Nematodes Parasites of Higher Plants Rev 10/

Plant parasitism evolved independently in three traditional nematode orders, Dorylaimida Pearse 1942, Triplonchida Cobb 1920 (both Adenophorea), and Tylenchida Thorne 1949 (Secernentea). In the Tylenchida, ribosomal RNA analysis suggests that plant parasites have evolved within clades that include many free-living nematodes (Baldwin et al, 2004). Insight from molecular phylogenies based on the nuclear small subunit ribosomal RNA indicate that the adenophorean Dorylaimida and Triplonchida lack a common ancestor. Tylenchida shares a most recent common ancestor with Cephalobina (primarily bacteriovore Cephaloboidea). Triplonchida appears to be the sister clade to the Enoplida Filipjev, 1929 (primarily marine omnivores and bacteriovores), and Dorylaimida are embedded within a clade that includes insect parasites (Mermithida), vertebrate parasites (Trichinellida), and predators (Mononchida, and other Dorylaimida). It cannot be assumed that plant parasitism arose only once in each order (Baldwin et al, 2004).

Anatomy of Plant-feeding Nematodes (Order Tylenchida)

Classification of plant parasites by feeding habit:

1. Ectoparasites of plant roots
     a. Migratory
     b. Sedentary
2. Semi-endoparasites of plant roots
    a. Migratory
    b. Sedentary
3. Endoparasites of plant roots
    a. Migratory
    b. Sedentary

4. Endoparasites of bulbs and stems

5. Endoparasites of leaves

6. Endoparasites of seeds

Definitions:

Migratory ectoparasites: Remain outside the root or other feeding area throughout the life cycle and feed on epidermal cells or on cells deeper in the root. Retain the ability to move to new feeding sites.

Sedentary ectoparasites: Remain outside the root or other feeding area throughout the life cycle but feed from a modified nurse cell or group of nurse cells for an extended period.

Migratory endoparasites: Enter the root system or other feeding area and feed from cells as they migrate.

Sedentary endoparasites: Enter the root system or other feeding area and feed from highly modified cells. Lose the ability to move and must maintain an active feeding site.

Migratory and sedentary semi-endoparasites: Subcategories of the above with only the anterior portion of the body within the root and the remainder on the root surface.


Feeding Habits of Nematodes on Plant Roots	Sedentary Forms of Root-feeding Nematodes
Drawings by Charles S. Papp, CDFA

Commonalities

Plant-parasitic nematodes with postcorpus (digestive glands) bounded by a membrane are all ectoparasites;
All genera that feed or that can feed endoparasitically have overlapping esophageal glands with dorsal gland usually more developed.
Nematodes with long stylets are ectoparasites; those with short stylets may be ectoparasites or endoparasites.
Nematodes with long stylets feed several cell layers deep and do not move frequently to new feeding sites; hence they are relatively sedentary.
Nematodes with long stylets and those that use the stylet actively to penetrate cells have a well-developed headframe.
Metacorpus size and feeding habit or feeding rate??
No apparent relationship between number of gonads and feeding habit.
No apparent relationship between mode of reproduction and feeding habit, although parthenogenesis is common.
Caudal alae and feeding habit??
Tail length (c') and number of ovaries??
Tail length and feeding habit??

Ref: Bongers (1988), Ferris and Bongers (communication), Luc et al.

Generalizations

Plant-feeding nematodes that have long bodies (>2mm):

are ectoparasites (or feed in natural channels in the root).
have long stylets and feed in deeper root tissues.
are sedentary ectoparasites and stimulate nurse cells.
have low ovary to body volume ratios and partition more assimilates to maintenance of somatic tissues
have lower fecundity than short-bodied nematodes.
are susceptible to mechanical soil disturbance.
inhabit coarse-textured soils or structured soils with high aggregate stability.
are components of structured and mature food webs.

Community Structure of Nematodes Parasites of Higher Plants:

Generalizations

A diversity of plant-feeding nematodes with different feeding habits and morphotypes occurs in nutrient-enriched agricultural systems.
In nutrient-enriched systems, plant-feeding nematodes are often in great abundance.
In natural, non-enriched systems, abundance of plant-feeding nematodes is usually much lower than in agricultural systems.
In natural, non-enriched systems, plant-feeding nematodes with small bodies and high ovary:body ratios predominate.
In natural, non-enriched systems, plant-feeding nematodes with long stylets, sedentary lifestyles, nurse cells and other features that increase the access to limited resources, predominate.
In natural, non-enriched systems, narrow host-range specialists rather than wide host range generalists, predominate.

Conversations among Ferris, Tom Bongers, Bart Verschoor, Ron de Goede.

References

Baldwin, J.G., S.A. Nadler and B.J. Adams. 2004. Evolution of plant parasitism among nematodes. Ann. Rev. Phytopathol, 42:83-105.

Plant-feeding Nematodes of the Phylum Nematoda
Class Secernentea		Class Adenophorea
Tylenchida		Dorylaimida	Triplonchida
Tylenchina Tylenchoidea Anguinidae Belonolaimidae Dolichodoridae Heteroderidae Hoplolaimidae Pratylenchidae Tylenchidae	Aphelenchina Aphelenchoidea Aphelenchoididae Aphelenchidae	Dorylaimina Dorylaimoidea Longidoridae	Diphtherophorina Diphtherophoroidea Trichodoridae
	Tylenchina Criconematoidea Criconematidae Tylenchulidae		Classification by Trophic Level
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Taxonomic classification of plant parasites:

Plant-feeding Nematodes of the Phylum Nematoda

Class Secernentea

Class Adenophorea

Anatomy of Plant-feeding Nematodes (Order Tylenchida)

Classification of plant parasites by feeding habit:

Definitions:

Commonalities

Generalizations

Community Structure of Nematodes Parasites of Higher Plants:

Generalizations

References

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Nematode Parasites of Higher Plants	Nematode Parasites of Animals and Humans