Genetic consequences of the intranuclear symbiosis
Paramecium-Holospora analyzed by PFGE
Potekhin, A.1, Brigge T.2, Rautian M.1
1Laboratory of Protozoan Karyology, Biological Research Instutute, Saint-Petersburg State University, Oranienbaumskoye sh. 2, 198904 Saint-Petersburg, Russia
2Biological Institute, University of Stuttgart, Pfaffenwaldring 57, 70550 Stuttgart, Germany
The symbiotic system formed by ciliates of genus Paramecium and their intranuclear bacterial endobionts of genus Holospora has been investigated. Such systems are of special interest for endobionts may produce some changes in their hosts genomes. The goal of this work was to investigate if any changes in Paramecium macronuclear (Ma) genome occur due to Holospora influence. The main method of analysis was pulsed-field gel electrophoresis (PFGE). This method allows to work with linear DNA molecules up to 10000 kb in length, which makes it very convenient in ciliates’ Ma genomes studies. The work was carried out in two stages. At the first stage it was shown by means of visual and densitometrical analysis that electrokaryotype of P. caudatum macronucleus consists of continuous spectrum of the DNA molecules ranging in size between 600 kb and 50 kb. It is in a good agreement with the data on molecular composition of Paramecium’ Ma genome [1]. Electrophoretic pattern doesn’t vary significantly neither between different preparations of one clone DNA samples, nor between different P.caudatum clones. Southern blot hybridization also improved that there is almost no detected difference in genome organization between the clones. These results allowed us to proceed with a second stage. Symbiotic «triplet» systems were created, each consisting of three derivatives of the same Paramecium clone - symbiont-free (initial one), symbiont-containing and symbiont-deprived ones. PFGE and Southern hybridization (specific sequences from Paramecium genome were used as the probes) were performed. No differences of a hybridization signal positioning were found within these «triplet» systems, which means that the endobionts do not cause total damage of their hosts’ DNA, though one can’t exclude a chance that some changes still take place. Possible explanations and other approaches to solve this problem are discussed.
[1] Caron F. (1992) A high degree of macronuclear chromosome polymorphism is generated by variable DNA rearrangements in Paramecium primaurelia during macronuclear differentiation. Journal of Molecular Biology 225: 661-678
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