Sybil A. Clark

Cuticle Structure in the Genus Heterodera

The cuticle structure of H. rostochiensis males and of the females of fourteen species or pathotypes of Heterodera was examined with light and electron microscopes. The layers of the male cuticle resemble those of the 2nd-stage larva and other vermiform Tylenchida so far described. The cuticle of females differs from this basic pattern in ways that fit their swollen form and sedentary habit, and their ultimate function as a protective cyst enclosing the quiescent eggs. The outer and inner layers, A and B, of males and larvae are supplemented by a third, fibrous layer, C, in lemon-shaped and round-cyst nematodes and by a fourth layer, D, in round-cyst nematodes only. The fibres in D are arranged helicoidally as in the chitin of insect endocuticle. The B layer of larvae and males is crystalloid, with disjunctions in the regular pattern at every half-annule. The periodicity of the pattern is 18-20 nm, similar to one of the forms of collagen. In females, the B layer ruptures as the body swells and ultimately occurs as separate islands at the junction of the A and C layers. Stained sections observed with bright field, phase, polarised light or interference microscopy, suggested that the thick fibre layers C and D are collagenous and that the A and C layers contain lipid. The composition and function of the various layers and the role of collagen are discussed. The differences in cuticle structure between the species with lemon-shaped and round cysts supports placing the round-cyst species (possibly excluding H. punctata) into the separate genus Globodera, already suggested by Skarbilovich as a subgenus.

Spicule structure in some Heterodera spp.

Observations with the scanning electron microscope on spicule structure of eleven species of Heterodera confirmed that the spicule tips in species of H. (Heterodera) were basically bifid whereas those of H. (Globodera) had single points. In all the species two small pores were located close to the tip of each spicule. Sections through the spicules of H. rostochiensis observed with the transmission electron microscope showed that they contained a large nerve which tapered towards the spicule tip, where two dendritic elements were associated with the pores. The shape of the spicule blades, which have incurved, interlocking wings, ensures an enclosed, tubular structure through which sperm can he transferred when the spicules are protruded and inserted into the female reproductive tract. In mating, the spicules may have a sensory function.

The development and life history of the false root-knot nematode, Nacobbus serendipiticus.

The development and life history of Nacobbus serendipiticus was studied from the egg to maturity and the early stages recorded by cinematography. There are four moults, the first in the egg. The four larval stages are described and their behaviour in relation to tomato roots. The sex of larvae can be determined at the end of the third stage by the development of the gonads. Damage to the host roots caused by the larvae feeding and invading is followed by necrosis of cortical cells. Galls occur only in association with adult females; they contain a spindle-shaped mass of small cells in which starch grains were found. Of 25 plant species tested for susceptibility, mature females were recovered from eight. The nematodes can survive the winter in England but are probably not indigenous.

Structure of the Anterior Alimentary Tract of Aphelenchoides Blastophthor Us (Nematoda: Tylenchida, Aphelenchina)

The stylet shaft of Aphelenchoides blastophthorus has a porous region associated with nervous tissue, suggesting that the function of the stylet includes sensitivity to taste. Between the shaft and the stylet protractor muscles there is a single microtubular sensillum, possibly a proprioceptor, which is the modified tip of a neuron that passes back through the oesophageal tissues to the region of the nerve ring. There are six stylet protractor muscle cells; the cell bodies of these and the other cells of the procorpus lie in the anterior metacorpus. The prominent metacorpus contains not only all the oesophageal nuclei but also an extensive system of obliquely radial pump muscles, plus an additional set of muscles occupying the apical region of the bulb at its junction with the procorpus. A valve in the lumen of the metacorpus, actuated by truly radial muscles, lies between the dorsal gland duct orifice and the pump chamber. The base of the bulb surrounds a very short isthmus which also contains a valve controlled by radial muscles, with associated circular and longitudinal myofilaments in the isthmus wall. The lining of the pump chamber is of complex shape. There is an extensive system of marginal-cell filaments in the procorpus and metacorpus. These, and the muscles in the anterior bulb and isthmus possibly counteract the pull of the pump muscles and stylet protractors on the other oesophageal tissues. The oesophago-intestinal valve consists of closely apposed, unlined cell membranes as already described in some other Tylenchida. The cell bodies of the oesophago-intestinal junction cells lie alongside the anterior intestine, which is distinguished from the mid intestine by having shorter microvilli. The oesophageal gland lobe is also close, but not joined to the anterior intestine. The nerve ring surrounds the anterior intestine. The structure and working of the feeding apparatus in A. blastophthorus is discussed and compared with that described for some other nematode groups.

Structure of the Anterior Alimentary Tract of the Passively Feeding Nematode Hexatylus Viviparus (Neotylenchidae: Tylenchida)

Following the observations of Doncaster & Seymour (1974) that H. viviparus passively ingests the contents of fungal cells, electron microscopy has confirmed that modifications to some features of the anterior alimentary tract are associated with this method of feeding. These we have interpreted as follows. The oesophagus is a straight tube with a very narrow, oval-shaped, cuticle-lined lumen, of the same cross-sectional area as the stylet lumen (about 0.02-0.03 μm2). At no point is the lumen triradiate. The oesophagus leads, via the oesophago-intestinal valve, into an anterior-intestinal region which also has a narrow lumen (about 1 μm across) lined with short microvilli. This in turn leads into the mid intestine, which has a much wider lumen lined with long microvilli that, unlike those of the anterior intestine, have a sculptured outer coat. The oesophageal glands lie alongside the anterior intestine and the gland ducts pass up through the oesophageal wall. The two subventral gland ducts enter the oesophageal lumen anterior to the oesophago-intestinal valve and the dorsal gland duct enters just behind the stylet base. There is no musculature or other contractile tissue associated with the oesophagus. The nerve ring is circum-intestinal.

Structure of the Spicules and Caudal Sensory Equipment in the Male of Aphelenchoides Blastophthorus (Nematoda: Tylenchida, Aphelenchina)

The spicules of Aphelenchoides blastophthorus are thorn-shaped, with a large dorsal and a smaller ventral limb, and a flattened base embedded in the tail tissues. When retracted, the spicule tips are not exposed through the cloacal opening as they are in some other genera. The large spicular nerve runs along the dorsal limb. Within the nerve a single dendritic process is differentiated and this passes to the outer edge of the dorsal limb, running just beneath the surface towards the tip, where there is a single pore. The spicules do not form an enclosed tube; the ventral limbs abut but the dorsal limbs gape open when extended. Two innervated cuticular processes from the wall of the cloacal sac are exposed when the spicule tips are protruded. These are probably mechanoreceptors. Three pairs of caudal papillae, in adanal, mid-tail and terminal positions, are probably chemosensory as the sensilla open to the exterior through a pore. The relationship between the morphology of spicules and accessory sensory structures and mating behaviour is discussed.

A Re-Examination of Oesophageal Ultrastructure in Ditylenchus Dipsaci (Nematoda, Tylenchida) With Some Observations On Intestinal Structure

The structure of the oesophagus of Ditylenchus dipsaci was examined by transmission electron microscopy of serial sections. The structure of the procorpus was essentially similar to that of other tylenchs examined; it houses the cell bodies of the stylet protractor muscles at its anterior end and has a circular lumen throughout surrounded by cells with a complex of tight junctions. Where it joins the metacorpus, a set of radial muscles attach to the basement membrane and to the cuticular lumen-lining; their function is discussed. In this nematode, which feeds both passively and actively, the metacorpus is a somewhat slender bulb with a rather weak musculature. The long isthmus lacks muscles and the postcorpus incorporates the three discrete oesophageal gland cells. The oesophago-intestinal junction contains a typical unlined valve formed by two specific cells. The few cells of the anterior intestine are characterised by thin microvilli with a smooth surface; the mid-intestinal microvilli are thicker and have a ridged surface.

Scanning electron micrographs of the head region of some species of Aphelenchoidea (Aphelenchina: Nematoda)

The head regions of eighteen species in eight genera, representing six subfamilies in three families, of the Aphelenchoidea were examined with a scanning electron microscope for characters which might be helpful in the taxonomy of this group. All species have the same basic hexaradiate pattern, with a cephalic plate with two subdorsal, two lateral and two subventral lip sectors. The amphidial apertures are widely spaced and located slightly dorsally on the lateral lip sectors, towards the edge of the cephalic plate. There is a prominent cephalic papilla on the outer margin of each of the other four lip sectors. Many specimens have six small papillae or depressions close to the stomatal aperture and these are the inner labial papillae. A second ring of six raised areas probably represents the lips (sensu stricto) which correspond with the position of the outer labial papillae. Several Aphelenchoides spp. have a lip region surrounded by a deep groove so that the lips are contained within a labial disc. The cephalic plate patterns of Aphelenchus and Paraphelenchus are similar to each other and without a labial disc. Huntaphelenchoides, Cryptaphelenchoides and Seinura have the most prominent lips and lip sectors and the Seinura sp. has additional lateral lobes between the stomatal aperture and lateral lips. All of the species, except one Aphelenchoides, have fine annular striations on the head region behind the cephalic plate.

Population Changes and Development of Meloidogyne Naasi in the Field

Experiments show that soil samples, especially those taken between October and April, yield more larvae if they are incubated for a week at 20°C than if they are processed on the day they are taken. Samples taken in autumn after a host crop and kept at 0°C for a week before incubation yield more larvae than incubated, non-chilled samples. The numbers of free larvae in field soil are few during winter, increase greatly as the soil temperature increases in spring, decrease sharply when the host roots are invaded, remain low during summer and show a moderate increase when some of the newly formed eggs hatch in late summer. In the absence of a host crop numbers increase in spring and decrease gradually during summer. The rate of development of M. naasi in host roots depends on temperature and is the same in barley and ryegrass. The amount of energy in the form of heat required for development is approximately the same throughout the year but the time taken varies greatly.

Observations On Gut ph aNd Absorption of Methyl Red and Neutral Red in the Intestinal Walls of Pelodera and Mesodiplogaster

The literature on gut pH in nematodes is reviewed; early work was on free-living marine nematodes and later, on animal parasites. Except for Ascaris lumbricoide.r and Strongylus edentatus the nematodes tested showed acid gut reactions. The free-living soil nematodes, Pelodera lambdiensis and Mesodiplogaster sp. had oesophageal and intestinal contents that were just acid and the granular contents of cells of the intestinal wall were acid to different degrees. In Pelodera, cells just behind the anterior tip of the intestine were usually most acid. Methyl red and neutral red pH indicator dyes were absorbed selectively by different regions of the intestinal wall and by the two species.