The exon/intron organization of the globin gene of Scapharca inaequivalvis homodimeric hemoglobin: unusual intron homology with other bivalve mollusc globin genes.

In this study, we have investigated the positions of introns in the globin gene of Scapharca inaequivalvis homodimeric hemoglobin. We found the three exon/two intron organization typical of vertebrate globin genes, with the two introns in highly conserved positions, as it occurs in the A and B globin genes of the tetrameric hemoglobin from the same organism, confirming the absence of the so-called 'central intron' found in the globin genes of plants and of some invertebrates. We identified two homodimeric globin genes (3207 and 2723 bp) that differ only with respect to the size of the first intron. Sequence analysis of the two first introns (1668 and 1364 bp) has revealed that they are highly homologous, except for a 569- and 296-bp insertion in each intron I. Interestingly, the two first introns contain regions with an unusually high identity (approximately 80%) with regions of the first intron of the congeneric clam Anadara trapezia and the related clam Barbatia reveana globin genes, suggesting that these uncoding regions may have played a regulatory role that has subsequently been lost during the course of the evolution.