Worms Have Mothers Too: Meet the Heteroderids

by Joseph DeSisto

Today’s story comes from another group of agricultural pests — not insects this time, but cyst-forming nematodes in the family Heteroderidae. What are nematodes, you ask? Nematodes are one of many animal phyla referred to as “worms.” These worms in particular are unsegmented, surrounded by a hard cuticle or exoskeleton, and often microscopic.

There are around 25,000 known species of nematodes, but as many as a million are likely undiscovered. Some are free-living and eat bacteria or other tiny animals, but most are parasites either on animals (including humans) or plants. The heteroderids belong to the last category — here are some:

A whole bunch of nematode cysts, among potato roots. Photo by Xiaohong Wang, in public domain.

A whole bunch of potato cyst nematodes, Globodera pallida. Photo by Xiaohong Wang, in public domain.

The wormy-looking things are not worms at all, but the roots of a potato plant. Instead, the worms are packed away in cysts, the tiny yellow spheres nestled among the roots.

Within each of these cysts is a cluster of eggs which, when conditions are right, will emerge as worms. If conditions do not support hatching, the eggs can survive in these cysts for up to 20 years! Below is a recently hatched cyst nematode, beside an unhatched egg. This image was taken with an electron microscope — the colors do not reflect the colors of the actual animals.

The soybean cyst nematode (Heterodera glycines). Photo from Agricultural Research Service, in public domain.

The soybean cyst nematode (Heterodera glycines). Photo from Agricultural Research Service, in public domain.

Although called “cyst nematodes,” the heteroderids are not unique in forming cysts. Lots of nematodes do it — the reason it’s so important to cook pork thoroughly is that parasitic nematode cysts can survive in meat long after the pig has been slaughtered. What’s unusual about many heteroderids is how and when they form their cysts, but to appreciate that we need to start at the beginning.

For now, let’s consider the soybean cyst nematode, the species in the image above (see Davis and Mitchum 2005). When the nematode first hatches, it is microscopic and barely small enough to burrow into the root. Once inside, it secretes chemicals that trick the plant into forming special tissues where the worm can feed.

After reaching a certain size, a male soybean cyst nematode leaves the inside of the root and migrates over the surface, looking for mates. The female is lazier — not leaving her special feeding site, she eats and grows until her body expands so much that her rear end bursts out of the root, exposed to the soil. At this point she has eaten so much that if you were to dig up the soybean plant, you could see her exposed body without a microscope.

The soybean (Glycine max) is one of the most economically important crop plants in the world -- and the most important food plant for the soybean cyst nematode. Photo by Harry Rose, licensed under CC BY 2.0.

Originally from East Asia, the soybean (Glycine max) is now one of the most economically important crop plants in the world — and the most important food plant for the soybean cyst nematode. Photo by Harry Rose, licensed under CC BY 2.0.

She’s not done yet, though. With her rear end now outside the root and able to swell freely, the nematode “lays” her eggs into a jelly-filled cavity, still inside her body. This hardens to form an egg sac until finally, the nematode herself dies and her dried cuticle becomes the cyst’s protective skin. For the female cyst nematode, laziness becomes the biggest sacrifice a mother can make. Her eggs, numbering up to 400, can now survive for years until it’s time to hatch.

Soybean cyst nematodes can be pretty damaging to crops, but pale in comparison to the potato cyst nematode (Globodera pallida — see what I did there?). Nicol et al. (2011) estimate that just in the United Kingdom, yearly losses to pallida total roughly £50 million. Fields infested by the potato cyst nematode have reported crop losses of up to 50%. That’s a lot of potatoes. Thanks to strict quarantine protocols, the worm has not successfully invaded the United States, but pallida is so scary that a small outbreak in Idaho caused Japan to boycott U.S. potatoes for decades until imports were resumed in 2006.

Controlling nematodes is not easy, and this is especially true for cyst-formers. Not only are the cysts extremely resilient, the adult worms are hard to monitor and control as they spend their lives sheltered inside the roots of their host plants. Conventional pesticides, even those designed to kill nematodes, are seldom effective. Including all species, nematode damage to crops worldwide is estimated to cost $80 billion every year (Nicol et al. 2011).

A newly hatched cyst nematode, Meloidogyne incognita, entering the root of a tomato plant. Photo by William Wergin and Richard Sayre, licensed under CC BY 2.0.

A newly hatched cyst nematode, Meloidogyne incognita, entering the root of a tomato plant. The colors aren’t real — this was taken with an electron microscope. Photo by William Wergin and Richard Sayre, licensed under CC BY 2.0.

Modern science, however, may provide solutions. Last year the complete genome of the potato cyst nematode was sequenced, giving scientists a first look at the role genes play in the worm’s complex life cycle (Cotton et al. 2014). Cotton et al. were able to isolate the genes responsible for several key proteins, each vital to the ability of cyst nematodes to invade, manipulate, and feed on their hosts. Hopefully these discoveries will pave the way for newer, more effective methods of controlling pallida and its relatives.

For a more technical description of the heteroderid life cycle, I recommend Davis and Mitchum’s 2005 paper in Plant Physiology. If you enjoy reading about life histories and especially chemical ecology, you will find I have barely scratched the surface of what there is to know about these strange and amazing creatures.

Cited:

Cotton J.A., C.J. Lilley, L.M. Jones, T. Kikuchi, A.J. Reid, P. Thorpe, I.J. Tsai, H. Beasley, V. Blok, P.J.A. Cock, S.E. den Akker, N. Holroyd, M. Hunt, S. Mantelin, H. Naghra, A. Pain, J.E. Palomares-Ruis, M. Zarowiecki, M. Berriman, J.T. Jones, and P.E. Urwin. 2014. The genome and life-stage specific transcriptomes of Globodera pallida elucidate key aspects of plant parasitism by a cyst nematode. Genome Biology 15(3):

Davis E.L. and M.G. Mitchum. 2005. Nematodes: Sophisticated parasites of legumes. Plant Physiology 137(4): 1182-1188.

Nicol J.M., S.J. Turner, D.L. Coyne, L. den Nijs, S. Hockland, and Z. Tahna Maafi. 2011. Current nematode threats to world agriculture. In: J. Jones, G. Gheysen, and C. Fenoll (Eds.), Genomics and Molecular Genetics of Plant-Nematode Interactions (21-43). Netherlands: Springer.

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