Scapharca inaequivalvis tetrameric hemoglobin A and B genes: evidence for a minigene

Vertebrate and many invertebrate globin genes have a three-exon/two-intron organization, with introns in highly conserved positions. According to the "intron early" hypothesis, introns are the vestigial segments which flank previously independent coding sequences, thus providing evidence for the assembly of the ancient proteins by "exon shuffling." In this paper, we report the analysis of the genes of the bivalve mollusk Scapharca inaequivalvis tetrameric hemoglobin (HbII), which support this hypothesis, at least for the hemoglobin genes. We show the existence of "minigenes" in the IIA and IIB globin genes, spanning part of the first and second introns, "in frame" with the heme-binding domain coded by the second exon. Further support for the exon shuffling hypothesis can be found in the degree of identity of the "new" translated sequences with those flanking the central protein domain of some invertebrate hemoglobins.