Dear Prom', It has been one month since the beginning of my internship in the Station Biologique of Roscoff in the team DIPO, mainly working on interactions within phytoplankton. Our interest stands on Amoebophrya, a dinoflagellate parasite that infects other species from its phylum[1]. As this may be completely new to most of you, I'll start this first insight on the subject by definitions ! Symbiotic association As none of living species lives totally isolated from others, biological interactions are considered as the main force for evolutionary processes. Among these biological interactions, symbiotic associations are characterized by the sustainability of a specific relationship between two different species, kept from one generation to another. These two partners are called the host (the one that is hosting the second species) and the symbiont (the one that is living inside or on the host). The host can be used by the symbiont (parasitism) or, on the contrary, the symbiont can be used by the host (endosymbiosis). These effects are often reversible (an endosymbiont can suddenly act as a parasite and reciprocally), and all possible effects on the host (from lethal to beneficial) are observed in Nature. Parasitism Until the middle of XXème century, parasitism was only considered if there was a unilateral need for the parasite to infect a host in order to survive. Generally, the parasite is smaller than its host. In this larger definition, viruses are thus considered as parasites. The term “parasitoids” is used for parasites that kill their host to accomplish their life cycle. For unicellular, it is sometime difficult to distinguish parasitoids from predators (or grazers). Both kill their host/prey, both may have similar functions in Nature, by controlling their host/prey populations. For instance, Ameobophrya is a parasite that controls some microalgae species (Scrippsiella) that are likely to create red-tides (Figure2). However, there is a big difference between a predator/grazer and a parasite. A predator/grazer generally consumes several preys during its life and its division release two daughter cells. In contrast, a parasite feeds generally on a single host and produces more than two daughter cells during sporogenesis. This sporogenesis will ends by the production of > 2 spores, which are released outside the host and used to infect a novel host. Phytoplankton Our host and parasite are part of the primary food source known on Earth as phytoplankton. The phytoplankton is composed by unicellular autotrophic organisms mostly composed of unicellular algae (microalgae) that freely float in the water and are transported by currents. There are several types of microalgae separated in five groups according to their shape and color : diatom, cyanobacteria, chlorophyceae, chrysophyceae, dinophyceae. Some algae can proliferate under favorable conditions (when temperature and /or nutrients such as phosphate and nitrate increase for example), causing blooms known as red-tides (Figure2). These phenomena are not always “red”, as the color of water is depending on the pigments of the microalga. Phytoplankton’s role as primary producer has an environmental impact on the food chain. As it uses solar energy directly as well as CO2 and mineral salts to produce organic matter and release oxygen, it is consumed by zooplankton. Zooplankton is mainly composed of tiny heterotrophic animals from marine surface. It is then consumed by fish or crustaceans (shellfish). Dinoflagellate A dinoflagellate is a unicellular organism that can be autotroph thanks to endosymbiose with a microalgae. A dinoflagellate is characterized (Figure3) by the presence of two perpendicular flagella, the absence of histones in the nucleus, sub-cortical alveoli that most of the time contain cellulosic slabs regularly arranged. The host belongs to the class Dinophyceae (photosynthetic dinoflagellates). Dinophyceae have a dinokaryon : their chromosome remained condensed during interphase stage of mitosis (interval between two successive mitoses). The parasite belongs to class Syndiniophyceae (order Syndiniales, family Amoebophryidae). Syndiniales are, up to now, only composed of marine parasites. The taxonomy of this group is still uncertain, as most of Syndiniales are only known by their SSU (small sub-unit) rDNA gene sequences issued from marine environment. Amoebophrya is a dinoflagellate from MALV Group II (one of the V orders of Syndiniales[4], Marine Alveolates) which is only composed of parasitic dinoflagellates. How does the parasite Amoebophrya survive in the natural environment ? Why is the life cycle of dinoflagellates so important to understand host-parasite relationship and advantages ? All those questions remains and are waiting to be solved !
To be continued… [1] Ultrastructure Des Amoebophrydae. 1st ed. Jean et Monique Cachon, 1969. Print. [2] Images.google.fr. N.p., 2016. Web. 28 Aug. 2016. [3] Miller, John J., Charles F. Delwiche, and D. Wayne Coats. "Ultrastructure Of Amoebophrya Sp. And Its Changes During The Course Of Infection". Protist 163.5 (2012): 720-745. Web. [4] Guillou, L. et al. "Widespread Occurrence And Genetic Diversity Of Marine Parasitoids Belonging To Syndiniales ( Alveolata )". Environmental Microbiology 10.12 (2008): 3349-3365. Web.
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