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

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Abstract
The highly repetitive satellite DNA family "ATOC180" is specific for the three closely related species Drosophila obscura, D. ambigua, and D. tristis but does not occur in their closest relatives D. subsilvestris and D. bifasciata. Approximately 10,000 copies/haploid genome of approximately 180-bp repetition units are tandemly arranged in the centromeric heterochromatin of all chromosomes of all three species. Molecular analysis of 29 cloned repeats shows much intra- and interspecific sequence homogeneity. Single nucleotide changes are the main source of variability and distinguish the sequence-, subfamily- and species-specific ATOC180 repeats from each other. Based on these nucleotide differences, phylogenetic dendrograms were constructed and compared with published trees for other traits. The data indicate that the sequences of the ATOC180 satellite 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 genome reorganization, followed by a "long period" of entirely gradual sequence evolution. For the latter period, an evolutionary rate of 3 x 10(-8) substitutions/site/year was calculated.

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