|Morphology and Anatomy||Life Cycle|
|Return to Heteroderidae Menu||Feeding||References|
|Go to Nemaplex Home Page|
Tylenchida Tylenchina Tylenchoidea Heteroderidae Heteroderinae Heterodera (Schmidt, 1871) Synonyms: Tylenchus (Heterodera) (Schmidt, 1871) Heterodera (Heterodera) (Schmidt, 1871) Heterobolbus (Railliet, 1896) Bidera (Krall' and Krall', 1978) Ephippiodera (Shagalina and Krall', 1978)
Refer to Subfamily Diagnostics (Heteroderinae) for taxonomic history and distinctions among related genera.
Slide show on Globodera and Heterodera spp.
Cyst of Heterodera glycines
|Sexual dimorphism; female swollen; males vermiform.
Female body forms a cyst.
Females: anterior neck-like region; swollen body shape - lemon, round, or pyroid shape (about 0.5-0.6 mm diam).
Weak cephalic framework.
Moderate stylet with small rounded knobs.
Metacorpus enlarged and fills neck region.
Diovarial; prodelphic; ovaries coiled or reflexed.
Vulva subterminal; anus terminal.
Posterior region important taxonomically:
Pre-parasitic stage: vermiform J2, 300-500 µm long.
Heavily sclerotized head framework; head offset.
Stylet prominent with anteriorly-directed knobs.
Ventro-lateral overlap of esophageal glands over intestine.
Genital primordia visible.
Post-parasitic stages: swollen.
Stylet weak, sometimes not visible in 3rd and 4th stage.
Developing gonads visible.
Males: Vermiform, 1-1.5mm length.
Sclerotized cephalic framework, rounded head cap.
Strong stylet; knobs project forward.
Esophagus tylenchid, overlaps ventro-laterally.
Reproductive system monorchic.
Tail bluntly rounded.
[Ref: H. Ferris.]
By 2009, more than 40 species of the genus Heterodera had been molecularly characterized by sequencing the ITS-rRNA genes and by PCR-RFLP profiles. These tools are so far the best available for identifying cyst-forming nematodes.
By restricting the lTS amplicons with one or a combination of seven restrietion enzymes (AIuI, Aval, Bsh 12361, BsuRI, C/ol, MvaI, and Rsal), h most of the agriculturally important eyst nematode species can be distinguished.
When it is not possible to use sequences of ITS-rRNA genes and PCR-RFLPs in diagnostic work, morphometrie eharaeteristies are still useful.
Intraspeeifie polymorphism in the lTS sequences can make identification diffieult and more eonclusive moleeular identification tools are needed.
(Ref: Waeyenberge et al., 2009)
Worldwide, usually with the definitive hosts of each species.
Juveniles enter root to region of developing vascular tissues by direct penetration of cells. Juveniles may feed from individual cells as they cut through cell walls while migrating to permanent feeding site region.
The permanent feeding site is a syncytium which is stimulated in a cortical or endodermal cell. The syncytium becomes multinucleate after 24 hours as adjacent cells merge. Cell wall dissolution is through a combination of physical stress (nematode head movement) and chemical action.
Syncytia associated with developing males are usually smaller than those associated with females.
Large sectors of the developing root, including areas that would have become vascular tissue are transfomed into syncytia. Syncytia have many plastids, mitochondria, ribosomes, increased rough endoplasmic reticulum and enlarged lobed nuclei.
Cell wall protruberances increase the surface area of the cell membrane for flow of solutes from the xylem to the syncytium - the transfer cell configuration (Endo, 1975).
More than 50 genes are upregulated to some extent in the development of giant cells (Meloidogyne) and syncytia (Heterodera/Globodera) (Gheysen and Fenoll, 2002). Both types of feeding cells have the genome amplified as a result of multiple shortened cell cycles; but the processes differ. Giant-cells go through repeated (acytokinetic) mitosis. Syncytia undergo repeated S-phase endoreduplication without mitosis or nuclear division.
The eukaryotic cell cycle has four stages.:
1. Nuclear DNA is replicated during synthesis phase (S-phase).
2. DNA synthesis is followed by an interval called the G2 phase (G=gap).
3. Mitosis occurs, the nucleus divides (M-phase).
4. The interval between the completion of mitosis and the beginning of DNA synthesis is the G1-phase,
In normal cell division, the cell divides (cytokinesis) after the mitosis phase.
In the root-knot nematode (Meloidogyne) feeding site there is repeated nuclear division (S and M phases of the cell cycle) but no cell division; this is called acytokinetic mitosis or karyokinesis without cytokinesis.
In the cyst nematode (Heterodera, Globodera) feeding site, the S phase of the cell cycle is activated but not the M phase. Instead, the cells repeatedly go through the S-phase (endoreduplication) and probably through parts of the G1 and G2 phases, but bypass mitosis.
|The Cell Cycle: modified from Gheysen and Fenell, 2002.|
Mundo and Baldwin showed that, in the Heteroderinae, syncytium formation and number varies with nematode species and genus in the same plant.
Since nematodes in the Heteroderidae become sedentary from the late second stage onwards (except for the metamorphosis to males), the feeding site in the plant must be maintained in a condition favorable for perhaps five or six weeks to allow the nematode to fulfill its reproductive potential. Besides stimulation of the cell cycle events, pathogen-triggered immunity (PTI) responses must be suppressed. The Hg30C02 effector protein of Heterodera glycines which may be involved in active suppression of host defenses. The same gene occurs in H. schachtii (Smant and Jones, 2011; Hamamouch et al., 2012).
Several forms of delayed hatch and diapause exhibited.
1. Host-mediated hatch in response to host root exudates. Vary in
degree of response to hatching factors:
a) Very sensitive - hatch in water only 5% of that in host root
G. rostochiensis, H. carotae, H. cruciferae, H. humuli
b) Intermediate - hatch in water 10-50% of that in diffusate.
H. schachtii, H. trifolii, H. galeopsidis.
c) Insensitive - apparently not stimulated by diffusate.
H. goettingiana, H. avenae, H. glycines. H. avenae requires
a chill period to break diapause.
Artificial hatching agents include Aminoacridine (Rivanol) and ZnCl for H. schachtii.
Ecological factors also influence hatch: temperature, moisture, aeration,
osmotic potential, pH, etc.
For an extensive list of host plant species and their susceptibility to this genus, copy the genus name
select Nemabase Genus Search and paste the name in the Genus box
J2 enters near root tip and takes position in cortex with head near vascular cylinder. Moves intra- and intercellularly towards vascular tissues.
During development, female breaks through cortex to surface so that most of the body of adult female remains outside root.
Sugarbeet cyst nematode molts at 6, 12, and 15 days after entering root; matures in 19 days at 25 C.
Males are needed for reproduction in most species.
Eggs are retained in cysts, but some are deposited in egg masses in many species. Distended uterus enlarges to fill body; eggs are packed in mucoid mass.
Cysts undergo a color change as they mature - from white to brown - due to action of polyphenol oxidase on polyphenols in the cyst wall. The wall remains permeable to chemicals and dissolved oxygen.
Cyst drops-off of root when dead; mucoid packing disappears.
|Life cycle diagram by Charles S. Papp, CDFA|
H. schachtii - about 2 generations per year in Europe.
about 3 generations per year in northern California.
about 5 generations per year in the Imperial Valley.
H. glycines - about 5 generations per year in North Carolina.
Little mechanical injury due to the parasitism, body of female on outside of the root and no cell division stimulated.
Branch rootlets may be stimulated near the point of infection.
General debilitation and reduction in efficiency of the root system. Chlorosis, stunted growth, wilted plants.
Management strategies vary with species and biology.
Management of this genus is usually difficult due to prolonged viability. Possibilities include prevention, crop rotation, soil fumigation, use of resistant varieties, and use of clean seed sources.
Endo, B. 1976. In Vistas on Nematology
Gheyson, G. and C. Fenoll. 2002. Gene expression in nematode feeding sites. Ann. Rev. Phytopathol. 40: 191-219.
Waeyenberge, L., Viane, N., Subbotin, S.A., Moens, M. 2009, Molecular identification of Heterodera spp., an overview of fifteen years of research. Pp 109-114 in Riely, I.T., Nicol, J.M., Dababat, A.A. First Workshop of the International Cereal Cyst Nematode InitiativeCereal cyst nematodes: status, research and outlook. CIMMYT.