Emerging Model Organisms

Paramecium tetraurelia: The Renaissance of an Early Unicellular Model

¹ Centre de Génétique Moléculaire, Centre National de la Recherche Scientifique, FRE3144, F-91198 Gif-sur-Yvette, France
² Laboratoire de Biologie Cellulaire 4, Centre National de la Recherche Scientifique, UMR 8080, Université Paris-Sud, 91405 Orsay Cedex, France
³ Laboratoire de Génétique Moléculaire, Centre National de la Recherche Scientifique, UMR 8541, École Normale Supérieure, F-75230 Paris, France
⁴ Laboratoire de Biométrie et Biologie Évolutive, Centre National de la Recherche Scientifique, UMR 5558, Université Lyon 1, F-69622, Villeurbanne, France
⁵ Laboratory of Molecular Biology and Department of Genetics, University of Wisconsin-Madison, WI 53706, USA
⁶ Department of Biology, Indiana University, Bloomington, IN 47405-3700, USA

⁷Corresponding author (emeyer{at}biologie.ens.fr).

INTRODUCTION

Paramecium tetraurelia is a widely distributed, free-living unicellular organism that feeds on bacteria and can easily be cultured in the laboratory. Its position within the phylum Ciliophora, remote from the most commonly used models, offers an interesting perspective on the basic cellular and molecular processes of eukaryotic life. Its large size and complex cellular organization facilitate morphogenetic studies of conserved structures, such as cilia and basal bodies, as well as electrophysiological studies of swimming behavior. Like all ciliates, P. tetraurelia contains two distinct types of nuclei, the germline micronucleus (MIC) and the somatic macronucleus (MAC), which differentiate from copies of the zygotic nucleus after fertilization. The sexual cycle can be managed by controlling food uptake, allowing the study of a developmentally regulated differentiation program in synchronous cultures. Spectacular genome rearrangements occur during the development of the somatic macronucleus. Their epigenetic control by RNA-mediated homology-dependent mechanisms, which might underlie long-known cases of non-Mendelian inheritance, provides evolutionary insight into the diversity of small RNA pathways involved in genome regulation. Being endowed with two alternative modes of sexual reproduction (conjugation and autogamy), P. tetraurelia is ideally suited for genetic analyses, and the recent sequencing of its macronuclear genome revealed one of the largest numbers of genes in any eukaryote. Together with the development of new molecular techniques, including complementation cloning and an easily implemented technique for reverse genetics based on RNA interference (RNAi), these features make P. tetraurelia a very attractive unicellular model.