Complete sequence, gene arrangement, and genetic code of mitochondrial DNA of the cephalochordate Branchiostoma floridae (Amphioxus)

We have determined the 15,083-nucleotide (nt) sequence of the mitochondrial DNA (mtDNA) of the lancelet Branchiostoma floridae (Chordata: Cephalochordata). As is typical in metazoans, the mtDNA encodes 13 protein, 2 rRNA, and 22 tRNA genes. The gene arrangement differs from the common vertebrate arrangement by only four tRNA gene positions. Three of these are unique to Branchiostoma, but the fourth is in a position that is primitive for chordates. It shares the genetic code variations found in vertebrate mtDNAs except that AGA = serine, a code variation found in many invertebrate phyla but not in vertebrates (the related codon AGG was not found). Branchiostoma mtDNA lacks a vertebrate-like control region; its largest noncoding region (129 nt) is unremarkable in sequence or base composition, and its location between ND5 and tRNAG differs from that usually found in vertebrates. It also lacks a potential hairpin DNA structure like those found in many (though not in all) vertebrates to serve as the second-strand (i.e., L-strand) origin of replication. Perhaps related to this, the sequence corresponding to the DHU arm of tRNAC cannot form a helical stem, a condition found in a few other vertebrate mtDNAs that also lack a canonical L-strand origin of replication. ATG and GTG codons appear to initiate translation in 11 and 2 of the protein-encoding genes, respectively. Protein genes end with complete (TAA or TAG) or incomplete (T or TA) stop codons; the latter are presumably converted to TAA by post-transcriptional polyadenylation.

Mitochondrial gene arrangement of the horseshoe crab Limulus polyphemus L.: conservation of major features among arthropod classes.

Numerous complete mitochondrial DNA sequences have been determined for species within two arthropod groups, insects and crustaceans, but there are none for a third, the chelicerates. Most mitochondrial gene arrangements reported for crustaceans and insect species are identical or nearly identical to that of Drosophila yakuba. Sequences across 36 of the gene boundaries in the mitochondrial DNA (mtDNA) of a representative chelicerate. Limulus polyphemus L., also reveal an arrangement like that of Drosophila yakuba. Only the position of the tRNA(LEU)(UUR) gene differs; in Limulus it is between the genes for tRNA(LEU)(CUN) and ND1. This positioning is also found in onychophorans, mollusks, and annelids, but not in insects and crustaceans, and indicates that tRNA(LEU)(CUN)-tRNA(LEU)(UUR)-ND1 was the ancestral gene arrangement for these groups, as suggested earlier. There are no differences in the relative arrangements of protein-coding and ribosomal RNA genes between Limulus and Drosophila, and none have been observed within arthropods. The high degree of similarity of mitochondrial gene arrangements within arthropods is striking, since some taxa last shared a common ancestor before the Cambrian, and contrasts with the extensive mtDNA rearrangements occasionally observed within some other metazoan phyla (e.g., mollusks and nematodes).

Deducing the pattern of arthropod phylogeny from mitochondrial DNA rearrangements.

The origins of arthropods and the phylogenetic relationships among their three major living groups (atelocerates, crustaceans and chelicerates) are vigorously contended. To help resolve this, we determined mitochondrial gene arrangements for a chelicerate, a myriapod, two crustaceans, an onychophoran, a mollusc and an annelid, and compared them with published gene orders of other species. The result strongly supports the monophyly of Arthropoda and of Mandibulata (atelocerates plus crustaceans) and refutes the Uniramia (atelocerates plus onychophorans). Gene arrangement comparisons are emerging as a powerful new tool for resolving ancient phylogenetic relationships.

Sequences with the potential to form stem-and-loop structures are associated with coding-region duplications in animal mitochondrial DNA.

Tandem duplications of gene-encoding regions occur in the mitochondrial DNA (mt DNA) of some individuals belonging to several species of whiptail lizards (genus Cnemidophorus). All or part of the duplicated regions of the mtDNAs from five different species were sequenced. In all, the duplication endpoints were within or immediately adjacent to sequences in tRNA, rRNA or protein genes that are capable of forming energetically stable stem-and-loop structures. In two of these mtDNAs, the duplication endpoints were also associated with a direct sequence repeat of 13 bp. The consistent association of stem-and-loop structures with duplication endpoints suggests that these structures may play a role in the duplication process. These data, combined with the absence of direct or palindromic repeats at three of the pairs of duplication endpoints, also suggest the existence of a mechanism for generating de novo duplications that is qualitatively different from those previously modeled.