Genome downsizing during ciliate development: Nuclear division of labor through chromosome restructuring

The ciliated protozoa divide the labor of germline and somatic genetic functions between two distinct nuclei. The development of the somatic (macro- ) nucleus from the germinal (micro-) nucleus occurs during sexual reproduction and involves large-scale, genetic reorganization including site- specific chromosome breakage and DNA deletion. This intriguing process has been extensively studied in Tetrahymena thermophila. Characterization of cis- acting sequences, putative protein factors, and possible reaction intermediates has begun to shed light on the underlying mechanisms of genome rearrangement. This article summarizes the current understanding of this phenomenon and discusses its origin and biological function. We postulate that ciliate nuclear restructuring serves to segregate the two essential functions of chromosomes: the transmission and expression of genetic information.