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Exercise: Free-living and Plant-Parasitic Nematodes (Roundworms)

Purpose:

Introduction to an important group of soil and water organisms and an opportunity to conduct some simple biodiversity experiments. To extract and examine free-living or non-parasitic nematodes, that feed on bacteria, fungi or other nematodes, and plant-parasitic nematodes from soil samples.

Background information:

Nematodes are translucent roundworms that live in all soil and water habitats and are the most abundant multicellular organisms on the earth. About 30,000 species are currently known. While most nematodes are so tiny that one needs a microscope to see them, many animal parasites are large enough to be seen easily with the naked eye. One species that parasitizes whales can reach 10 m (25 feet) in length.

The majority of nematode species are free living in soil and water and feed on microorganisms (bacteria, fungi, algae, other nematodes) and organic debris. Of these, 50% are marine salt water and 25% dwell in soil and freshwater. The bacterial feeding nematode, Caenorhabditis elegans, has the distinction of being the first multicellular organism to have its entire genome (genetic code) sequenced. This nematode serves as a wonderful model organism for basic molecular biological research. (See APSnet Feature, Implications of sequencing the nematode Caenorhabditis elegans genome for plant nematology.)  The fungal feeding nematode Aphelenchus is commonly found in soil and moss. The predatory nematode, Mononchus, uses its large tooth to prey on other nematodes and small creatures living in the soil. Certain nematodes living in sulfide-rich marine sediments are coated with ectosymbiotic epibacteria, which are sulfur-oxidizing chemoautotrophs. The nematode benefits by having a portable, renewable food source, the epibacteria. The epibacteria benefit because the nematode transports the bacteria to oxygen-sulfide gradient conditions which favor their growth as the nematode migrates through the sediment.

Figure 1. Caenorhabditis elegans adult and two juveniles. (Courtesy J. Esnard) Click image for an enlarged view.

Other species of nematodes (about 15% of the known species) are parasites of animals, including humans and insects. One nematode species found in water and sediments parasitizes mosquito larvae, while another species parasitizes bumblebee queens when they are overwintering underground. The nematodes Heterorhabditis bacteriophora and Steinernema glaseri are used for biological control of Japanese beetles. The nematodes harbor bacterial symbionts internally. When these nematodes enter the insect larvae, they introduce the bacteria, which subsequently kill the insects with toxins. If you have pets, you may already have heard about nematodes from a veterinarian. Dogs are routinely treated for heartworm, which is transmitted by mosquitoes. Domestic cats are often treated for the nematode Toxicara cati, which may be acquired when the cat catches and eats a mouse. In the tropics, there are a number of human-parasitic nematodes; people may become infected with hookworms by walking barefoot, with schistosomes by swimming in still or slow-moving bodies of fresh water, with various kinds of filaria when bitten by insect vectors, and with a variety of other nematodes by eating contaminated raw vegetables. Regulations governing production and handling of meats help to prevent the occurrence in pork of Trichinella, a nematode that causes trichinosis in humans.

About 10% of the known nematode species are plant parasites. The life cycle of a nematode begins as an egg. The embryo develops into a juvenile. Juveniles grow and molt (shed their outer body wall or cuticle) to the next juvenile stage. Usually, the first molt occurs inside the egg so that it is the second-stage juvenile that hatches from the egg. After four molts and four juvenile stages, nematodes mature into adult males and females. Plant-parasitic nematodes are obligate parasites, meaning they must feed on living plant tissue at some point in their life to complete their life cycle.

Plant-parasitic nematodes have rigid, pointed mouth-spears or stylets, usually hollow, that are used to penetrate plant cells and feed. The presence of a stylet differentiates plant parasites from free-living nematodes in soil (although some fungal-feeding nematodes also possess stylets).

	Figure 2. Head of plant-parasitic ring nematode Mesocriconemella xenoplax  with prominent, pointed stylet. (Used by permission of R.S. Hussey) Click image for an enlarged view.
	Figure 3. Head of Caenorhabditis elegans showing cylindrical mouth cavity for feeding on bacteria. Note that a stylet is absent. (Courtesy J. Esnard) Click image for an enlarged view.
Figure 4.Typical morphology of a plant-parasitic nematode. (Courtesy C. Jasalavich) CLICK HERE FOR IMAGE.

Plant-parasitic nematodes are generally categorized into those that feed from the outside of plant tissue (ectoparasites) and those that enter plant tissue to feed and live (endoparasites). The stubby-root nematode, the sting nematode, and the ring nematode are examples of plant-parasitic nematodes that are ectoparasites.

Figure 5. ECTOPARASITE: Ring nematode juvenile (left) and adult female (right) feeding on a plant root. (Courtesy S. W. Westcott III) Click image for an enlarged view.	Figure 6. ENDOPARASITE: Lance nematodes feeding inside soybean roots. (Used by permission of G. L. Tylka) Click image for an enlarged view.

Several important plant-parasitic nematodes are endoparasites. The lesion nematode, Pratylenchus, is named because it migrates through root tissue leaving a trail of dead cells, which form a dark, dead spot or lesion. Root-knot nematodes (Meloidogyne) and cyst nematodes (Heterodera) are different from most plant-parasitic nematodes because the adult females become swollen and lose their worm (vermiform) shape. Root-knot nematodes greatly modify roots to form small knots or galls around the swollen adult female nematodes; these galls are easily seen on infected roots.

Plant-parasitic nematodes damage plants in a number of ways. Feeding causes wounding, which creates openings through which other pathogens may enter the plant. Nematodes also retard root growth and disrupt the vascular tissue of the roots, which reduces the transport of water and minerals from the root system up to the leaves and stems of the plant. Evidence of nematodes feeding on roots includes the following symptoms: lesions, stubby, curled, or galled roots, and a reduced root system.

Figure 7.  Key to some of the common plant-parasitic nematodes  (Courtesy E.B. Nelson) CLICK HERE  TO SEE THIS FIGURE.
	Figure 8. Strawberry root with lesions caused by the lesion nematode Pratylenchus penetrans. (Courtesy J. L. Townshend) Click image for an enlarged view.
Figure 9. Swollen root tips caused by sting nematode Belonolaimus feeding on potato roots. (Courtesy D. P. Weingartner) Click image for an enlarged view.	Figure 10. Galls on roots of carrot infected with Meloidogyne hapla, the northern root-knot nematode. (Courtesy G. L. Tylka) Click image for an enlarged view.
Figure 11. Poor root growth of rose infected by nematodes (right) compared with healthy roots (left). Click image for an enlarged view.	Figure 12. White adult soybean cyst nematode females (arrows) on a soybean root. (Used by permission of G. L. Tylka) Click image for an enlarged view.

Although the majority of plant-parasitic nematodes feed on the roots of plants, there are some that feed on above ground plant parts. Two important types are the foliar nematodes (Aphelenchoides) and stem and bulb nematodes (Ditylenchus dipsaci). Two important diseases of rice, white tip and ufra, are caused by foliar-feeding nematodes. The nematodes swim up from the soil in water films when plants are wet.

Figure 13. V-shaped necrotic lesions caused by Aphelenchoides ritsemabosi. Click image for an enlarged view.	Figure 14. Panicle distortion typical of ufra, caused by Ditylenchus angustus. Click image for an enlarged view.

Feeding nematodes use water, minerals, and nutrients made by the plant during photosynthesis that would otherwise be available to support plant growth. Nematodes produce damaging enzymes and other disease-inducing compounds. Such damage causes stunting of plant growth, reduced crop yields, and may lead eventually to plant death.

Figure 15. Aerial view of soybean cyst nematode damage in a soybean field. The yellow areas are soybean plants infected by the soybean cyst nematode. (Courtesy G. L. Tylka). Click image for an enlarged view.

Plant-parasitic nematodes may attack plants together with other plant-pathogenic microorganisms, such as fungi. The result is often synergistic, in that the incidence and severity of the disease caused when both organisms are present are much greater than that caused by either organism alone. An example of this is the nematode-fungus disease complex caused by root-knot nematodes and Fusarium wilt on tomato plants. Some plant-parasitic nematodes transmit viruses; they acquire the virus when feeding on a virus-infected plant and later pass the virus on when they feed on other plants.