Gradual evolution of a specific satellite DNA family in Drosophila ambigua, D. tristis, and D. obscura

The highly repetitive satellite DNA family "ATOC180" is specific for thethree closely related species Drosophila obscura, D. ambigua, and D.tristis but does not occur in their closest relatives D. subsilvestris andD. bifasciata. Approximately 10,000 copies/haploid genome of approximately180-bp repetition units are tandemly arranged in the centromericheterochromatin of all chromosomes of all three species. Molecular analysisof 29 cloned repeats shows much intra- and interspecific sequencehomogeneity. Single nucleotide changes are the main source of variabilityand distinguish the sequence-, subfamily- and species-specific ATOC180repeats from each other. Based on these nucleotide differences,phylogenetic dendrograms were constructed and compared with published treesfor other traits. The data indicate that the sequences of the ATOC180satellite DNA family probably arose in a phylogenetically "short period"during the anagenetic evolution of the common ancestor of D. obscura, D.tristis, and D. ambigua, as a consequence of a process of genomereorganization, followed by a "long period" of entirely gradual sequenceevolution. For the latter period, an evolutionary rate of 3 x 10(-8)substitutions/site/year was calculated.