2013 Volume 72 Issue 6 Pages 326-334
The author previously established a molecular tree based on the cDNA-derived primary structures of globins in two giant tortoises, Geochelone nigra and G. gigantea. The divergence time was estimated to be 15-21 million years ago. In the present study, the author reexamined the divergence time of these two species using another source of genetic information--globin-introns--including those from the Chaco tortoise (G. chilensis), a close relative of G. nigra. The previously determined divergence time was supported by the findings of this intron study. However, the inter-relationships based on the intron nucleotide sequences of the globins from the three Geochelone species remain controversial, because it is difficult to determine which of the three is the ancestral species. In addition, the nucleotide sequences reveal the following interesting characteristics: (1) an abnormal GC dinucleotide sequence located at the 5'-splicing site of the second intron of αD globins instead of a consensus GT--this finding is common to all studied Geochelone species; (2) a repeated sequence 5'-GCCCCGCGCCCCGC-3' found only in the first intron of the G. nigra a A globin gene, is a unique feature distinguishing the Galapagos giant tor-toise from the other Geochelone tortoises that have non-repeated GCCCCGC sequences.
