engleski

Interactions in green hydra symbiosis

Symbiotic associations are of a wide significance in evolution and biodiversity. Green hydra (Hydra viridissima Pallas, 1766) is a cosmopolitan freshwater cnidarian, a valuable model system for studying symbiotic interactions and it represents a typical example of endosymbiosis. In its gastrodermal myoepithelial cells it harbors individuals of Chlorella-like photoauotrophic algae. Up to 20 algae in one cell occupy 10% of the cellular volume. This symbiotic relationship is stable and well determined and it represents a biological advantage over the free-living brown hydra (Hydra oligactis Pallas, 1766). The bidirectional flow of metabolites between the host and the symbionts, and also the genetic flow through horizontal gene transfer might be the reason for the selective advantages. Endosymbiotic algae can be permanently isolated from its green hydra host showing that different genera and species of unicellular green algae could be present as symbionts in green hydra, but only one at a time, depending on the natural habitat of a particular strain of green hydra. All endosymbiotic algae isolated from green hydra with the characteristic of permanent stable lab growth belong to the Chlorella zagrebiensis group. The isolated zoochlorellae are also capable of modifying their morphology from coccoidal to cenobial, with transitional forms present. Newly described mechanisms of the hydra-alga symbiosis are widening of perialgal space, degradation and loss of symbiosome, fusion of symbiosomes and perialgal spaces, and their role in regular re-assembly of the symbiosis. Using the Chlorella bioassay huge differences are noticed in the growth rate of the endosymbiotic algae isolated from green hydra when compared to the growth rate of the free-living relatives (3.3% : 36.7%). These results indicate that isolated algae exhibit lower viability and are less adapted to unfavourable (micro-)environmental conditions than their free-living relatives. During millions of years of coevolution, preadaptations and interactions, the first phase of parasitic interactions secondarily became obligatory mutual, today being facultatively mutual. Endosymbiotic algae perform as the “stronger” symbiotic partner in green hydra symbiosis, pointing out also the evolutionary reversal. The results refer to the fact that symbiogenesis in green hydra has probably not been terminated yet and that hydra symbiosis biodiversity is much more complex than previously thought. Furthermore, if we consider the hydra as a holobiont that comprises three types of organisms (hydra host-algae-bacteria), we find ourselves in an exciting period of time for studying interactions in green hydra symbiosis.

green hydra, interactions

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