All nematodes pass through an
embryonic stage, four
juvenile stages (J1–J4) and an adult stage. Juvenile
Meloidogynes parasites hatch from
eggs as vermiform, second-stage juveniles (J2), the first
moult having occurred within the egg. Newly hatched juveniles have a short free-living stage in the soil, in the
rhizosphere of the host plants. They may reinvade the host plants of their parent or migrate through the soil to find a new host root. J2 larvae do not feed during the free-living stage, but use
lipids stored in the gut. The
Arabidopsis roots are initially small and transparent, enabling every detail to be seen. Invasion and migration in the root was studied using
M. incognita. Briefly, second stage juveniles invade in the root elongation region and
migrate in the root until they became sedentary. Signals from the J2 promote
parenchyma cells near the head of the J2 to become
multinucleate to form feeding cells, generally known as giant cells, from which the J2 and later the adults feed. Concomitant with giant cell formation, the surrounding root tissue gives rise to a gall in which the developing juvenile is embedded. Juveniles first feed from the giant cells about 24 hours after becoming sedentary. After further feeding, the J2s undergo morphological changes and become saccate. Without further feeding, they moult three times and eventually become adults. In females, which are close to spherical, feeding resumes and the reproductive system develops. The relationship between rate of development and temperature is linear over much of the root-knot nematode life cycle, though it is possible the component stages of the life cycle, e.g. egg development, host
root invasion or growth, have slightly different optima. Species within the genus
Meloidogyne also have different temperature optima. In
M. javanica, development occurs between 13 and 34 °C, with optimal development at about 29 °C.
Gelatinous matrix Root-knot nematode females lay eggs into a gelatinous matrix produced by six rectal glands and secreted before and during egg laying. The matrix initially forms a canal through the outer layers of root tissue and later surrounds the eggs, providing a barrier to water loss by maintaining a high moisture level around the eggs. As the gelatinous matrix ages, it becomes tanned, turning from a sticky, colourless jelly to an orange-brown substance which appears layered.
Egg formation and development Egg formation in
M. javanica has been studied in detail, and is similar to egg formation in the well studied, free-living nematode
Caenorhabditis elegans. Embryogenesis has also been studied, and the stages of development are easily identifiable with a phase contrast microscope following preparation of an egg mass squash. The egg is formed as one cell, with two-cell, four-cell and eight-cell stages recognisable. Further cell division leads to the tadpole stage, with further elongation resulting in the first stage juvenile, which is roughly four times as long as the egg. The J1 stage of
C. elegans has 558 cells, and the J1 of
M. javanica likely has a similar number, since all nematodes are morphologically and anatomically similar.
Cyst nematodes, such as
Globodera rostochiensis, may require a specific signal from the root
exudates of the host to trigger hatching. Root-knot nematodes are generally unaffected by the presence of a host, but hatch freely at the appropriate temperature when water is available. However, in an egg mass or
cyst, not all eggs will hatch when the conditions are optimal for their particular species, leaving some eggs to hatch at a later date.
Ammonium ions have been shown to inhibit hatching and to reduce the plant-penetration ability of
M. incognita juveniles that do hatch.
Reproduction Root-knot nematodes exhibit a range of reproductive modes, including sexuality (
amphimixis),
facultative sexuality, meiotic parthenogenesis (
automixis) and mitotic parthenogenesis (
apomixis). ==Species==