Abstract
Digestion of Rana graeca italica DNA with Asp 718I produces highly repetitive fragments of 281 and 385 bp that were cloned and sequenced. The shorter fragment corresponds to the unit repeat (RgiS1b) of a satellite DNA. The longer fragment was found to be part of a 494-bp repeat of another satellite DNA (RgiS1a) that was cloned intact as an EcoRV fragment. RgiS1b is 97% homologous to RgiS1a, from which it seems to be derived by a single deletion. Among all species tested, only the related brown frog Rana dalmatina contained homologous repetitive DNA. The overall number of RgiS1a and RgiS1b repeats per R. graeca italica haploid genome was estimated to be 2.7 x 10(5). RgiS1a and RgiS1b repeats are organized in separate arrays, but repetitive units formed by various combinations of the two repeats were also observed on Southern blots. The amount of these extra repeats varies greatly among animals from the same population, representing a rare case of individual variability in the satellite DNA organization. FISH with probes specific for both satellites, or for RgiS1a only, labeled the centromeric and pericentromeric heterochromatin of all chromosomes. This indicated that RgiS1a and RgiS1b are interspersed within the same heterochromatic regions of the chromosomes.
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