Molecular characterization of a centromeric satellite DNA in the hemiclonal hybrid frog Rana esculenta and its parental species.

Hybrid water frogs Rana esculenta reproduce by hybridogenesis: one parental genome (of Rana lessonae) is excluded in the germ line, the other (of Rana ridibunda) is clonally transmitted to haploid gametes. The two parental species differ in that the amount of centromeric heterochromatin revealed by differential staining is much higher in Rana ridibunda. An abundant, tandemly arrayed, centromeric satellite DNA, designated RrS1, is revealed in Rana ridibunda genomes by the restriction endonuclease Stul, which generates a major repetitive sequence fragment of 300 and a minor one of 200 bp. This AT-rich (68%) satellite family is located at the centromeres of the five largest chromosomes (1-5) and of a medium to small heterobrachial one (8 or 9); it thus constitutes only part of the centromeric heterochromatin that characterizes all Rana ridibunda chromosomes. RrS1 represents about 2.5% of the genome of Rana ridibunda; it may represent as little as 0.2% of the genome of Rana lessonae, and cannot be detected in Xenopus laevis frogs or Salamandra salamandra and Triturus carnifex salamanders. Segments of the satellite sequence are similar to sequences of yeast centromeric DNA element CDEIII and of the mammalian CENP-B box. A role for RrS1 and other centromeric satellite DNAs in the germ line genome exclusion of the hybridogenetic frog hybrids, although suggested, has not yet been demonstrated.