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Ultrastructural mechanisms of green hydra symbiosis: morphometrical characterization

Hydras are simple freshwater invertebrates, members of the phylum Cnidaria, class Hydrozoa, order Hydroida, family Hydridae. Green hydra (Hydra viridissima Pallas, 1766) is a classical example of symbiosis. It forms an endosymbiotic relationship with individuals of unicellular green algae that reside in the gastrodermal myoepithelial cells of hydra. Each alga is surrounded by perialgal space and placed in a separate vacuole symbiosome. Ultrastructural modifications of green hydra symbiosis in an unfavorable environment perform as protective mechanisms in green hydra symbiosis and point towards a certain degree of endosymbiont independency (Figure 1). In this work quantitative characterization was performed (Figure 2). Individuals of green hydra (strain S1J-J1) were used for the experiment in the laboratory conditions. Ultrastructural changes were observed by means of TEM. For TEM standard preparation methods were used. Hydras were fixed in 2 % glutaraldehyde. After post-fixation the material was dehydrated through an ethanol series, embedded into resin and contrasted with uranyl-acetate and Pb-citrate. Micrographs were obtained using the Zeiss EM10A and FEI Morgagni 269D electron microscopes. After the treatment with norflurazon and cinoxacin and period of recovery after the treatment with norflurazon, morphometrically were worked out the following parameters: width of perialgal spaces, area and perimeter of symbiosomes. In all the parameters numerical values were changed according to the control. The numerical values of the three measured parameters showed oscillations in the size of perialgal spaces and symbiosomes (Figure 2), as the result of disturbance of symbiosis. Damages were often so huge that the measurements could not be performed, because the values would be endlessly high. It could be supposed that in the longer period of recovery the result would be closer to the control values. After the recovery period a shift towards the measured values was traced, in relation to the impossibility of measurement of endless values in the treatments themselves. A little is known about the molecular basis of survival of endosymbiotic algae inside the vacuoles and the control of interactions of both partners. This opens possibilities for further investigations on mechanisms of green hydra symbiosis.

symbiosis; green hydra; morphometry

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