Introduction to Nematodes
Place among living organisms
Phyum Porifera: sponges, colonies of cells without apparent
specialization of function
|multicellular animals not possessing a true
diploblastic -ectoderm and endoderm)
- Hydrozoa (e.g., Hydra)
- Scyphozoa (e.g., Aurelia,
- Actinozoa (e.g., corals)
unsegmented, triploblastic (ectoderm, endoderm
- Turbellaria (freeliving
flatworms, e.g., Planaria, Bipalium)
- Trematoda (parasites with
suckers, e.g., Fasciola hepatica -liver fluke, Schistosoma
- Cestoda (intestinal parasites
of vertebrates, have no gut and absorb nutrients directly into
tapeworms, Taenia); proglottids are asexually produced
buds in which eggs are produced and which can break off as egg sacs.
worms, but have circulatory system and anus; unsegmented,
- like nematodes, but no excretory or lateral chords; unsegmented,
||like nematodes but
hooked proboscis; unsegmented, triploblastic
||have ciliated disks
wafting food particles; may anchor at posterior end; have features
of platyhelminthes and nematodes. Among the smallest of the
metazoa; unsegmented, triploblastic
have features of Rotifera and Nematoda; unsegmented, triploblastic
||bristles on cuticle,
no cilia; have similarities with Rotifera and Gastrotricha; unsegmented,
dwellers; have posterior appendages; unsegmented, triploblastic
have tentacles; unsegmented, triploblastic
|Note: in the older literature all these phyla
were included as classes of the Aschelminthes, e.g., L.H. Hyman (1930s)
|All the above phyla are
unsegmented and have no coelom.
Other invertebrates (Annelida,
Arthropoda, etc.) are coelomate, and
- Unsegmented is the condition of lacking
- Segmentation or Metameric Segmentation is the
repetition of elements of the main organ systems of the body along the length
of the body. For example, in the earthworms (Annelida), each of the
externally visible rings marks a segment (or metamere) of the body that
contains a similar pattern of blood vessels, nerves, excretory organs,
external features, etc.; similar patterns are evident in Arthropoda. In
the Vertebrata, segmentation is most obvious during embryonic development; in
adults it is confined to mesoderm derivatives such as muscle and skeleton,
e.g., repetition of vertebrae and ribs.
- Germ Layers: The layers of cells that are
distinguishable in the embryo immediately after gastrulation. In most
animals, the layers give rise to similar tissues and organs.
Diploblastic animals, such as Coelenterata, have two layers, ectoderm and
endoderm. The ectoderm gives rise to external structures and the
endoderm to internal structures such as intestine and reproductive organs.
Triploblastic animals have a third layer, mesoderm, which gives rise to
muscle and skeletal structures and which surrounds the body cavity.
- Coelom, the main body cavity in which the intestine
is suspended. Surrounded entirely by tissues of mesoderm origin.
- Acoelomate: The condition of lacking a true
coelom. That is, the body cavity is not entirely surrounded by mesoderm.
- Pseudocoelomate: Possessing a body cavity that is not entirely surrounded
- Cephalization: The tendency for sensory structures and the nervous system
to be aggregated anteriorly.
Name of the Phylum
Nematoda and Nemata are variously used. Both Cobb and Chitwood supported
the contraction "nema", and the shorter phylum name. Current
argument is for standard use of Nematoda.
(i) unsegmented worms, basic elongate body shape
often have cuticular markings - annulation, striation
ratio of axes may change with stage and sex, loss of mobility
(ii) bilateral symmetry, radial symmetry superimposed anterior
(iii) Triploblastic - 3-cell layers - ectoderm is a cellular
hypodermis (see Bird and Bird, 1991 - prefer epidermis).
Endodermal central portion of alimentary canal,
ectoderm in anterior and posterior region. Mesoderm cells
(e.g., muscles) do not completely surround a body cavity -
pseudocoelomate. (Pseudocoelom is vaguely defined as a body
cavity not completely surrounded by mesoderm).
(iv) Cuticle - proteinaceous (not chitin) with outer lipid layers;
extends into body cavities. Secreted by hypodermis;
Usually molted 4 times.
(v) Hypodermis (epidermis) thickened into chords, thicker laterally
Excretory tubules in lateral chords, nerve cells associated with
Muscle groups between chords in four quadrants.
(vi) Movement - generally dorso-ventral
undulation, but 3-dimensions (flexibility)
- internal hydrostatic pressure counters
(vii) Excretory system is simple and tubular or epidermal glands, no cilia or flame cells
(viii) No respiratory system - surface-to-volume ratio is important
for gaseous diffusion
- consider relative to activity.
Note: surface of a cylinder: s=2.pi.r.l
volume of a cylinder: v=pi.r.r.l
s/v ratio for a vermiform nematode = 2/r
- as r increases, s/v decreases, independent of length.
surface of a sphere: s=4.pi.r.r
volume of a sphere: v=(4/3).pi.r.r.r
s/v ratio for a spherical organism = 3/r
- as r increases, s/v decreases, but is greater than for
a vermiform nematode of the same radius. However, movement
(ix) No circulatory system - note reversible effects of nemastatic
(e.g., carbamates Aldicarb)
(x) Sexes usually separate; sexual dimorphism - females with separate
anus and gonopore, male with common
cloaca for intestine and
gonad. Sexual dimorphism may also be exhibited in differences
in size and shape of males and females. Gonads tubular, single or double; Amoeboid sperm.
Parthenogenesis common - useful adaptation to parasitism,
but no genetic recombination. Males often few, or reduced
and do not feed.
(xi) Development by total cleavage of egg - no yolk. Direct develop-
ment, no metamorphosis.
(xii) Size range 0.2 mm to 9 meters. Most terrestrial forms <2 mm.
Paralongidorus maximus longest plant parasite, 12 mm.
(xiii) Trophic levels - primary consumers, secondary consumers,
decomposition food chains (biodegradation, env markers)
C:N ratios vs bacteria - mineralization.
Letters to the Editor Nematology
Newsletter (June 2000)
Phylum Nematoda or Nemata
I enjoyed reading the articles in Nematology Newsletter 45 (4), especially
the article on "The English word "Nema"
Revised." Since this
article draws again the attention to the existence of two names for the phylum,
I would like to make some comments and suggestions.
Maggenti (1981) in his book on General Nematology stated, "I will hold
to the concept that nematodes belong in a phylum of their own, Nemata, as first
proposed by Cobb, 1919, and reinstated by Chitwood
in 1958." Later, Maggenti
et al (1987) in their article "A reappraisal of Tylenchina (Nemata).
2. Classification of the suborder Tylenchina
Revue de Nématologie 10(2) drew our attention to the fact that nematodes, being
widely accepted as a phylum, have two different phylum names in use: Nemata
(Cobb, 1919) and Nematoda (Potts, 1932).
At the NATO Advanced Research Workshop on Morphological Identification of
Plant Parasitic Nematode Genera, Raleigh, June 1988, Maggenti
again raised the
problem of correct nomenclature and spelling upon phylum status recognition (see
Maggenti, 1988, Teaching Nematology: to read is to learn, pp. 313-322 in R.
Fortuner, ed. in Nematode Identification and Expert System Technology) and
convinced the attendants to use Nemata in the future and, so several of us did.
From that period on, you will notice an increase in the use of Nemata compared
to a level use of Nematoda (see for example Fundamental and Applied Nematology
However, there are no rules for higher taxa nomenclature. According to
Article 1 of the Code of Zoological Nomenclature, the code excludes taxa above
the family group. Further , Nemata appeared confusing for my non -nematologist
colleagues and also for other nematologists working in other fields, for example
on free-living marine nematodes. All textbooks on general zoology use Nematoda
as well as do most non-nematologists (for example in recent articles on
classification based on molecular data). Both terms for the phylum remained in
use in Fundamental and Applied Nematology for example, until volume 20, 1997 in
which only Nematoda appears upon action of the chief -editors who systematically
changed Nemata into Nematoda.
By writing the article on the English word nema in the newsletter showing a
preference for the word nema to nematoid or nematode, I believe it is time that
nematologists decide on the use of the phylum name. Since Nematoda is the most
commonly used name, although not the oldest one, I propose to indicate it as THE
NAME of the phylum to use. A statement could be given by editors of important
Looking forward to comments, I send you my best regards.
Koninklijk Belgisch Instituut voor Natuurwetenschappen
|How Humans are Related to Flies and Worms
22 April 2002 -- The most comprehensive genetic study
to date concerning the evolutionary relationships among the three animal
species whose genes have been completely sequenced--the human, the fruit
fly, and the nematode worm--has determined that the human species is
more closely related to the fruit fly than to the nematode. "We compared
100 genes that are common among these three species--the largest data
set ever used to address this question--and obtained a result that is
unambiguous," says S. Blair Hedges, an evolutionary biologist at Penn
State, whose research team includes other scientists from Penn State and
"Cost estimates for acquiring the human genome alone
range between 300 million and 3 billion," Hedges comments. "After
spending all that money and effort, we now should at least be able to
know for sure how these animals are related." These three species, which
were singled out for the extensive genome effort, each represent much
larger groups of animals: vertebrates are represented by humans,
arthropods are represented by the fruit fly, and nematodes are
represented by one species of nematode.
The results of the study by Hedges and his colleagues
overturn a popular recent hypothesis, based primarily on the study of a
single gene, and have important implications for research in fields such
as medicine and developmental biology. The study, published in the
current edition of the web-based journal
BMC Evolutionary Biology, also is expected to impact the content of
"About five years ago, the journal Nature published an
analysis of one gene common to these three species that inexplicably
persuaded many scientists to abandon the classic hypothesis of their
relationships, which was based on the long-standing method of comparing
structural similarities," Hedges says. The new hypothesis, named "Ecdysozoa,"
argued that fruit flies and nematodes are more closely related to each
other than to humans. The classic hypothesis, named "Coelomata," argued
instead that humans and fruit flies are more closely related to each
other than to the nematodes.
The name "Ecdysozoa" alludes to the fact that insects
(and other arthropods) and nematodes both shed their outer covering, a
process called ecdysis. The name "Coelomata" alludes to the presence of
a true body cavity (coelom) in humans and fruit flies, whereas nematodes
have a false body cavity (pseudocoelom).
"At first a few developmental biologists began using the
new Ecdysozoa hypothesis of species relationships in studies involving
the HOX genes, which control the development of body parts like fingers,
toes, and wings. Soon afterwards, a lot of people started using it and
now it is included in introductory biology textbooks even though it
hasn't been adequately tested," Hedges says. "Many scientists are
surprised by this uncharacteristically rapid abandonment of the
long-standing Coelomata hypothesis and acceptance of the new Ecdysozoa
hypothesis without the intense scrutiny that is typical in science,"
Hedges says. How textbooks arrange the relationships among HOX genes is
particularly important, the researchers say, because it has an effect on
how crucial events in the development of animals are understood by
future generations of scientists.
To resolve the controversy, Hedges and his colleagues
decided to apply that much-needed scrutiny by tapping into the wealth of
data now available in the completely sequenced genomes of the three
species. "We could have looked at more than the 100 genes we selected,
but it takes a long time to inspect the genes as carefully as we wanted
to in order to avoid the errors that can creep into automated analyses,"
comments Jaime E. Blair, a graduate student at Penn State and first
author of the paper. "You want to be sure, for example, that you are
comparing the nematode version of gene A with the human version of gene
A with the fruit fly version of gene A--not with the fruit fly version
of gene B."
The researchers also rigorously tested the genes to
eliminate a number of possible biases that could occur by chance, or by
natural selection, or for other reasons. "We did every relevant analysis
known to molecular evolutionists, and took every known precaution to
account for intrinsic biases in the data, "Blair says. Every test the
researchers performed supported the classic Coelomata hypothesis, not
the Ecdysozoa hypothesis. "With 100 genes, we could perform analyses
that you just can't do with a single gene, which simply doesn't give you
enough data," she explains.
Blair says most individual genes don't contain enough
amino-acid molecules to provide the amount of data necessary for
producing statistically valid relationship diagrams, known as trees.
Working with 100 genes allowed the researchers to work with groups of 10
or 20 genes stuck together--the equivalent of several thousand amino
acids--which they say is enough for the statistical analyses required to
obtain a significant result.
"We also ordered the groups from the 10 slowest-evolving
genes to the 10 fastest-evolving genes, which allowed us to test the
claim of the Ecdysozoa supporters that the slowest-evolving genes should
yield the correct tree because the faster-evolving genes incorporate
more opportunities for mistakes and biases," says Blair. "What we found,
instead, was that the slowest-evolving genes gave us the classic
Coelomata tree, not the Ecdysozoa tree, at 100-percent significance. In
addition, we analyzed the species of nematode designated by the
Ecdysozoa supporters as slow-evolving and found that it, too, rejects
the Ecdysozoa hypothesis," explains Blair. "Even the original emphasis
on shedding in Ecdysozoans has been misleading, because the structure
that is shed in arthropods is completely different from that in
nematodes, and some vertebrates such as snakes also shed their skin."
The nematode and fruit fly are among the most widely
used model organisms of medical and genetics researchers because they
can be bred very easily and produce new generations quickly. "A lot of
our understanding of human medicine is based on these species because we
can do experiments with them that you wouldn't do with humans," Hedges
says. Scientists' understanding of how those species are related will
determine how that history is reconstructed and whether a mutational
change is interpreted as relatively recent or ancient. "If you assume
that the nematode and fruit fly are more closely related than either is
to human, as under the Ecdysozoa hypothesis, we now know that you would
probably be making a mistake in understanding the history of the
mutations," Hedges explains.
The study impacts any field that is concerned with the
inheritance of traits in major groups of animals, Hedges says. "Besides
medicine and biology, these results also affect fields such as
astrobiology, where scientists need to know how complex organisms
developed on Earth to better understand how life develops elsewhere in
the Universe." At the same time, he cautions, "we could be completely
wrong. I prefer to view our result as the best supported, based on the
weight of the evidence, rather than as a proven fact. It is always
better to keep an open mind about these things, not to become married to
one hypothesis or another, and to let the data speak for themselves."
This research was supported by the National Aeronautics
and Space Administration (NASA) Astrobiology Institute and the U. S.
National Science Foundation. In addition to Hedges and Blair,
researchers involved in the study include Kazuho Ikeo and Takashi
Gojobori, of the National Institute of Genetics in Japan.
CONTACTS: Barbara K. Kennedy (PIO), 814-863-4682,
Source: Penn State University
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