FNZ 55 - Criconematina (Nematoda: Tylenchida) - Morphological characters
Wouts, WM 2006. Criconematina (Nematoda: Tylenchida). Fauna of New Zealand 55, 232 pages.
(
ISSN 0111-5383 (print),
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no.
55.
).
Published 24 Mar 2006
ZooBank: http://zoobank.org/References/088E84BD-3DE3-4EA2-B6D9-C45BBC0A12F1
Morphological characters
Identification of plant-parasitic nematode species is usually based on characters of the adult female. Characters used for the identification of Criconematina are: the body shape, characters of the body wall, lateral field, secretory–excretory pore, stylet, oesophagus, and reproductive system and the morphology of the lip region and the tail (Fig. 1a).
Shape and size of the body
In Criconematina the body shape of the female depends on the feeding habit of the species. In species where feeding causes the plant cell to disintegrate, as in Criconematidae, the females continuously search for new cells to feed on and remain slender. The body length of such females may be diagnostic if they reach extreme values for the genus. Species that cause the plant cell to transform into a feeding site that actively accumulates nutrients suitable for nematode development, as in Tylenchulidae Skarbilovich, 1947 (Kirjanova, 1955) and Sphaeronematidae, generally have swollen, sessile females. In females of swollen species cuticular markings, internal characteristics, and the shape of the head are usually grossly distorted and unsuitable for diagnosis of the species. Swollen females, therefore, generally have fewer diagnostic characteristics than females of slender species. Their offspring, however, are produced in large quantities over a short period of time, and are plentiful when females are available and their characteristics are easily accessible, uniform, and reliable, and more than make up for the loss of characters in the females.
Stylet
The stylet consists of a conus, shaft, and base, representing the conical anterior part, a tubular central piece, and the knobs at the base of the stylet respectively. It is typical for the suborder that the length of the shaft (including the base of the stylet) is independent of the length of the conus. Wouts (2000) established that in the subgenus Nothocriconemella Ebsary, 1981, the length of the shaft is directly related to the size of the base of the stylet — this may be true also for the other genera of Criconematidae.
Diagnostic characters associated with the stylet are: the length of the stylet, the size and shape of the base of the stylet, and the length of the posterior part of the stylet relative to its total length.
Body wall
In Criconematoidea and Hemicycliophoroidea, the annulation of the cuticle is very strong. The number of annules, and the frequency with which they intersect, especially in the lateral region, is often characteristic for the species. A transparent cover may surround the cuticle. This cover may vary from a tight, rather brittle, easy to remove layer visible only as indistinct extensions of the ornamentions of the annules, as is characteristic in Blandicephalanema, to the persistent permanent double cuticle in Hemicriconemoides Chitwood & Birchfield, 1957 and Hemicycliophora de Man, 1921. In Criconematidae the annules may be adorned with an ornate posterior edge varying from a simple irregular fringe to a complex series of rows of overlapping or alternating smooth or palmate scales or spines. In this family, generally, lateral fields are present only in the males. When a lateral line or a lateral field is present in the female, even if it is restricted to part of the body, it is diagnostic at species level. Deirids and phasmids are lacking.
Tylenchuloidea have a cuticle with fine transverse striation and may have deirids, mechanoreceptors, in the lateral field on the anterior part of the body at the level of the secretory–excretory pore. In Tylenchulidae the secretory–excretory pore is located posteriorly, close to the vulva, and produces a gelatinous substance into which the eggs are laid.
Phasmids, chemoreceptors regularly present laterally on the posterior part of the body of tylenchs, usually in the vicinity of the anus or on the tail, have not been reported in Criconematina but may be present in species of the genus Sphaeronema Raski & Sher, 1952 (personal communication D. Sturhan, 2001).
Lip region
The lip region usually extends from the anterior end of the body to the base of the cephalic framework and may be 1 to 3 annules long. The relative size of the individual annules and the space between them is diagnostic for the species. At the base of the cephalic framework a constriction of the body or the sudden narrowing or widening of an annule may set the lip region characteristically off from the rest of the body. The anterior view of the lip region generally consists of a low labial disc around the mouth opening with 6 pseudolips symmetrically around it. The various modifications of this basic arrangement, best observed through scanning electron microscopy, can be characteristic for the species. Submedian lobes, raised growths on either side of the lateral lips, are well-documented variations of the lip region and usually characteristic at the genus level.
Cephalic framework
The cephalic framework is a hexaradiate, rigid structure in the head of tylenchs for the attachment of the muscles that protract the stylet. The shape and density of the cephalic framework may be diagnostic for some taxa, but rarely identifies a species. The depth it extends into the body may be used to establish the height of the lip region and the number of lip annules (Orton Williams 1985).
Oesophagus – intestine
The oesophagus consists of the procorpus, metacorpus (= postcorpus, median bulb), isthmus, and basal bulb; it does not include the stylet, which is part of the stoma s.l. In the measurements, however, the length of the oesophagus is taken from the anterior end of the animal and encompasses the stylet. In Criconematidae and Hemicycliophoridae the procorpus has amalgamated with the median bulb. The median bulb contains a valvular structure for pumping secretions from the dorsal oesophageal gland into the host cell and subsequent sucking up the cell’s predigested contents. The distance the opening of the dorsal oesophageal gland is located behind the base of the stylet may be diagnostic for other tylenchs but not for Criconematina. The size of the metacorpus may be of diagnostic value. The isthmus is a tube-shaped connection between the median and the basal bulb. Its diameter is characteristic at the family level. A nerve ring surrounds the isthmus. The length of the oesophagus can be diagnostic. It is used to calculate the “b” value of the de Man ratios, which expresses the length of the oesophagus as a proportion of the total length and is universally used as a character state in the description of a new species.
The intestine is syncytial, lacking a lumen, the rectum is obscure and short and the anus is a small pore. The only character of significance amongst these is the position of the anus relative to the end of the body, which may be diagnostic in terms of number of tail annules (RA) and tail length.
Reproductive system
In the suborder Criconematina the female reproductive system is monodelphic/prodelphic without a remnant of a posterior branch being present. It consists of the ovary, oviduct, spermatheca, uterus, vagina, and vulva. Sperm in the spermatheca, as an indicator of the presence of males, may be diagnostic. The position of the vulva is a diagnostic character, both as a percentage of the total length (V) and in number of body annules anterior to the tail terminus (RV). The vulva may be open or closed and the anterior lip may overlap the posterior lip.
The male has a single testis. Spicules and a gubernaculum, structures facilitating copulation, are present. Their shape, size, and position have not yet been studied sufficiently to appreciate their significance in the diagnosis of species. The cloacal region may lie in the contour of the body or may exhibit a protrusion characteristic for a genus. The tail may be ventrally enclosed by caudal alae that are difficult to observe and without diagnostic significance for the species. Characters of the male generally are of little use for the identification of the species.