Evolution of the genomes of two nematodes in the ... - Ken Wolfe

Figure 1.1: The model organism Caenorhabditis elegans: an adult with two juveniles.
1.2 THE ORIGIN OF NEMATODES
The first nematode was probably a free-living marine animal that fed on bacteria (Poinar, 1983). The
relationship of this early nematode to other animals is uncertain. Based on phylogenetic trees, some argue
they are most closely related to arthropods (Aguinaldo et al., 1997; Mushegian et al., 1998; Figure 1.2).
Conflicting phylogenetic results suggest they are an outgroup to a clade that includes arthropods and
chordates (Blair et al., 2002).
Doubts also surround when the first nematode lived. Related animal phyla such as the arthropods ap-
pear in the fossil record ∼550 million years ago (Mya). However, because they lack hard body structures,
nematode fossils are scarce, and most fossils found are recent, from 20–120 Mya (Poinar, 1983). Based
on both fossil and phylogenetic evidence, paleontologists believe that the animal kingdom split into a few
basal clades ∼1000 Mya, and that later, ∼600–800 Mya, animal groups acquired new body plans, giving
rise to the modern phyla including the nematodes and arthropods (Benton and Ayala, 2003).
1.3 A MODEL NEMATODE, Caenorhabditis elegans
Caenorhabditis elegans belongs to the order Rhabditida: small (1–2 mm), free-living worms that feed on
bacteria in decaying organic matter. The closest parasitic relatives of the rhabditids are the strongylids,
small gut parasites such as the human hookworm Necator americanus (Blaxter et al., 1998; Figure 1.3).
In 1963 Sydney Brenner realised that species from the genus Caenorhabditis would be ideal model
organisms. They have many cell types involved in complex functions in mammals, including intestine,
muscle and excretory cells, and neurons. Since then study of C. elegans has contributed to understanding
of central biological processes: apoptosis, signalling pathways, cell movement and polarity, sex determi-
nation, and synaptic signalling (Riddle et al., 1997). C. elegans was the first animal for which we had a
complete description of development, anatomy, and a neural wiring diagram; and, as a crowning glory,
the first to have its genome sequenced (Riddle et al., 1997; The C. elegans Sequencing Consortium, 1998).
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