Abstract
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.
Highlights
The ciliate Paramecium tetraurelia is one of a few traditional objects of genetic studies of unicellular eukaryotes [1]
We studied the Spinning Top mutant of P. tetraurelia strain 51, which appeared
DsRNA feeding with copine or NimA sequences was much less efficient at inducing screwy phenotypes; presumably the distances between these genes and the Spade gene result in a smaller fraction of induced deletions affecting the latter
Summary
The ciliate Paramecium tetraurelia is one of a few traditional objects of genetic studies of unicellular eukaryotes [1]. Sex occurs through one of two different processes: conjugation, where two partners exchange gametic pronuclei to produce a pair of genetically-identical heterozygous progeny, and autogamy, a self-fertilization process resulting in entirely homozygous progeny. These features are highly advantageous for genetic analysis. The germline, diploid micronucleus (MIC) acts as a silent transmitter of chromosomes across sexual generations, while the somatic, highly polyploid macronucleus (MAC) is responsible for all gene expression. The extensive programmed genome rearrangements that occur during the development of a new MAC from the zygotic nucleus in each sexual generation have led ciliates to become recognized models for epigenetics [2,3]
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