Abstract
Paramecium bursaria is a single-celled model organism for studying endosymbiosis among ciliates and green algae. Most strains of P. bursaria bear either Chlorella variabilis or Micractinium conductrix as endosymbionts. Both algal genera are unicellular green algae characterized by cup-shaped chloroplasts containing a single pyrenoid and reproduction by autospores. Due to their size and only few morphological characteristics, these green algae are very difficult to discriminate by microscopy only. Their cultivation is laborious and often unsuccessful, but we developed a three-step isolation method, which provided axenic cultures of endosymbionts. In addition to the time-consuming isolation, we developed a simple diagnostic PCR identification method using specific primers for C. variabilis and M. conductrix that provided reliable results. One advantage of this approach was that the algae do not have to be isolated from their host. For a comparative study, we investigated 19 strains of P. bursaria from all over the world (new isolates and available laboratory strains) belonging to the five known syngens (R1–R5). Six European ciliate strains belonging to syngens R1 and R2 bore M. conductrix as endosymbiont whereas C. variabilis was discovered in syngens R1–R5 having worldwide origins. Our results reveal the first evidence of C. variabilis as endosymbiont in P. bursaria in Europe.
Highlights
Symbiosis of green algae with protists and invertebrates has been studied for more than 100 years ([1] and references therein)
TheThe isolation of of thethe green algal endosymbionts using thethe method was successful of the Paramecium strains27.96, (SAGCIL-16, 27.96, CIL-19, CIL-16, and CIL-19, and CIL-20). it successful for somefor of some the Paramecium bursariabursaria strains
Loefer [14] was the first to obtain the endosymbiont in culture. He isolated this alga by taking green algae from the sediment of an axenic P. bursaria culture and spreading it on agar plates containing tap water with unknown organic compounds
Summary
Symbiosis of green algae with protists and invertebrates has been studied for more than 100 years ([1] and references therein). The algae are protected against infection by chloroviruses, which lyse the endosymbionts outside of their hosts [2,3,4]. Such mixotrophic ciliates are widely distributed in many freshwater habitats and belong to different phylogenetic lineages [5,6,7]. Paramecium bursaria is the most studied model ciliate in endosymbiosis research because of its easiness of cultivating and cloning and it can be identified rather from its morphology [10,11]. Identification of the endosymbionts of P. bursaria based solely on morphology is almost impossible
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