Evolution of DNA organization in hypotrichous ciliates.

Profound changes have been introduced into the germline (micronuclear) and somatic (macronuclear) genomes of hypotrichous ciliates during evolution. First, multiple, short, unique, noncoding sequences, called IESs, have been inserted into micronuclear genes. IESs are spliced out of each gene, and the gene segments, called MDSs, are ligated during conversion of the micronuclear genome to a macronuclear genome after cell mating. The IESs in a particular gene can change dramatically in number, position, length, and sequence during speciation. Once inserted, IESs can shift along the DNA of a gene 1 or 2 bp at a time by a mutational mechanism that does not alter the coding sequence of the gene. Second, the MDSs in the same genes have been rearranged into random or nonrandom, scrambled disorder. The origin of nonrandom scrambling patterns can be explained by a model of simultaneous insertion of multiple IESs into a germline gene during evolution. Subsequent recombinations among the IESs may change the MDS arrangement from a nonrandomly scrambled to a randomly scrambled pattern, including inversions of MDSs. Third, during the conversion of a micronucleus to a macronucleus after cell mating, MDSs are ligated in the unscrambled, orthodox order in association with IES excision, and the genes are then removed from the chromosomes as individual, short DNA molecules. These gene-size molecules are amplified many-fold to produce a mature macronucleus. All of these phenomena attest to a remarkable fluidity of the hypotrich genome both over evolutionary time and in the conversion of a germline genome to a somatic genome. The significance of this fluidity for the life and evolution of these organisms is still obscure. Recombination among IESs could shuffle MDSs and facilitate faster evolution of new genes.