engleski

From Mutualism to Parsititsm

Symbiotic associations are of wide significance in evolution, providing biological advantages and biodiversity. Mutualism is a form of symbiosis where both partners benefit from the relationship, while in parasitism only one partner benefits on the count of the other. It is presumed that symbiotic relationships arose from parasitic attempts. In this work an example of a rare process of evolutionary reversal was described on green hydra symbiosis. Green hydra (Hydra virdissima Pallas, 1776) is a typical example of symbiosis. In its gastrodermal myoepithelial cells it contains the individuals of unicellular green alga Chlorella, each alga surrounded by perialgal space and placed in a separate vacuole called symbiosome. Triggers for observing the described changes were herbicide norflurazon (nf) and antibiotics chloramphenicol (cfc), cinoxacin (cin) and ciprofloxacin (cip). Individuals of green hydra (sort S1J-J1) were treated with aqueous solutions of nf (2x10-4, 2x10-5, 2x10-6, 2x10-7 and 2x10-8 mol/L ; SAN 9789, Sandoz, Switzerland), cfc (100 and 250 mg/L ; Pliva, Croatia), cin (62.5 and 117.6 mg/L ; Lilly Deutschland, Germany) and cip (1 and 5 mg/L ; Bayer AG, Germany) in the laboratory conditions (21o C, photoperiod 10 hs daylight and 14 hs dark) in subacute exposure for 3-21 days. They were compared to the control groups of organisms. For TEM, standard preparation methods were used. Micrographs were made by using Zeiss EM10A and FEI Morgagni 269D microscopes. Toxicity resulted in numerous ultrastructural changes, showing a strong deleterious effect upon both hydra and alga FIG. 1. Hydra suffered from overall greater extent of damages than alga did. Well-defined green hydra endosymbiotic relationship was disturbed. Alga showed interesting and independent protective mechanisms by widening of perialgal space to the final extent, which resulted in symbiosome reduction or loss FIG. 1. In this way alga regulated itself a more viable environment than the host. Formations of two to three algae contained inside a single symbiosome in which algae “ joined forces” for their protection were noticed as well. By these mechanisms algae showed a certain degree of independence from their host. When it came up to survival, alga seemed to be the stronger component in this symbiotic relationship. It seemed that during millions of years of coevolution endosymbiotic alga gained the specific preadaptations by which it defended itself from environmentally harmful effects better than the host green hydra itself. The following question arose: could we discuss the possibility of alga to transform the green hydra endosymbiotic relationship from mutualism to parasitism, for at least a very short interval where it survived longer and better than its host. The lateral gene transfer from alga to hydra is well supposed. But, could it be that a part of genetic material of hydra became dependent on alga? It came to re-questioning of the host-symbiont relationship, which opens many research possibilities. The results supported the ever-stronger idea that Chlorella from green hydra represents a separate species of algae.

mutualism; parasitism; symbiosis; green hydra; Chlorella; perialgal space; symbiosome; preadaptations

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