There is probably
only one nice thing to be said about the soybean cyst nematode (or
Heterodera glycines): it has a very interesting life cycle.
The soybean cyst
nematode begins life in the soil. It hatches from its egg case (A)
along with several hundred brothers and sisters and begins to migrate
through the soil in search of a soybean plant. When it finds a soybean
root, the nematode uses its stylet to puncture the root and secrete
enzymes that degrade cell walls as it moves through the root towards
the vacular tissue of the plant (B). There the nematode stops and
selects a cell to transform into its feeding site by secreting molecules
from the esophageal gland cells (C). (This feeding site is called
a syncytium.) Once it starts feeding, the nematode becomes sedentary,
or immobilized, in the root. (This is where it gets the name “sedentary
endoparasite.” The term “endoparasite” refers to
the fact that the nematode gets all the way into the root. This is
in contrast to other parasitic nematodes that feed from the exterior
of the root.)
As the nematode
feeds, its body begins to swell with each molt. A female nematode
becomes so large that she eventually breaks through the root, exposing
herself on the surface of the soybean (D). Besides becoming larger,
her body turns from white to yellow and eventually brown as she matures.
Male nematodes, in contrast, migrate back out of the root and into
the soil, where they will fertilize the females and then die (E).
The female, on the other hand, remains attached to the root system,
where she will continue to feed and will begin to produce eggs. Unlike
other nematodes that produce an egg sac outside the body, the female
soybean cyst nematode keeps the majority of her eggs inside her body.
Once she dies, her body cavity - filled with several hundred eggs
- hardens to become a cyst (F). At this point, these cysts are actually
visible to the naked eye as tiny yellow balls on the roots of the
soybean plant. Eventually, the cyst dislodges from the root and falls
off into the soil, where it will protect the eggs in the soil until
conditions are favorable to hatch. The whole life cycle takes about
thirty days, allowing it to go through multiple generations in a single
growing season.
