Table of Contents

Volume 509 Number 7501 pp399-526

22 May 2014

About the cover

A conjugating pair of Paramecium cells visualized by confocal microscopy: the mating-type O cell was transformed to express fluorescent-labelled ciliary protein (shown in yellow), and both O and E cells are decorated with the TAP952 anti-monoglycylated tubulin antibody (shown in red). The protozoan Paramecium, widely studied as a typical ciliate, multiplies asexually by binary fission much of the time, but under certain conditions will reproduce sexually. Two mating types, E and O, were discovered in 1937 but only now has the molecular basis for maternal inheritance of mating type been elucidated. Eric Meyer and colleagues show in Paramecium tetraurelia that mating type E depends on expression of the transmembrane protein mtA, and the default type O is determined during development by excision of the mtA promoter by scnRNAs, a class of small ‘scan’ RNA that reprograms the Paramecium genome during sexual reproduction by recognizing and excising transposable elements. A similar switch mechanism, involving a different gene, mtB, has evolved independently in a sibling species P. septaurelia, implying that exaptation of the scnRNA pathway may be a general mechanism for transgenerational epigenetic inheritance of differentiated states in Paramecium. Cover: Anne Aubusson-Fleury /Centre de Génétique Moléculaire Gif-sur-Yvette.

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    The molecular basis for mating-type determination in the ciliate Paramecium has been elucidated, revealing a novel function for a class of small RNAs — these scnRNAs are typically involved in reprogramming the Paramecium genome during sexual reproduction by recognizing and excising transposable elements, but they are now found to be co-opted to switch off expression of the newly identified mating-type gene mtA by excising its promoter, and to mediate epigenetic inheritance of mating types across sexual generations.

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