Inheritance of Mitochondrial DNAs and Allozymes in the Female Hybrid Lineage of Two Japanese Pond Frog Species

In order to elucidate the nucleotide sequence divergence of mitochondrial DNAs between two Japanese pond frog species Rana nigromaculata and R. brevipoda, and the mode of inheritance of cytoplasmic genomes in the female hybrid lineages of the two species, the cleavage patterns of mtDNAs digested with 10 restriction endonucleases were examined by agarose gel electrophoresis using a total of 52 frogs including Rana nigromaculata and R. brevipoda, their reciprocal hybrids and the backcross offspring (B1 and B2) derived from female hybrids by crossing with the paternal species. The cleavage patterns for mtDNA of Rana nigromaculata were different from those of R. brevipoda digested with all the restriction endonucleases used except EcoRV. The nucleotide sequence divergence of mtDNAs between these two species was roughly estimated to be 8.5%. The cleavage patterns for mtDNAs of the reciprocal hybrids and the B1 and B2 offspring were clearly similar to those of the maternal species, and paternal mtDNAs could not be detected. On the other hand, the proportions of original maternal nuclear genes at the 22 allozyme loci were 50% in the reciprocal hybrids, 21.9% or 25.6% in the B1 offspring, and 7.5% in the B2 offspring. These results demonstrate that nuclear genomes decrease the original maternal constitution in the female hybrid lineages generationally, whereas the mtDNAs are inherited maternally during repeated backcrossing.