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Life History
The stages in
the life cycle of plant parasitic nematodes start with the egg, the
embroynic as well as post-embryonic stages generally following more or
less a similar pattern in all the cases. After a series of transverse
and longitudinal cell divisions, the first-stage larva, lacking stylet
is formed. However, in the dorylaims (species of Trichodorus, Xiphinema,
Longidorus, etc.), a week stylet may de discernible. In majority
of the plant parasitic nematodes, except the dorylaim group, the first
moult occurs within the egg shell and it is only the second-stage larva
which normally emerges as the infective stage. In the dorylaim
nematodes, the first-stage larva itself emerges from egg followed by a
quick moult into second-stage larva.
Before
breaking through the egg shell, the second-stage larva moves within the
egg shell and probe the wall surface with the help of its stylet, for
locating a weak spot for continued stylet thrusts for puncturing the
wall. In other cases, enzyme secretions like lipase, chitinase, etc.,
may help in the weakening and dissolution of multilayered egg shell to
release the larva.
In the
endoparasitic species of genera Heterodera, Meloidogyne etc., the
second-stage larva enters the host roots and further life processes
proceed inside the roots. These larvae require feeding on host roots
before proceeding with the other developmental stages. In the case of
Rotylenchulus reniformis, however, the
Second-stage
larva does not require feeding for further development. Therefore,
further superimposed moult occur in the soil itself. In most of the
cases, a certain period of feeding may be necessary before the second
moult lakes place. This feeding period may be a short or prolonged one,
depending on the nematode species. In the case of Anguina tritici,
the second-stage larva requires at least 60 days feeding on growing
point of wheat seedings before it is able to moult to third-stage larva,
while in other cases it may vary from 3-10 days or more depending upon
the environment and availability of host1. Generally, feeding
is considered an essential pre-requisite to moulting. However, in some
cases, the larvae, after successive moults, retain the previous cuticle
(e.g., in Meloidogyne) and when this occurs, the nematode is not
able to feed and there is very little growth in size.
Normally,
there are three moults outside the egg before the larva becomes adult.
Thus, there are four moults, four larval stages and an adult stage. In
several species, both male and female adult nematodes occur in almost
equal numbers in the population. The role of the male nematodes is only
for copulation with the females. In these bisexually reproducing
species, sperm from the male is transferred to the female during
copulation. Fusion of sperm and egg occurs within the female body
resulting in the formation of an embryo. In some species, even though
the males may be present in the populations, they have not been recorded
participating in fertilization of females, which develop
parthenogenetically. Males of the most of the plant parasitic nematodes
are not known to feed on host root tissues.
The
second-stage larvae, in most of the cases, are the infective f stage
which then initiate, the life processes. In the case" of Rotylenchulus
reniformis, however, the pre-parasitic young female nematode is
the infective stage. The other nematode stages, in the case of
ectoparasites and also in some endoparasites like Pratylenchus
are also capable of causing infection.
More On
Life History
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FEEDING
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General Symptoms
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Above Ground
Symptoms
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Below Ground
Symptoms
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