Nematodes are small, usually microscopic, unsegmented roundworms
with a narrow cylindrical body. They are incredibly numerous in soil, water, and inside other animals
and plants throughout the world. Approximately 10,000 species are known. Four of every five animals
on the planet are nematodes, outnumbering even insects. A single gram of soil can contain more than
1,000 plant parasitic nematodes.
Nematodes are simple animals with a body plan that resembles a tube within a tube with a head, mouth,
tail, and anus. The outer tube is comprised of the cuticle, hypodermis, and muscle; and the inner tube
contains the pharynx and intestine. Nematodes also have several hundred nerve cells and a prolific
reproductive system. They vary in size from a fraction of a millimeter to many centimeters for some
animal/human parasitic species.
Many nematodes are free-living in terrestrial and aquatic environments where they feed on bacteria.
Numerous other species are parasitic, specializing in the invasion of plants, mammals, or even insects.
Parasitic nematodes are major challenges to human health and
agriculture. Parasitic nematodes, including
whipworm, Ascaris, hookworm, and filarial worms, currently infect about 3 billion people. Plant parasitic
nematodes, such as root knot nematode, cause an estimated 80 billion dollars in crop damage annually.
Animal parasites infect livestock and pets and their treatment
is a mainstay of veterinary practice (e.g. heartworm prevention in dogs and cats). Nematodes are an
ancient phylum dating back hundreds of millions of years and their diversification likely predates the
arrival of life on land. Parasites are likely to have evolved at least seven separate times from
Animal and human parasitic nematodes have adapted complex life cycles to take advantage of their hosts.
Hookworm larvae, for instance, can enter the skin from the soil, burrow into the blood stream, travel
through the circulation to the lungs, transit up the bronchial tree and down the esophagus, finally reaching
the small intestine where they drink blood from capillaries, and reproduce by laying eggs in the feces. They
take several weeks to complete this cycle. Filarial worms that cause river blindness have a very different
life cycle, entering humans by the bite of a fly, growing to adulthood in nodules under the skin where they
can live for more than 8 years, generating millions of larva which migrate through the skin and eyes. Other
filarial worms are transmitted by mosquitoes.
Plant parasitic nematodes such as root knot and cyst nematodes are sedentary endoparasites, invading plant
roots, establishing feeding sites where they divert plant nutrients toward their own growth. Once established
at a feeding site, the female worm gradually changes to a pear shape and fills with hundreds of eggs. The life
cycle generally takes several weeks. Other plant parasites migrate through roots, stems, or leaves. Many
parasites have long-lived forms; for instance, soybean cyst nematode can survive five or more winters, making
crop rotation ineffective.
The free-living nematode C. elegans has become an incredibly useful model organism for biologists and medical
scientists. Selected in the mid-1960's by Sydney Brenner as a genetic specimen for the general study of animal
development and behavior, the worm is now studied by nearly 500 laboratories worldwide. The advantages of C.
elegans as a model include its short generation time (4 days), large reproductive size (350 offspring), and
relatively simple body structure (959 somatic cells). It was the first animal for which the complete cell
lineage was determined and the first to have its entire neuronal wiring deciphered. C. elegans was also the
first multicellular organism to have its genome fully sequenced and annotated
(www.wormbase.org). An international
collaboration between Washington University, St. Louis and the Sanger Institute near Cambridge, England completed
the 100 megabase genome in 1998, identifying ~20,000 genes. In parallel with these descriptive efforts, thousands
of genetic mutations have been characterized affecting intercellular signaling, cell migration and death, sex
determination, dauer formation, germ line development, muscle structure, mechanosensation, and developmental
pathways involved in cell fate and timing. Numerous genes involved in these processes are highly conserved with
The importance of research using C. elegans has been widely recognized. Six nematologists studying C. elegans
have been awarded the Nobel Prize this decade. The 2002 Nobel Prize in physiology or medicine was awarded to
Sydney Brenner, John Sulston, and Robert Horvitz for their use of C. elegans in understanding cell development
and programmed cell death. The 2006 Nobel prize in physiology or medicine was awarded to Craig
Mello and Andrew Fire for their use of C. elegans in the discovery of gene expression silencing by RNA interference (RNAi).
The 2008 Nobel prize in chemistry was awarded to three scientists including Martin Chalfie who introduced the use
of green fluorescent protein (GFP) as a luminous genetic tag for tracking individual cells in C. elegans.
"If all the matter in the universe except nematodes were swept away, our world would still be recognizable, and if,
as disembodied spirits, we could then investigate it, we should find its mountains, hills, vales, rivers, lakes, and
oceans represented by a film of nematodes. The location of towns would be decipherable, since for every massing of
human beings there would be a corresponding massing of certain nematodes. Trees would still stand in ghostly rows
representing our streets and highways. The location of the various plants and animals would still be decipherable, and
had we sufficient knowledge, in many cases even their species could be determined by an examination of their erstwhile
~ Nathan Cobb, 1914