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Ultrastructure of the Eggshell of Heterodera Schachtii and H

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Ultrastructure of the Eggshell of Heterodera Schachtii and H View metadata, citation and similar papers at core.ac.uk brought to you by CORE provided by Horizon / Pleins textes Ultrastructure of the eggshell of Heterodera schachtii and H. glycines (Nematoda : Tylenchida) Roland N. PEFUXY and Marcus W. TRETT Department of Nematology, Rothamsted Experimental Station, Harpenden, Ultrastructure de la coque de l’cswf de Heterodera schachtii et H. glycines (Nematoda : Tylenchida) TJhrastructural studies demonstrate that the eggshells of Heterodera schachtii and H. glycines consist of a chitinous layer and a lipid layer. The chitinous layer comprises an osmiophilic outer portion and a more substantial, less-densely staining inner portion. The lipid layer is composed of a thin, amorphous outer layer external to a predominantly tetra- or pentalaminate imrer layer. An outer vitelline membrane was not identified in either species. During embryonation, a secondary vitelline membrane, initially indistinguishable from the inner lipid layer, separates to form the epicuticle as the fïrst-stage juvenile cuticle is secreted. The inner lipid layer was not detected in eggs of H. schachtii from cysts with extensive fungal contamination and this is discussed in context of possible fungal enzyme action and mechanical abrasion by the enclosed juvenile. Ultrastructure de la coque de l’œuf de Heterodera schachtii et ,H. glycines (Nematoda : Tylenchidal L’étude ultrastructurale de la coque de l’œuf chez Heterodera schachtii et H. glycines a montré que celle-ci comprend une couche chitineuse et une couche interne lipidique. La couche chitineuse est constituée d’une partie externe osmiophile et d’une partie interne, plus importante, moins densément colorée. La couche lipidique est composée d’une fine couche basale et d’une membrane plus importante comportant elle-même quatre ou cinq couches. Lors de la formation de l’embryon, une membrane vitelline secondaire est formée, qui au début ne se distingue pas de la membrane de l’œuf et qui se sépare ensuite pour former I’épicuticule lorsque la cuticule du juvénile de premier stade est secrétée. Ces membranes de la coque de l’œuf n’ont pu être distinguées sur les œufs de H. schachtii provenant de kystes très contaminés par des champignons; ce point est discuté dans le contexte d’une éventuelle action enzymatique du champignon et d’une abrasion mecanique par le juvénile. The precursor of enhanced juvenile metabolism and (1981) and recently Tefft and Bone (1985) reported the subsequent activity leading to hatch of Globodera occurrence of leucine aminopeptidase activity in the egg rostochiensis appears to be a change in eggshell supernatent of H. glycines. permeability which is likely to be governed primarily by Eggshell permeability changes may, therefore, be the lipid layer (Perry, 1986). Following the discovery of central to the hatching of H. schachtii and H. glycines but this layer in the eggshell of G. rostochiensis (Perry, the presence of lipoprotein membranes remains to be Wharton & Clarke, 1982), the nature of the permeability confirmed. We have now examined the eggshell change was examined by Clarke and Perry (1985). They ultrastructure of these two species using a novel considered that hatching agents, by binding to or cryofracture-fixation technique (Trett, in preparation). replacing membrane bound Ca’+, may change lipoprotein membrane structure. In several respects, the Materials and methods changes in Gaz+ content of Heterodera schachtii eggshells in response to various treatments parallel the Cysts of H. glycines were obtained from pot-grown pattem for G. rostochiensis indicating the existence of soybean plants. Cysts of H. schachtii originated from two lipoprotein membranes in eggshells of H. schachtii sources : either from a field population after sugar beet (Clarke & Perry, 1986). stored in moist soi1 for 12 months at 5” or from the same The hatching mechanism of H. glycines appears to population increased on cabbage plants (cv. Hispi) grown involve zinc rather than Ca” and eclosion may be in pots containing steam-sterilised loam. The latter cysts mediated by a zinc-dependent enzyme (Tefft & Bone, had virtually no fungal contamination. In both cases, 1984). The possible involvement of enzymes in altering cysts were extracted from moist soi1 (Shepherd, 1970). eggshell permeability was reviewed by Perry and Clarke Difficulties in preparation of nematode eggs for Revue Nématol., 9 (4) : 399-403 (1986) 399 R. N. Pery & A4 IK Trett transmission electron microscopy (Bird, 1971) have led flocculant material adhering to the outer surfaces (UM; to the development of a cryofracture-fixation technique Fig. Id). This frequently formed a continuous outer that has given better results with H. schachtii and layer and closely resembled degenerating utcrine cells. H. glycines than earlier methods (Pen-y, Wharton & Clarke, 1982). In brief, cysts, held between aluminium foi1 sheets and frozen in liquid nitrogen (- 170’), were Discussion fractured and permitted to thaw at 4” in 2.5 % glutaraldehyde in 0.2 M sodium cacodylate buffer Despite some confusion in the terminology used, the (pH 7.2; 280 mOs kg- ‘) containing 11 mM calcium fine structure of the eggshells of H. schachtii and H. chloride and 56 mM magnesium chloride (Trett, in glycines appears to conform to that described for other preparation). Subsequent processing followed the tylenchids @rd, 1968; Bird & McClure, 1976; methods of Trett (1984) and Trett and Perry (1986). Wharton, 1980; Pen-y, Wharton Sr Clarke, 1982). Ultrathin sections were viewed in a Philips 201 electron However, the outer vitelline membrane, present in the microscope at 100 kV accelerating voltage. eggs of hleloidogyne javanica, Rotylenchus reniformis, T~~lenchulus semipenetrans and Pratylenchzrs minyus (Bird & McClure, 1976), was not identified in eggs of Results either species examined in thc present study. The significance of this is uncertain. It is possible that the The eggshells of H. schachtii and H. glycines appeared outer vitelline membrane degenerates within thc uterus identical in structure. In each the chitinous layer (CL) after eggshell formation. This may not be true of a11 comprises a densely-staining outer layer (OCL), members of the Heteroderidae as an outer “ vitelline 0.02-0.05 pm thick and a more substantial inner layer Iayer ” has been reported in the eggshell of (ICL), 0.25-0.30 um thick (Figs la, b, d and 2). G. rostochiensis (Perry, Wharton & Clarke, 1982). Throughout, the chitinous layer is fine textured and The sub-division of the chitinous layer into distinctive granular. High magnification of this layer indicated a outer and inner components (Fig. 2) is justifïed on the fibrous sub-structure. In both species the chitinous layer grounds of their differing chemical composition. Whilst lies immediately external to the lipid layer (LL) which both have the same fine, granular sub-structure, which is composed of a thin, amorphous outer layer (OLL) and would appear to reflect the presence of fibrous a multi-laminate, membrane-like structure, termed here sub-units, the outer chitinous layer is markedly more the inner lipid layer (ILL; Figs lb and 2). Unlike the osmiophilic than the inner layer. This is probably not an chitinous layer, the outer lipid layer has a less compacted artefact resulting from slow penetration of osmium structure that probably results from leaching of fiiative into the eggshells as prolonged osmification did saturated lipids during processing; different not increase the observed thickness of the outer layer. It dehydration regimes significantly affected the is possible that the outermost layer represents a zone appearance of this layer. into which unsaturated lipids, or substances with similar Although the inner lipid layer of some specimens was chemical properties, have diffused from the uterus. found to comprise six laminae, tetra- and pentalaminate Steric hindrance or cross-reaction with the chitin and/or conditions predominate (Fig. lc). In both species the proteins of this region, during eggshell formation, may inner lipid layer measures between 0.02 and 0.03 um determine the thickness of this layer. The specific thick and exhibits a lamina separation of approximately hydrophilic/lipophilic balance of this boundary layer Will 30 mn (Fig. lc). This layer was frequently seen to be be of considerable importance in determining the detached from the outer lipid layer (Figs lb and d) but, relative mobility of diffusable substances, such as root with the exception of small remnants, could not be diffusates, within the eggshell. located in eggshells of H schachtii taken from the field The presence of the lipid layer may also place cyst population. constraints on the movement of substances through the Examination of embryonating eggs showed that, eggshell in either direction. Eggshell permeability during early stages of devclopment, the secondary changes after initial embryonation have been observed vitelline membrane (SVM) is closely apposed against the in hookworms of the genus Ancylostoma (Matthews, h-mer lipid layer, from which it is indistinguishable. 1985) and it is possible that similar changes occur in However, where embryonating cells (EJ) have separated H. schachtii and H. glycirzes during embryonation but from the eggshell, discrete membranes cari be identified prior to permeability changes induced by hatching (Fig. la). Later in development, the secondary vitelline agents (Clarke & Perry, 1986). If such changes do occur membrane becomes more permanently separated as the during egg developmem,
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