This new paper comments on several interesting matters (e.g., an annelid-
mollusk clade that excludes arthropods) , so it was difficult to decide what
to put in the subject line ;-), but, anyway, watch for this one coming soon to
a library near you.
Boore, J.L., & W.M. Brown. 2000. 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. -- Molecular Biology and
Evolution, 17(1):87-106.
ABSTRACT. We report a contiguous region of more than half (>7,500 nt)
of the mitochondrial genomes for Platynereis dumerii (Annelida:
Polychaeta), Helobdella robusta (Annelida: Hirudinida), and Galathealinum
brachiosum (Pogonophora: Perviata). The relative arrangements of all 22
genes identified for Helobdella and Galathealinum are identical to one
another and to their arrangements in the mtDNA of the previously studied
oligochaete annelid Lumbricus. In contrast, Platynereis differs from these
taxa in the positions of several tRNA genes and in having two additional
tRNA genes (trnC and trnM) and a large noncoding sequence in this
region. Comparisons of relative gene arrangements and of the nucleotide
and inferred amino acid sequences among these and other published taxa
provide strong support for an annelid- mollusk clade that excludes
arthropods, and for the inclusion of pogonophorans within Annelida, rather
than giving them separate phylum status. Gene arrangement comparisons
include the first use of a recently described method on previously
unpublished data. Although a variety of alternative initiation codons are
typically 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
the noncoding region of Lumbricus, although each contains runs of TA
dinucleotides and of homopolymers, which could potentially serve as
signaling elements. There is strong bias for synonymous codon usage in
Helobdella and especially in Galathealinum. In this latter taxon, 5 codons
are completely unused, 13 are used three or fewer times, and G appears at
third codon positions in only 26 of the 2,236 codons. Nucleotide
composition bias appears to influence amino acid composition of the
proteins.
--
Geoff Read <g.read at niwa.cri.nz>
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