A MONOPHYLETIC ORIGIN OF HEART-PREDOMINANT LACTATE-DEHYDROGENASE (LDH) ISOZYMES OF GNATHOSTOME VERTEBRATES - EVIDENCE FROM THE CDNA SEQUENCE OF THE SPINY DOGFISH (SQUALUS-ACANTHIAS) LDH-B

Duplication of a single lactate dehydrogenase locus early in vertebrat e evolution has been proposed to have given rise to Ldh-A and Ldh-B, t he encoded isozymes of which predominate in skeletal and heart muscle, respectively. This view has been challenged recently by phylogenetic analyses of LDH sequences. One question that has been raised is whethe r the heart-predominant isozyme (LDH-B) of cartilaginous fishes is ort hologous to that of bony fishes and their derivatives. To address this issue, we determined the complementary DNA sequence of the LDH-B of t he chondrichthyan Squalus acanthias. Phylogenetic analysis of this and other LDH isozyme sequences provided strong support for a single orig in of LDH-Bs prior to the divergence of cartilaginous and bony fishes.