Symbiosis is an evolutionary strategy that may provide biological
advantages. The most complex and tight cooperation between symbiotic
organisms is achieved in endosymbiosis, as it is the symbiosis between
green hydra (Hydra viridissima Pallas, 1766) and unicellular
photoautotrophic alga. We aimed to evaluate whether this symbiotic
cooperation bestows higher resistance to environmental stressors of
different nature, in terms of higher preservation of DNA integrity,
compared to free-living brown hydra (Hydra oligactis Pallas,
1766). Hydras were exposed to herbicide norflurazon at of
2×10-7 or 2×10-6 mol/L and UV-B
light of 254 nm, 0.023 mW/cm separately or simultaneously to detect
additive or synergistic effect. We used alkaline comet assay to
determine the level of primary DNA damage and fluorescent staining to
detect apoptosis and necrosis. Norflurazon at 2×10-6 mol/L significantly
increased both comet assay descriptors in brown hydra compared to
negative control (6.17±0.6µm and 5.2±1.7% vs. 2.9 ± 0.2µm and 1.2 ± 0.2
%). The frequency of apoptotic and necrotic cells was significantly
elevated either, being higher in brown hydra (25.7±3.5% and 8.2±0.2%)
than green hydra (20.3±2.5% and 2.8±0.2%). UV-B irradiation induced
significant DNA damage in brown hydra only (13.5±1.0µm, 4.1±1.0%).
Simultaneous exposure to UV-B light and norflurazon led to synergistic
increase in intensity of adverse effects. A concentration of
2×10-7 mol norflurazon/L combined with UV-B light
induced significantly increased DNA lesion-level in brown hydra (15.8 ±
3.1%) and at concentration of 2×10-6 mol/L in both
species (brown: 15.0±2.6%, green: 22.0±3.0%). Significant cytotoxicity
and increased hedgehog nucleoids frequency were recorded, significantly
more pronounced in brown (78.3±9.4% and 56.4±6.0%) compared to green
hydra (34.7±2.5% and 24.2±0.6%). Besides genotoxicity, comet assay
descriptors were significantly affected by and reflected cytotoxicity of
norflurazon and/or UV-B light. We proved that evolutionary established
symbiotic cooperation may contribute to higher resistance to
cyto/genotoxic stressors.