tides) of the mitochondrial DNA (mtDNA) from the sea Furthermore, the availability of the entire sequences urchin Arbacia lixula is presented. The comparison of of mtDNA from different organisms demonstrated that gene arrangement between different echinoderm orthe genetic content is conserved. The mt genome conders of the same class provides evidence that the gene tains genes for a complete set of tRNAs, for two rRNAs organization is conserved within the same echinoderm and for 13 proteins components of the oxidative phosclass. The peculiarities of sea urchin mtDNA features, phorylation system, the subunits I, II, and III of cytoalready described, are confirmed by the A. lixula chrome oxidase (COI, COII, COIII), the cytochrome b mtDNA sequence. The comparison of the entire se-(cyt b), the subunits 6 and 8 of the ATPase complex quences of mtDNA among A. lixula, Paracentrotus liv-(ATPase6 and ATPase8), and the subunits 1-6 and 4L idus, and Strongylocentrotus purpuratus allowed us to of the respiratory chain NADH dehydrogenase (ND1detect peculiar features, common to the three sea ur-ND6 and ND4L). Only in nematode mtDNA the gene chin species, that can represent the molecular signafor ATPase subunit 8 has not been found (Okimoto et ture of the mt genome in the sea urchin group. Analysis al., 1992). In addition, there are one or more noncoding of the nucleotide composition indicates that A. lixula regions which are retained or have been demonstrated mtDNA, in contrast with the mtDNA of other sea urto contain signals for the replication and for the tranchins, shows a bias in the use of T and tends to avoid the use of C, most evident in the neutral part of the scription of the mt genome. molecule, such as the third codon positions. This ob-
The distribution of genes along the mt genome differs servation indicates that the three sea urchin mtDNAs from phylum to phylum, but is conserved within the evolve under different mutation pressure. Analysis of same phylum, where only few transpositions are tolerthe sequence evolution allowed us to confirm the phyated. logenetic tree. However, the absolute divergence time,
In the phylum Echinodermata, the mtDNA complete calculated on the basis of paleontological estimates, sequences have been obtained in two sea urchins, largely diverged from the expected one.