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Meloidogyne (Goeldi, 1892)
Descriptions and taxonomic history important - name changes and taxonomic uncertainty negated much of the physiological and ecological early work.
The name Meloidogyne is derived from two Greek words meaning "apple-shaped" and "female".
Now host range test (J.N. Sasser), chromosome counts (Triantaphyllou), juvenile head structure (Eisenback), protein (gel electrophoresis patterns -Esbenshade) and DNA patterns are used to separate species. Many of these approaches came out of International Meloidogyne Project.
North Carolina (Sasser) Differential Hosts Test for four common species of Meloidogyne
|Meloidogyne species and races||Cotton (Deltapine 61)||Tobacco (NC 95)||Pepper (California Wonder)||Watermelon (Charleston Gray)||Peanut (Florunner)||Tomato (Rutgers)|
Note that the current 75+ spp. probably include some that were lumped in the original 1 species and in Chitwood's 4 spp., hence compounding concern that some nematologists have with taxonomic revisions; e.g. McKenry and Lamberti - a dilemma.
Slide show on Meloidogyne spp.
Female has 2 ovaries, prodelphic; adults swollen; eggs deposited in matrix secreted by six rectal glands, eggs not retained in female body.
Female body does not form cyst. Cuticular striations in posterior of female form a fingerprint-like perineal pattern.
Overlap of esophageal glands over intestine
Sedentary obligate parasites of roots, usually forming galls.
[Note predominance of parthenogenisis - is this associated with endoparasitism, in contrast to semi-endoparasites - is amphimixis more common in species with small galls; e.g., M. hapla)?; how about M. chitwoodi?]
International Meloidogyne Project focused on this single genus - J.N. Sasser, US-AID, from about 1975 to 1985.
Objectives of the project:
There are about 75 described species in the genus; 68 species listed by Luc, Maggenti, et al., 1988..
Genus has world-wide distribution, and is the most widely recognized plant-parasitic nematode because it causes characteristic galling symptoms.
All species are C-rated pests in California, except M. naasi and M. chitwoodi which are B-rated.
J2 are attracted to the root tip in the zone of elongation. They are also attracted to areas of lateral root emergence.
They are attracted by CO2, and apparently by small molecules that are dialysable - perhaps amino acids. Recent studies suggest that the attraction may not be to CO2 per se but to lower pH resulting from carbonic acid formed from the CO2 in solution. When CO2 is injected on a surface of agar that is buffered to prevent pH shift, the CO2 appears less attractive (V.M. Williamson, pers comm).
Detailed studies have been conducted in the model plant system, Arabidopsis, by Wyss (Nematologica 38:98-111).
J2 penetrates zone of elongation by mechanical (stylet thrusts) and probably chemical (cellulase and pectinase) means. It moves between, rather than through, cortical cells towards root apex, turns at the meristem, and migrates back to the vascular cylinder in the zone of cell differentiation. (Heterodera J2 moves through cells directly to vascular tissue).
The J2 penetrates cells with the stylet and initiates the giant cell from potential vascular tissue. Subventral glands become prominent 2 days after penetrating root. The subventral glands are most visible and active in J2; they shrink and atrophy as nematode becomes an adult - so what is their role? There is strong evidence that they produce enzymes responsible for giant cell initiation even though their structure suggests that their secretions would pass back into intestine.
As nematode grows and molts, the dorsal esophageal gland (DEG) becomes more prominent and secretions are thought to stimulate multinucleate giant cell. Secretions are packed in secretory granules, 800nm diameter. The DEG canal is a cytoplasmic extension of the single-cell DEG and opens into the lumen of the esophagus just behind the stylet. The secretions of the DEG are injected into the cell. If the nematode is removed, the giant cell atrophies.
There are ongoing attempts at chemical analysis of secretory granules; also the ability of the nematode to initiate giant cells following laser surgery of the glands has been investigated. Monoclonal antibodies have been used to identify sources of nematode secretions.
Note the issue of definition - a syncytium is formed by the breakdown of neighboring cell walls to form a multinucleate cell. However, the Meloidogyne spp. feeding sites are giant cells induced by synchronous mitosis without cell wall deposition (Endo in Vistas on Nematology) - following initial findings of karyokinesis without cytokinesis by Huang and Maggenti. Karyokinesis without cytokinesis, also known as acytokinetic mitosis, might be regarded as a specific form of endoreduplication, the process of an interupted and repeated cell cycle that results in increase in the number of chromosomes and consequently the amount of DNA in a cell.
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.
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, including activation of the salicylic acid pathway, must be suppressed. The salicylic acid pathway leads to production of active oxygen molecules and hypersensitive cell death. In the Meloidogyninae, a possible candidate for effector-triggered suppression of PTI is chorismate mutase, produced in the nematode esophageal glands. In PTI responses, chorismate is converted to salicylic acid to iniate the defense events. Chorismate mutase from the nematode reduces chorismate, and thus salicylic acid (Smant and Jones, 2011).
Increase in tissue size varies with both plant and nematode species - M. hapla and M. incognita cause different sized galls on tomato. Mundo and Baldwin showed that, in the Heteroderinae, syncytium formation and number varies with nematode species and genus in the same plant (note they were not working with Meloidogyninae).
Plant growth regulators occur at higher levels in galled tissue. Both auxins (promoters of cell growth) and cytokinins (promoters of cell division) have been implicated - roots of susceptible tomato cultivars contain higher levels of cytokinin than resistant roots. Cytokinins are also reported from eggs, juveniles and adults; however, this may not be a source, but a result of feeding. When cytokinins are applied to nematode-resistant plant roots, resistance may be partially or completely reversed; e.g., Nemaguard rootstocks in peach.
Yellowing; mid-day wilting; symptoms of water and nutrient stress; sometimes death, especially if interacting with other organisms; gall formation (note convenience of bioassay - ease of study but cautionary implications in screening for resistance); root branching; J2 and eggs in soil.
As a genus, they are reported as parasites of over 3000 host plants, and individual species often have a wide host range. Jensen et al. (1977) listed some 874 crop species as hosts of the 7 or 8 species commonly occurring in the western U.S.
Extreme differences in host range occur within the genus. Meloidogyne incognita is extremely polyphagous, with a host range of up to 3,000 plant species, while M. megatyla and M. pini are restricted to Pinus spp. (Castagnone-Sereno, 2002; Jepson, 1987).
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
Life stages of Meloidogyne spp. Infective J2 on left, young female on right. Most of the growth occurs during the second stage.
Mature Meloidogyne female (on head of pin for size perspective).
Meloidogyne male still coiled within the J4 cuticle.
Third and 4th stage within 2nd stage cuticle, passed fairly rapidly, no stylet, do not feed. Usually 4-500 eggs per egg-mass, Tyler reported a high count of 2,800.
|Mass invasion of second-stage juveniles in a grape root. Only a few will establish a feeding site at this location.|
|Mature female at feeding site with egg mass on root surface.|
|Enlarged metacorpus of adult female.
Photo by Hussey
Life cycle diagram by Charles Papp, CDFA
Orion discovery of cellulases in egg-mass matrix - suggests that a hole is enzymatically digested to the root surface by the developing egg-mass. Nematodes are thought to have acquired cellulases via horizontal gene transfer from bacteria.
Sexual differentiation starts in late 2nd stage, heart shaped gonad. Sex reversal can occur under adverse conditions resulting in males with two testes (Triantaphyllou et al.).
Nematode exhibits a high reproductive rate.
Melakerberhan and Ferris - increase in body weight 250 fold, from 0.11µg for J2 to 300µg for total weight of female and egg mass. Total energy demand = 1 calorie, but consider repair costs, increased root metabolism, leakage, control of partitioning - effects which outweigh that of feeding.
|Feeding site||Multinucleate syncytium||Multinucleate giant cell|
|Reproductive strategies||Sexual||Mainly parthenogenic|
|Eggs||Mainly retained in female body||Deposited in egg mass|
|Female body||Becomes hardened cyst||Does not form cyst|
|Hatching factors||From host root exudates||Favorable environmental conds.|
|Cell cycle||Endoreduplication||Acytokinetic mitosis andendoreduplication|
See mechanistic details in Feeding section.
|Meloidogyne spp. damage in grape vineyards in California|
Number of vegetable cultivars with resistance to common species (Fassuliotis, 1976).
|M. arenaria||M. hapla||M. incognita||M. javanica|
These numbers are now higher, but the proportions are similar.
The Mi gene is a single dominant gene that confers resistance to M. incognita, M. javanica, and M. arenaria. It is located near the centromere of chromosome 6. Bailey (1940) provided an early report of the wild tomato species Solanum peruvianum as a source of resistance to root-knot nematodes. Due to reproductive incompatibilities between the Solanum lycopersicum and S. peruvianum, embryos resulting from crosses do not reach maturity. Consequently, techniques for embryo rescue techniques were developed in which immature embryos are dissected from seed and cultured axenically. The technique appears to have been first used to transfer the Mi gene from wild tomato into commercial cultivars by Smith (1944) in crossing Solanum lycopersicum var. Michigan State with S. peruvianum PI128.657.
Dr. Charles Rick and colleagues at UC Davis discovered that an isozyme, acid phosphatase, is coded by the gene Aps-1 which is located on chromosome 6 of tomato close to, and tightly linked with, Mi (Rick and Fobes, 1974). The isozyme provides a tool for tomato breeders to determine whether they have successfully transferred Mi into commercial varieties and has facilitated the development of processing varieties with root-knot nematode resistance.
The Mi gene has been cloned and sequenced in the laboratory of Dr. Valerie Williamson at UC Davis. Using Agrobacterium as a carrier, the resistance gene has been transferred to a susceptible tomato cultivar, which expresses the resistance. Plants grown from seeds of the transgenic plant are also resistant to M. incognita. However, after the second generation of plant offspring, the expression of resistance is progressively reduced in seed batches from some plants but not from others. In both cases, the gene is still present and is still coding for RNA (Goggin et al, 2004).
The resistance conferred by the Mi gene breaks down at soil temperatures >28C.
With repeated use of the single source of resistance in California tomato production, aggressive strains of the nematode are being selected (Kaloshian et al. 1996).
In the early 1990s, farm advisors and entomologist Dr. Harry Lange noticed that tomatoes with the Mi gene appeared to be also resistant to the potato aphid, Macrosiphum euphorbiae. Initial determination was that a gene tightly linked to Mi and designated Meu1 was responsible for the potato aphid resistance. Current research indicates, however, that the two genes are identical and that Mi confers resistance to both root-knot nematodes and the potato aphid. A more recent development is the discovery that the Mi gene also confers resistance against the white fly Bemisia tabaci (Nombela et al., 2003). The gene is located near the centromere of tomato chromosome #6.
As with the resistance to M. incognita, the resistance to the potato aphid is also progressively reduced after the the second generation of plant progeny (Goggin et al, 2004).
Bailey, D.M. 1940. The seedling test method for root-knot nematode resistance. Proc. Amer. Soc Hort. Sci. 38:573-575.
Castagnone-Sereno, P. 2002.
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.
Goggin FL, Shah G, Williamson VM, Ullman DE. 2004. Instability of Mi-mediated nematode resistance in transgenic tomato plants. Molecular Breeding 13:357-364.
Kaloshian, I., V.M. Williamson, G. Miyao, D.A. Lawn and B.B. Westerdahl. 1996. "Resistance-breaking" nematodes identified in California tomatoes. California Agriculture 50(6):18-19.
Nombela, G., V. M. Williamson, and M. Muniz. 2003. The root-knot nematode resistance gene Mi-1.2 of tomato is responsible for resistance against the whitefly Bemisia tabaci. Mol. Plant Microbe Int. 16:645-649.
Rick, C.M. and Fobes, J.F. 1974. Association of an allozyme with nematode resistance. Rep. Tomato Genet. Coop 24:25.
Smant, G., Jones, J. 2011. Suppression of plant defences by nematodes. Chapter 13, pp 273-286. In Jones, J., Gheysen, G., Fenoll, C. (eds). Genomics and Molecular Genetics of Plant-Nematode Interactions. Springer, NY.
Smith, P.G. 1944. Embryo culture of a tomato species hybrid. Proc. Amer. Soc Hort. Sci. 44:413-416.
Wyss, U. (Nematologica 38:98-111).