Nematode Pests of Horticultural Crops and their Management (1+1)

Biology of Plant Parasitic Nematodes

Biology of a typical plant parasitic nematode

• The life cycle of a typical nematode is simple and direct.
• It starts with a one celled egg, passes through four juvenile stages to finally convert in to a matured female/ male adult making it to total of six stages.

• Nematode Egg:

• The Nematode eggs are usually oval or elliptical in shape, enclosed in an egg shell and are deposited either singly or in an egg mass.
• Egg shell is secreted by the egg itself and comprise three layers. they are:
  

• Outer most vitelline layer made up of lipoprotein
• Middle chitin-protein membrane
• Innermost lipoprotein/only lipid membrane

• Oogenesis: Formation and maturation of egg is defined as oogenesis.

• After insemination, the sperms are stored in the spermatheca of the female.
• As the egg cell passes through spermatheca, it is fertilized.
• As soon as the sperm enters the egg cell, a fertilization membrane appears around the egg and the egg protoplasm contracts. The shell begins to form endogenously immediately after fertilization.
• Nematode embryo shows determinate cleavage.
• Perhaps nematodes are the only animals in which the first cleavage is equatorial, cutting the egg axis horizontally.

Embryogenesis starts with in a few hours after deposition.

• Embryogenesis:

Embryogenesis is the process through which a single celled egg transforms in to a multicellular embryo through a series of cell divisions.

• A series of transverse and longitudenal mitotic cleavages of the single celled egg leads to the formation of a bunch of cells, each with a predetermined function in body organization
  
• Once all the cell types have been differentiated,only one particular cell retains the full chromosome compliment that will form the gonads in future.
• The embryo at this stage is a hollow ball of cells, the Blastula.
• It is followed by gastrulation and reorganization which eventually produces a worm shaped embryo i.e. first stage juvenile with in the egg shell itself.
• Since destiny of each dividing cell is predetermined (i.e. which cell will form which organ), the cleavage is said to be ‘Determinate’.

• Eclosion/Hatching: (Greek word; e=out; clauses=shut i.e. shut out meaning hatching)

• Once the embryogenesis is complete, first or second stage juvenile (J1 or J2), as the case may be, emerges out of the egg shell.
• While in most of the secernenteans, first stage juvenile (J1) undergoes first moult within the egg, in most of the adenophoreans, J1 comes out of the egg and undergoes first moult to develop in to second stage juvenile (J2).
• Hatching in certain genera of plant parasitic nematodes occurs under normal suitable conditions, but some like Globodera rostochiensis require stimulus in form of root exudates from the host plant to hatch out. In Xiphinema, certain secretions from the oral aperture of nematode itself bring some structural changes in the egg which leads to its rupture.
• Once the juvenile is ready to hatch, it starts moving slowly within the egg.
• The process of breaking down of egg shell may be mechanical or enzymatic or both.
• The secretions from the oesophageal glands dissolve the lipid layer of the multilayered egg shell to facilitate the hatching process.
• Juveniles of Globodera rostochiensis repeatedly hammer their stylet tip mechanically to make a row of close perforations which merge to form a slit in the egg shell. It is through this slit that the juvenile emerges out of egg shell. Similarly, juveniles of Meloidogyne, Pratylenchus, Paratylenchus etc. use mechanical force to hatch out of the egg. Emergence is generally head first, though tail first is also not uncommon.

Figure 7.1 Hatching