Journal of Endocytobiosis and Cell Research (2010) 13-15 | International Society of Endocytobiology zs.thulb.uni-jena.de/content/main/journals/ecb/info.xml t and Cell Journal of Endocy obiosis Research VOL 20 | 2010 13 Journal of Endocytobiosis and Cell Research Review: Endosymbiotic alga as the stronger evolutionary partner in green hydra symbiosis Goran Kovačević*, Mirjana Kalafatić, Biser- ka Jelenčić and Damjan Franjević Division of Biology, Faculty of Science, University of Zagreb, Rooseveltov trg 6, HR-10000 Zagreb, Croatia; *correspondence to: goran@zg.biol. pmf.hr; Phone: +385 1 4877700; Fax: +385 1 4826260 Green hydra (Hydra viridissima Pallas, 1766) is a clas sical example of endosymbiosis. Described ultrastruc tural features of green hydra symbiosis are extensive widening of perialgal spaces, degradation, loss and fusion of symbiosomes. These changes are considered as defensive and protective mechanisms and the endo symbionts show greater viability than their host. Also, a certain degree of independency was noticed and for the first time endosymbiotic algae from green hydra have been successfully isolated and permanently main tained in pure lab culture. Microscopical and molecular analyses of the isolated endosymbionts resulted in the discovery of two hitherto undescribed endosymbiotic species in green hydra. In final conclusion, endosymbi otic algae from green hydra perform as the “stronger” symbiotic partner in green hydra symbiosis. We keep on searching for more evidence and results. Journal of Endocytobiosis and Cell Research ȋʹͲͳͲȌ ͳ͵‐ͳͷ Category: review eywords: green hydra, endosymbiotic algae, aposymbiotic K algae, adaptation eceived: ʹ͸ August ʹͲͳͲ; Accepted: ͳͳ October ʹͲͳͲ ___________________________________________________________________ R _ Introduction Symbiotic associations Symbiotic associations are of wide significance in evolutio‐ nary biology. Symbiosis is seen as one of the driving forces in evolution and leads to competitive advantages in particu‐ lar habitats. )t represents a long‐term relationship between at least two species. The relationships vary from mutualism to parasitism ȋOsborne ʹͲͲ͹Ȍ. )n mutualism both partners benefit from the relationship and in parasitism only one partner benefits at the cost of the other. )t is presumed that symbiotic relationships arose from parasitic attempts. First ideas about symbiosis are known from the time of (erodot, who stated that mutualism is the balance of nature. The term "symbiotismus" was defined by Albert Bernhardt Frank in ͳͺ͹͹. The first definition of symbiosis was given by (einrich Anton de Bary in ͳͺ͹ͻ according to which symbiosis is the phenomenon where unequal organisms live together ȋSapp ͳͻͻͶȌ. Endosymbiosis requires the close physical contact of symbionts, where at least two genomes of different evolutionary origin exist inside the same cytoplasm ȋEbringer and Krajčovič ͳͻͺ͸, ͳͻͻͶȌ. Green hydra symbiosis Green hydra ȋHydra viridissima Pallas, ͳ͹͸͸Ȍ is a simple aquatic invertebrate and represents a model for research on the endosymbiotic relationship between two organisms. )t is a member of the phylum Cnidaria, class (ydrozoa, order (ydroida, family (ydridae ȋ(olstein and Emscher‐ mann ͳͻͻͷȌ. This cnidarian harbors the individuals of photoautotrophic microalgae in its gastrodermal myoepi‐ thelial cells and presents a mutualistic symbiosis. This is the only hydra species that is a host to Dzzoochlorellaedz. This term was first used by Brandt ȋͳͺͺʹȌ, besides others, also for endosymbiotic algae from green hydra. Later, Beijerinck ȋͳͺͻͲȌ used the term as a synonym for the genus Chlorella. (e described them as the Dzgreen ballsdz and until today this term denotes all endosymbiotic algae in freshwater inver‐ tebrates. Up to ʹͲ individuals of the unicellular green algae occupy about ͳͲ% of the cellular volume ȋ(olstein and Emschermann ͳͻͻͷȌ. The adaptation to a new intracellular ȋendosymbioticȌ niche is not easy ȋMoulder ͳͻͺͷȌ. Each alga is placed into one vacuolar membrane symbiosome. Algae are regularly placed to form columns, one alga above the other, in the basal part of the cell. A once successfully established endosymbiosis disqualifies an endosymbiotic event with other species of algae ȋRahat ͳͻͻͳȌ. A constant number of algae in the cell is controlled by inhibition of algal mitosis, expulsion, or digestion of algae ȋBaghdasarian and Muscatine ʹͲͲͲȌ. The maintenance of symbiosis is bi‐ directional and strengthens the fitness of the organism. )nside hydra, algae find their protection and habitat and also use the products of the host metabolism ȋMcAuley et al. ͳͻͻ͸Ȍ. (ydra, the heterotrophic host, uses the photosyn‐ thate released by the algae. Characteristic of endosymbiotic alga is export of maltose which prevents the fusion of sym‐ biosomes with lysosomes ȋ(ohman et al. ͳͻͺʹȌ. )t is impor‐ tant to mention that microalgae have a significant role as primary producers in aquatic ecosystems. Combination of algal partners and selection of host’s populations can con‐ tribute to better adapted symbionts ȋKl“etter et al. ʹͲͲ͸Ȍ. )n symbiotic associations, size and placement of the symbi‐ otic partners, necessity, duration, stability, mode of main‐ tenance, specificity, recognition, interaction ȋSmith and Douglas ͳͻͺ͹Ȍ, and integration ȋMargulis and Sagan ʹͲͲʹȌ are important. Besides endosymbiotic, there exist non‐symbiotic spe‐ cies of hydras that do not establish symbiosis, and aposym‐ biotic species that once formed a symbiotic relationship.