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The evolutionary position of nematodes.

ABSTRACT: BACKGROUND:The complete genomes of three animals have been sequenced by global research efforts: a nematode worm (Caenorhabditis elegans), an insect (Drosophila melanogaster), and a vertebrate (Homo sapiens). Remarkably, their relationships have yet to be clarified. The confusion concerns the enigmatic position of nematodes. Traditionally, nematodes have occupied a basal position, in part because they lack a true body cavity. However, the leading hypothesis now joins nematodes with arthropods in a molting clade, Ecdysozoa, based on data from several genes. RESULTS:We tested the Ecdysozoa hypothesis with analyses of more than 100 nuclear protein alignments, under conditions that would expose biases, and found that it was not supported. Instead, we found significant support for the traditional hypothesis, Coelomata. Our result is robust to different rates of sequence change among genes and lineages, different numbers of taxa, and different species of nematodes. CONCLUSION:We conclude that insects (arthropods) are genetically and evolutionarily closer to humans than to nematode worms.

SUBMITTER: Blair JE

PROVIDER: S-EPMC102755 | biostudies-literature | 2002 Apr

REPOSITORIES: biostudies-literature

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The evolutionary position of nematodes.

Blair Jaime E JE Ikeo Kazuho K Gojobori Takashi T Hedges S Blair SB

BMC evolutionary biology 20020408

<h4>Background</h4>The complete genomes of three animals have been sequenced by global research efforts: a nematode worm (Caenorhabditis elegans), an insect (Drosophila melanogaster), and a vertebrate (Homo sapiens). Remarkably, their relationships have yet to be clarified. The confusion concerns the enigmatic position of nematodes. Traditionally, nematodes have occupied a basal position, in part because they lack a true body cavity. However, the leading hypothesis now joins nematodes with arthr ...[more]

PMID: 11985779

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Project description:BackgroundHow various host-parasite combinations have been established is an important question in evolutionary biology. We have previously described two nematode species, Rhigonema naylae and Travassosinema claudiae, which are parasites of the xystodesmid millipede Parafontaria laminata in Aichi Prefecture, Japan. Rhigonema naylae belongs to the superfamily Rhigonematoidea, which exclusively consists of parasites of millipedes. T. claudiae belongs to the superfamily Thelastomatoidea, which includes a wide variety of species that parasitize many invertebrates. These nematodes were isolated together with a high prevalence; however, the phylogenetic, evolutionary, and ecological relationships between these two parasitic nematodes and between hosts and parasites are not well known.ResultsWe collected nine species (11 isolates) of xystodesmid millipedes from seven locations in Japan, and found that all species were co-infected with the parasitic nematodes Rhigonematoidea spp. and Thelastomatoidea spp. We found that the infection prevalence and population densities of Rhigonematoidea spp. were higher than those of Thelastomatoidea spp. However, the population densities of Rhigonematoidea spp. were not negatively affected by co-infection with Thelastomatoidea spp., suggesting that these parasites are not competitive. We also found a positive correlation between the prevalence of parasitic nematodes and host body size. In Rhigonematoidea spp., combinations of parasitic nematode groups and host genera seem to be fixed, suggesting the evolution of a more specialized interaction between Rhigonematoidea spp. and their host. On the other hand, host preference of Thelastomatoidea spp. was not specific to any millipede species, indicating a non-intimate interaction between these parasites and their hosts.ConclusionsThe two nematode superfamilies, Rhigonematoidea and Thelastomatoidea, have phylogenetically distinct origins, and might have acquired xystodesmid millipede parasitism independently. Currently, the two nematodes co-parasitize millipedes without any clear negative impact on each other or the host millipedes. Our study provides an example of balanced complex symbioses among parasitic nematodes and between parasitic nematodes and host millipedes, which have been established over a long evolutionary history.

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The evolutionary position of nematodes.

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The evolutionary position of nematodes.

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