Genome Reorganization in Tetrahymena

Abstract

This chapter presents a discussion of the genome reorganization in tetrahymena. In many respects tetrahymena is an ideal organism for molecular biological investigations. The concept of reorganization and elimination of DNA sequences as a developmental mechanism for producing stable changes in cell lineages was a prominent idea during the early part of this century. Genome reorganization as a developmen tal process is now well established, but it is not a major feature of most developmental programs. Among the ciliates, however, genome reorganization is a way of life. These organisms have a dual nuclear system. Each cell possesses one or more micronuclei and one or more macronuclei. The macronuclei are responsible for gene expression during the vegetative growth of the cell and the micronuclei are functional during the sexual phase of its life cycle. As pointed out by Gorovsky, tetrahymena affords a unique opportunity to investigate the functions and interactions of germline and somatic nuclei. In the sphere of the dominant role of genome reorganization in the life cycle of the ciliates, tetrahymena is a logical choice for an in-depth study of this process. Tetrahymena, like the hypotrichs, undergoes substantial genome reorganization during conjugation. The micronuclei of tetrahymena contain five chromosomes, which can be observed during meiosis and cytologically, the chromosomes all appear to be in the same size range. The best characterized case of genome reorganization in tetrahymena is the removal of the rRNA gene from an integrated site in the micronuclear chromosome and its rearrangement into a 21-kb palindrome in the macronucleus.