Origin, structure and function of millions of chromosomes present in the macronucleus of unicellular eukaryotic ciliate, Oxytricha trifallax: a model organism for transgenerationally programmed genome rearrangements

Abstract

The unicellular eukaryotic ciliate protists characteristically contain a germline micronucleus (MIC) and a somatic macronucleus (MAC) in their cytoplasm. The MAC, which is crucial for the pursuit of cellular growth and mitotic divisions, is derived from a postzygotic MIC. The transition from MIC to MAC involves extensive editing of the MIC genome, followed by massive amplification of the residual MAC genome. The spirotrichous ciliate, Oxytricha trifallax, has been most extensively deployed as the model system to reveal the mechanism(s) of origin of the architectural complexities of MAC. This has been possible by the application of genome sequencing, synthetic RNA transgenesis and a variety of other genetic techniques. Here, we summarize and discuss the current observations of MAC genome derived from MIC genome, properties of MAC genome structure, function and significance of the novel features described in O. trifallax. The differences in the genome organization of O. trifallax and its relative alveolate species Paramecium tetraurelia and Plasmodium falciparum have been described. Aspects of programmed genome rearrangements, MAC genome structure and function requiring further analyses in different ciliate protest have been pointed out.