that non-migrating butterflies are able to maintain a constant direction with respect to a simulated sun. Interestingly, they ignored the spectral component of the stimulus and relied on the intensity instead. When a panoramic skyline was presented as the only orientation reference, the butterflies maintained their direction only for short time windows probably trying to stabilize their flight based on optic-flow information. Next, I investigated whether the butterflies combine celestial with local cues by simulating a sun stimulus together with a panoramic skyline. Under this conditions, the animals’ directedness was increased demonstrating that they combine multiple visual cues for spatial orientation.
Following up on the observation that a sun stimulus resulted in a different behavior than the panoramic skyline, I investigated in my third project which orientation strategies the butterflies use by presenting different simulated cues to them. While a bright stripe on a dark background elicited a strong attraction of the butterflies steering in the direction of the stimulus, the inverted version of the stimulus was used for flight stabilization. In contrast to this, the butterflies maintained arbitrary directions with a high directedness with respect to a simulated sun. In an ambiguous scenery with two identical stimuli (two bright stripes, two dark stripes, or two sun stimuli) set 180° apart, a constant flight course was only achieved when two sun stimuli were displayed suggesting an involvement of the animals’ internal compass. In contrast, the butterflies used two dark stripes for flight stabilization and were alternatingly attracted by two bright stripes. This shows that monarch butterflies use stimulus-dependent orientation strategies and gives the first evidence for different neuronal pathways controlling the output behavior.