Paramecium

Abstract

Members of the genus Paramecium (from the classical Greek, παραμηκης, meaning oblong or oval shaped) are ciliated protozoa with an elongated shape (length approximately three to four times the width), a uniform distribution of cilia over the cell surface and a ciliated oral groove leading from the anterior of the cell to a midventral deep oral cavity. The oral apparatus is shaped like a funnel, with 12 rows of oral cilia in a helical array inside. Each cell has two distinct types of nuclei: one large, transcriptionally active, polycopy macronucleus and one or more small, transcriptionally inactive micronuclei. Many features make paramecia favourable organisms for the study of diverse cellular/developmental/genetic aspects of cells. Their large size allows microscopic observations as well as microinjections and cell surgery. They show Mendelian and non‐Mendelian inheritance and several different epigenetic inheritance patterns. The genomes of Paramecium tetraurelia and others have been sequenced and are available for analysis. Key Concepts Species of Paramecium are distributed worldwide in freshwater habitats and are easy to cultivate in the laboratory. Morphological differences are not enough to distinguish some species of Paramecium. The genomes of the macronucleus and micronucleus are not identical, even though both are derived from the same zygote nucleus. The genomes of Paramecium tetraurelia have been sequenced; 30 000 genes have been identified. Paramecia do not use two of the three ‘stop codons’ for translation termination. The trichocysts of paramecia are examples of a regulated exocytotic process. The cortex of the cell contains the cilia, basal bodies and associated accessory structures. Paramecia can carry a number of different types of endosymbiotic organisms.