Mitochondrial genomes of Galathealinum, Helobdella, and Platynereis: Sequence and gene arrangement comparisons indicate that Pogonophora is not a phylum and Annelida and Arthropoda are not sister taxa
Jl. Boore et Wm. Brown, Mitochondrial genomes of Galathealinum, Helobdella, and Platynereis: Sequence and gene arrangement comparisons indicate that Pogonophora is not a phylum and Annelida and Arthropoda are not sister taxa, MOL BIOL EV, 17(1), 2000, pp. 87-106
We report a contiguous region of more than half (>7,500 nt) of the mitochon
drial genomes for Platynereis dumerii (Annelida: Polychaeta), Helobdella ro
busta (Annelida: Hirudinida), and Galathealinum brachiosum (Pogonophora: Pe
rviata). The relative arrangements of all 22 genes identified for Helobdell
a and Galathealinum are identical to one another and to their arrangements
in the mtDNA of the previously studied oligochaete annelid Lumbricus. In co
ntrast, Platynereis differs from these taxa in the positions of several tRN
A genes and in having two additional tRNA genes (trnC and trnM) and a large
noncoding sequence in this region. Comparisons of relative gene arrangemen
ts and of the nucleotide and inferred amino acid sequences among these and
other published taxa provide strong support for an annelid-mollusk clade th
at excludes arthropods, and for the inclusion of pogonophorans within Annel
ida, rather than giving them separate phylum status. Gene arrangement compa
risons include the first use of a recently described method on previously u
npublished data. Although a variety of alternative initiation codons are ty
pically used by mitochondrial protein-encoding genes, ATG appears to be the
initiator for all but one reported here. The large noncoding region (1,091
nt) identified in Platynereis has no significant sequence similarity to th
e noncoding region of Lumbricus, although each contains runs of TA dinucleo
tides and of homopolymers, which could potentially serve as signaling eleme
nts. There is strong bias for synonymous codon usage in Helobdella and espe
cially in Galathealinum. in this latter taxon, 5 codons are completely unus
ed, 13 are used three or fewer times, and G appears at third codon position
s in only 26 of the 2,236 codons. Nucleotide composition bias appears to in
fluence amino acid composition of the proteins.