Nematoda

Introduction to Nematoda

NEMATODA: the phylum of ecdysozoans that contain members commonly either called roundworms or nematodes
nematodes are triploblastic, bilateral, vermiform, unsegmented, and blastocoelomates
many species are of agricultural or medical importance
there is a poor fossil record
recent molecular studies have placed Nematoda and its sister group Nematomorpha as the closest relatives of Panarthropoda

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most ancient reference of nematodes as human parasites in the Huang Ti Nei Ching
'fiery serpent' mentioned by Moses in the Bible could refer to the guinea worm
first discovery of free-living nematodes was in 1656 by Pierre Borel
- also noted by Robert Hook in 1667
first discovery of a plant parasitic nematode made by Needham in 1743

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EUTELY: the condition of having a body made up of a constant number of cells
apomorphies of Nematoda:
- four moults
- body musculature consists of four bands of obliquely striated longitudinal muscles
- excretory system consists of between one and five cells
- female genitals open at midbody
- males have complex copulatory structures
- aflagellate and amoeboid sperm
- potentially direct development
have a 3-layered cuticle
lack segmentation or appendages
lack a fat body/liver
body plan is described as a tube inside a tube, separated by a fluid-filled body pseudocoelom that forms a hydrostatic skeleton
main sense organs are a pair of lateral amphids near the front end of the animal
muscle arms can be found in all nematodes
no gas exchange system, but a complete digestive system
have neural innervation and neuromuscular junctions, as well as a circumoral collar brain and dorsal and ventral nerve cords

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the most abundant animal group, with around 0.4 sextillion nematodes found worldwide
most species are small, but some larger species can grow up to 8.4 metres long
over 28,000 species described, but possibly one million undescribed
three major clades: Enoplia, Dorylaimia, and Chromadoria
- Chromadoria contains Rhabditida
  - Rhabditida is split into three major clades of terrestrial and parasitic species: Spirurina, Tylenchina, and Rhabditina
can be bacterial feeders, fungal feeders, omnivores, predators, or plant feeders

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can be free-living or parasites
C. elegans is a soil nematode which is the model organism for developmental biology
- when it is disturbed by light it starts spitting
- light receptors of nematodes are taste receptors in insects
have high fecundity, and except in Enoplia, there is considerable variation in reproductive mode
- in some species, different reproductive modes are present in different generations
sex determination can be chromosomal or environmental
ANDRODIOECY: a reproductive system characterised by the coexistence of males and hermaphrodites
has 4 larval stages,and 4 moults, and start life as an egg
chromosomal diminution occurs during development

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soil nematodes feed on bacteria, fungi, algae, diatoms,and some are predators
- an important part of the topsoil mesofauna
- deep earth nematodes are the deepest living animals on the planet
- nematodes near hydrothermal vents grow chemoautotrophic bacteria in their gut and cuticle, which they can then feed on
predatory nematodes include parasitic nematodes
- parasitic nematodes have evolved independently at least 5x in vertebrates, 3x in plants, and 7x in invertebrates
- ENTOMOPATHOGENIC NEMATODES: nematodes that have evolved an association with insect-pathogenic bacteria
- most common human nematode infection worldwide is caused by Ascaris lumbricoides
- Filarioidea nematodes cause diseases known as filariasis
  - over 1 billion are at risk from infection by lymphatic filariasis
  - filarial nematodes are vector-borne
  - recent studies have implication Wolbachia as the cause of inflammatory-mediated filarial disease

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