Sequences lead to tree of worms (original) (raw)

Systematics

Nature volume 392, page 25 (1998)Cite this article

Nematode worms are found just about everywhere, often in enormous numbers. Parasitic species live in almost all animals and plants, and only arid soils and the open oceans seem to be unsuitable for free-living species. Yet only about 15,000 species have been formally described, and some textbooks treat nematodes as one of the ‘minor phyla’. In spite of this, it is now believed that the number of living species should be counted by the million. Classification has been difficult because most of the species are small to microscopic in size, and they lack obvious distinguishing characteristics. But the first attempt at a phylogenetic classification — based on small-subunit ribosomal DNA sequences from 53 species — is presented by Blaxter et al. on page 71 of this issue1. And, according to their findings, only one of the two classes of nematode that are recognized in the traditional classification is natural, consisting of an ancestral species and all its descendants.

Most (but not all) nematodes are small and nondescript. For example, Placentonema gigantissima, which lives as a parasite in the placenta of sperm whales, grows to a length of 8 m, with a diameter of 2.5 cm. The free-living, marine Draconema has elongate adhesive organs on the head and along the tail, and moves like a caterpillar. But the general uniformity of most nematode species has hampered the establishment of a classification that includes both free-living and parasitic species. Two classes have been recognized (the Secernentea and Adenophorea), based on the presence or absence of a caudal sense organ, respectively. But Blaxter et al.1 have concluded from the DNA sequences that the Secernentea is a natural group within the Adenophorea. Based on studies of free-living species, a paraphyletic nature for the Adenophorea — that is, a group comprising an ancestor but not all of its descendants — has previously been suggested (for example, by Lorenzen2), but the position of the various parasitic groups has always caused trouble.

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References

  1. Blaxter, M. L.et al. Nature 392, 71–75 (1998).
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  2. Lorenzen, S. in Concepts in Nematode Systematics (eds Stone, A. R., Platt, H. M. &Khalil, L. F.) 11-23 (Academic, New York, 1983).

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  1. the Zoological Museum, University of Copenhagen, Universitetsparken 15, Copenhagen, DK-2100, Denmark
    Claus Nielsen

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Nielsen, C. Sequences lead to tree of worms.Nature 392, 25 (1998). https://doi.org/10.1038/32058

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