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Open AccessArticle

Loss of a Fragile Chromosome Region leads to the Screwy Phenotype in Paramecium tetraurelia

1
Department of Microbiology, Faculty of Biology, Saint Petersburg State University, Universitetskaya nab. 7/9, 199034 Saint Petersburg, Russia
2
IBENS, Département de Biologie, Ecole Normale Supérieure, CNRS, Inserm, PSL Research University, F-75005 Paris, France
3
Institute for Integrative Biology of the Cell (I2BC), CNRS, CEA, Univ. Paris-Sud, Université Paris-Saclay, 91198 Gif-sur-Yvette CEDEX, France
4
Friedrich Miescher Institute for Biomedical Research, Maulbeerstrasse 66, 4058 Basel, Switzerland
*
Author to whom correspondence should be addressed.
Genes 2019, 10(7), 513; https://doi.org/10.3390/genes10070513
Received: 31 May 2019 / Revised: 28 June 2019 / Accepted: 2 July 2019 / Published: 6 July 2019
(This article belongs to the Special Issue Ciliate Genetics and Epigenetics)
A conspicuous cell-shape phenotype known as “screwy” was reported to result from mutations at two or three uncharacterized loci in the ciliate Paramecium tetraurelia. Here, we describe a new screwy mutation, Spinning Top, which appeared spontaneously in the cross of an unrelated mutant with reference strain 51. The macronuclear (MAC) genome of the Spinning Top mutant is shown to lack a ~28.5-kb segment containing 18 genes at the end of one chromosome, which appears to result from a collinear deletion in the micronuclear (MIC) genome. We tested several candidate genes from the deleted locus by dsRNA-induced silencing in wild-type cells, and identified a single gene responsible for the phenotype. This gene, named Spade, encodes a 566-aa glutamine-rich protein with a C2HC zinc finger. Its silencing leads to a fast phenotype switch during vegetative growth, but cells recover a wild-type phenotype only 5–6 divisions after silencing is stopped. We analyzed 5 independently-obtained mutant alleles of the Sc1 locus, and concluded that all of them also lack the Spade gene and a number of neighboring genes in the MAC and MIC genomes. Mapping of the MAC deletion breakpoints revealed two different positions among the 5 alleles, both of which differ from the Spinning Top breakpoint. These results suggest that this MIC chromosome region is intrinsically unstable in strain 51. View Full-Text
Keywords: Paramecium; micronuclear deletion; chromosome fragile sites; cortical inheritance; trichocysts; epimutation; maternal inheritance Paramecium; micronuclear deletion; chromosome fragile sites; cortical inheritance; trichocysts; epimutation; maternal inheritance
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Nekrasova, I.; Nikitashina, V.; Bhullar, S.; Arnaiz, O.; Singh, D.P.; Meyer, E.; Potekhin, A. Loss of a Fragile Chromosome Region leads to the Screwy Phenotype in Paramecium tetraurelia. Genes 2019, 10, 513.

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