Vitt, Simon: The influence of environmental UVB radiation on growth, immunity, behavior and sexually selected traits in the three-spined stickleback (Gasterosteus aculeatus). - Bonn, 2020. - Dissertation, Rheinische Friedrich-Wilhelms-Universität Bonn.
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author = {{Simon Vitt}},
title = {The influence of environmental UVB radiation on growth, immunity, behavior and sexually selected traits in the three-spined stickleback (Gasterosteus aculeatus)},
school = {Rheinische Friedrich-Wilhelms-Universität Bonn},
year = 2020,
month = jul,

note = {Changing photic conditions are, next to global warming, the major aspect in terms of ongoing global climate change and mainly occur within the short-wave and high-energetic ultraviolet-B (UVB) part of the electromagnetic spectrum. Studies investigating the effect of UVB on organisms mostly use short-term exposure, i.e. single doses, and/or conducted experiments under laboratory settings, inevitably accompanied by artificial photic conditions. Studies using ecologically relevant levels of UVB in combination with standardized settings, e.g. genetic background of experimental animals or food availability, under outdoor conditions are scarce. In my thesis, I used a semi-natural outdoor setup to create two photic conditions, including either natural sunlight in combination with an additional UVB source or exclusively natural sunlight to examine the consequences of enhanced ambient UVB on the biology of three-spined sticklebacks (Gasterosteus aculeatus). I used different time spans of UVB exposure, applied at various life stages, to study short- and long-term phenotypic plastic responses of a teleost fish species confronted with a stressful environment. Three-spined sticklebacks inhabit shallow lentic and flowing aquatic habitats and are naturally exposed to ambient ultraviolet radiation. Whereas ultraviolet-A (UVA) radiation is used during visual communication, sticklebacks are incapable of perceiving UVB which makes them prone to disproportional increases in UVB.
Male sticklebacks develop a conspicuous red breeding coloration and face a high risk of sperm competition due to alternative reproductive strategies, i.e. stealing of fertilizations. Thus, in addition to studying effects on growth and immunity, male sticklebacks are suitable for studying resource allocation to pre- and post-mating sexually selected traits, which both determine an individual’s reproductive success.
In my experiments, I showed that about two months as well as about seven and nine months of exposure to enhanced ambient UVB seriously impair growth. UVB affects various physiological processes, ranging from damage to DNA, enzymes, membrane proteins and lipids as well as increased photo-oxidative stress (formation of reactive oxygen species). Consequently, stressed individuals likely face higher energetic needs for costly repair mechanisms which may retard other physiological processes, e.g. digestion or growth.
I showed that exposure to enhanced ambient UVB for about two months has immunomodulatory effects in non-reproductive sticklebacks of both sexes. Stressed individuals had a lower relative spleen weight, i.e. splenosomatic index (SSI) together with a higher granulocyte-to-lymphocyte ratio of head-kidney leukocytes. The spleen and head-kidney represent major immune organs in teleost fishes. A lower SSI can be interpreted as a downregulated adaptive immunity and a higher proportion of granulocytes indicates an upregulated innate immune response. Chronic exposure to enhanced UVB radiation may have promoted inflammatory processes (e.g. erythema), potentially accompanied by an intensification of the innate immune system. Consequently, in terms of the assumed trade-off between innate and adaptive immunity, the adaptive immunity may be reduced. However, whether the adaptive immune response ultimately is weakened by UVB needs further research.
About two months of exposure to enhanced ambient UVB caused considerable negative effects on post-mating sperm-related traits, i.e. smaller testes, lower sperm number and shorter sperm in reproductively active male sticklebacks. In contrast, UVB exposure for about nine months, from early-life during the major growth phase, had no significant effects on the measured post-mating traits. Though, after nine months of enhanced ambient UVB, I revealed a trade-off between the investment in a pre- and a post-mating trait in males raised under enhanced levels of UVB. More precisely, the intensity of the breeding coloration in UVB-stressed males was negatively correlated with sperm numbers, whereas this relationship was reversed for the control. In contrast to sticklebacks raised under natural sunlight, individuals exposed to stressful conditions were not able to uphold the honesty of a pre-mating sexually selected ornament. Thus, stressed individuals seem to invest resources in a pre-mating trait (breeding coloration) at the expense of a post-mating trait (sperm number). The absent differences between both exposure groups regarding the measured color traits suggest that stressed individuals maintain their sexual attractiveness in order to enhance the access to potential matings. By revealing this trade-off, I could provide the first experimental evidence for a differential allocation to costly pre- and post-mating traits as a result of exposure to a key environmental stressor in terms of climate change.
In summary, I showed the serious effects of enhanced but ecologically relevant levels of ambient UVB, representing an abiotic environmental stressor related to climate change, on multiple fitness-relevant traits in three-spined sticklebacks. I showed phenotypic plastic responses to enhanced UVB on physiological processes, i.e. growth, immunity, sperm-related traits as well as resource allocation to pre- and post-mating traits. Furthermore, I observed consequences of enhanced UVB on the behavior of sticklebacks regarding predator-prey interactions. In my thesis, I demonstrated that chronic exposure to enhanced ambient UVB affects fitness-relevant traits and may influence sexual selection processes under natural conditions. Moreover, enhanced ambient UVB has the potential to affect population and community dynamics in aquatic ecosystems.},

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