Abstract
Ciliates are a highly divergent group of unicellular eukaryotes with separate somatic and germline genomes found in distinct dimorphic nuclei. This characteristic feature is tightly linked to extremely laborious developmentally regulated genome rearrangements in the development of a new somatic genome/nuclei following sex. The transformation from germline to soma genome involves massive DNA elimination mediated by non-coding RNAs, chromosome fragmentation, as well as DNA amplification. In this review, we discuss the similarities and differences in the genome reorganization processes of the model ciliates Paramecium and Tetrahymena (class Oligohymenophorea), and the distantly related Euplotes, Stylonychia, and Oxytricha (class Spirotrichea).
Highlights
Regulated genome rearrangements (DRGRs) involve the elimination of specific DNA sequences somatic cell lineages. This phenomenon is associated with two forms of DNA elimination either: (a) chromosome elimination where the entire chromosome is lost [1] or (b) chromosome diminution, a process characterized by loss of chromosome portions through chromosome breakage and repair during the developmental transformation from germline to soma [2, 3]
The injection of 27 nt piRNAs corresponding to internal eliminated sequences (IESs) into developing Oxytricha leads to the retention of those IES in the new somatic genome [83]. These data, combined with the apparent parental MAC origin of the piRNAs, suggest that they are responsible for identifying macronuclear destined sequences (MDS) to protect against excision, rather than targeting IESs for excision, as in Tetrahymena and Paramecium
Ciliates are a diverse group of organisms that have deeply contributed to our recent knowledge about the regulatory role of epigenetics in development
Summary
Regulated genome rearrangements (DRGRs) involve the elimination of specific DNA sequences (from the germline) somatic cell lineages. IESs are present in all the ciliate germline genomes studied to date [30, 31, 48, 51,52,53], in varying amounts
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