Developmental DNA rearrangements and micronucleus-specific sequences in five species within the Tetrahymena pyriformis species complex.

In Tetrahymena thermophila, the development of a transcriptionally active macronucleus from a transcriptionally inert micronucleus includes the elimination of many segments of DNA, the bulk of which belong to repetitive sequence families. Two approaches were used to study the interspecies variations in developmentally eliminated DNA segments. First, the occurrence of restriction fragments crosshybridizing to developmentally eliminated DNA segments isolated from T. thermophila was examined in other species of Tetrahymena. Most micronucleus-specific sequence families examined showed large differences in numbers and intensities of crosshybridizing bands in different species, indicating the possibility of gain or loss of repeats within each of the sequence families. Second, the presence of developmentally excisable DNA segments, i.e., of rearrangement sites, was examined in the same set of species at a number of unique loci. This was carried out by comparing the hybridization patterns of seven unique macronucleus-retained sequences in the micro- and macronuclei of each of the species. Essentially all of the loci displayed variability with respect to the presence of rearrangement sites among the species examined. Results from the two approaches indicate that generation or loss of developmental rearrangements can occur among the species examined here.