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Abstract
The complete mitochondrial genome sequences were determined for two species of human
hookworms, Ancylostoma duodenale (13,721 bp) and Necator americanus (13,604 bp). The
circular hookworm genomes are amongst the smallest reported to date for any metazoan
organism. Their relatively small size relates mainly to a reduced length in the AT-rich
region. Both hookworm genomes encode 12 protein, two ribosomal RNA and 22 transfer
RNA genes, but lack the ATP synthetase subunit 8 gene, which is consistent with three
other species of Secernentea studied to date. All genes are transcribed in the same
direction and have a nucleotide composition high in A and T, but low in G and C. The
AT bias had a significant effect on both the codon usage pattern and amino acid composition
of proteins. For both hookworm species, genes were arranged in the same order as for
Caenorhabditis elegans, except for the presence of a non-coding region between genes
nad3 and nad5. In A. duodenale, this non-coding region is predicted to form a stem-and-loop
structure which is not present in N. americanus. The mitochondrial genome structure
for both hookworms differs from Ascaris suum only in the location of the AT-rich region,
whereas there are substantial differences when compared with Onchocerca volvulus,
including four gene or gene-block translocations and the positions of some transfer
RNA genes and the AT-rich region. Based on genome organisation and amino acid sequence
identity, A. duodenale and N. americanus were more closely related to C. elegans than
to A. suum or O. volvulus (all secernentean nematodes), consistent with a previous
phylogenetic study using ribosomal DNA sequence data. Determination of the complete
mitochondrial genome sequences for two human hookworms (the first members of the order
Strongylida ever sequenced) provides a foundation for studying the systematics, population
genetics and ecology of these and other nematodes of socio-economic importance.