Tail-to-head linkage of alpha- and beta-globin genes in the Antarctic skate Bathyraja eatonii.

Introduction: Extensive investigations of the organisation of globin genes has greatly contributed to the understanding of universal mechanisms of gene evolution and of regulation of gene expression. A single primordial globin gene is believed to have duplicated to produce an ?-like and a ?-like globin gene approximately 450 million year ago. The duplication event that led to the evolution of the vertebrate ?- and ?-globin genes would initially have left the two genes adjacent. The ?- and ?-globin genes are tightly linked in teleosts, where they are organised as consecutive ?/? sets with the genes linked in head-to-head (5' to 5') orientation, and in amphibians, where they form ?- and ?-type clusters with the genes having the same transcriptional direction. In mammals and birds the ?- and ?-gene clusters are separated and located on different chromosomes, and usually arranged along the order of ontogenic expression. The duplication and differentiation of the ancestral globin gene into the ? and ? genes, including the evolution of the hemoglobin tetramer, the formation of the subunit interfaces, and the mechanism of the quaternary structure change, took place within a comparatively short period between the branching points of hagfish and lampreys from cartilaginous fish. Up to now, there was complete lack of data about the organisation of globin genes in chondrichthyans. Results: In Antarctica the chondrichthyan genera are represented in the modern fauna with only three species of sharks and eight species of skates, that account for about 4% of the total fauna. Bathyraja (six species) is the dominant rajid south of 60°S. In the framework of the investigations on globin genes in Antarctic red-blooded and hemoglobinless fish, by RT-PCR, we obtained the cloning of the ?- and ?-globin cDNAs of the main hemoglobin of the skate Bathyraja eatonii. Then, PCR-based gene linkage analysis was used to assess the relative orientation of the globin gene pair. A genomic fragment of about 6700 bp was obtained where the ? and ? genes are linked in a tail-to-head (3' to 5') orientation. To confirm this finding and to contribute to our Antarctic/Arctic comparison studies, work is currently in progress with the aim to elucidate the organisation of globin genes in another chondrichthyan: the Arctic skate Raja hyperborea. Conclusions: The finding of a tail-to-head linkage of ?- and ?-globin genes in the Antarctic skate B. eatonii is the first characterisation of the organisation of globin genes in chondrichthyes; such finding will offer a substantial contribution to the understanding of the general evolution of this class of genes.