Surface ultrastructure and cytochemistry of parasitic helminths.

In this review recent morphological and histochemical descriptions have been compiled of (mainly outer) features of all developmental stages of the Acanthocephala as well as what is known about the host's defence measures directed against these worms. From acanthors, for intance, it is documented how they escape melanization inside the haemocoel of a suitable intermediate host after they have been activated and released from their eggshell enclosure in the gut of the arthropod. Acanthors possess a complex set of eggshell-envelopes and interstices that fulfill different tasks. While the sequence of events inside the intermediate host's haemocoel is rather well known, what happens in paratenic hosts has been little studied. In final hosts the host-parasite interactions depend on the systematic affiliation of the parasite as well as the host and on the depth of penetration of each acanthocephalan species in the intestinal wall of the host. The mode of attachment also influences the microhabitat preference inside the gut. Mammals often reveal symptoms of high morbidity when infected with acanthocephalans, while fish seem to tolerate high intensities of worms deeply penetrating into their intestinal wall without showing pronounced symptoms of disease. The review also treats the subjects of host specificity, nutrient uptake and metabolism of the worms as well as their absorbance of metals and other elements. The high absorbance capacity for heavy metals suggests the usefulness of acanthocephalans as bioindicators.

Host-parasite interactions in Acanthocephala: a morphological approach.

The epidermis of both species of Seison is syncytial and has a characteristic internal layer divided into two distinct sublayers. Sublayer I is very thin (0.03 μm) and bounded to the outer cell membrane of the epidermis. Sublayer II is 0.5 μm thick and separated from sublayer I by a thin layer of cytoplasm. Intrusions of the outer cell membrane of the epidermis perforate the internal layer, before terminating within the cytoplasm. The intrusions of the cell membrane of S. annulatus are coated by an electron-dense material, the annulus, when passing through the internal layer. Bundles of filaments are present in the epidermis of S. nebaliae. A comparison of epidermal structures within the Gnathifera Ahlrichs, 1995, confirms phylogenetic relationships earlier proposed by the author.

Epidermal ultrastructure of Seison nebaliae and Seison annulatus, and a comparison of epidermal structures within the Gnathifera

We investigated possible relationships among pseudocoelomates (aschelminths) by using cladistic analysis to uncover all most-parsimonious trees from a data set comprising 45 morphological characters of 11 phyla (3 acoelomate, 7 pseudocoelomate, and 1 coelomate). Analysis of our matrix yielded 2 most-parsimonious trees, differing only in relative placement of 3 taxa (Kinorhyncha, Loricifera, and Priapulida). Tree topology suggests that the pseudocoel either evolved twice, or that it evolved once and was then independently lost in Gastrotricha and in Gnathostomulida. Tree topology also is consistent with derivation of the pseudocoel from a coelom. From our analysis and that of other morphologically-based studies, we conclude that there are 2 main lines of pseudocoelomate evolution: (1) a minor clade (Acanthocephala + Rotifera) defined principally by a syncytial epidermis, intracytoplasmic lamina, and sperms with anterior flagella; (2) a major clade (the 5 remaining phyla), defined primarily by a cuticle that is molted. Within the major clade are 2 subclades (Nematoda + Nematomorpha; Kinorhyncha + Loricifera + Priapulida). Using morphological data, the chief issues remaining to be resolved are placement of Gastrotricha and Gnathostomulida, and evolution of the pseudocoel. No analysis based solely on morphological data has so far produced a tree congruent with those based on molecular data. Nevertheless, there is a general consensus that (1) the pseudocoel is of little phylogenetic significance and (2) the pseudocoelomates are probably polyphyletic. Additional key words: evolution, phylogeny "In the face of this bewildering array of conflicting opinions about the interrelationships of the aschelminth phyla, it is impossible to form a coherent picture of the evolution of the animals." (Clark 1964) Does Clark's observation made some 30 years ago still offer a realistic view of our understanding of pseudocoelomate (aschelminth) phylogeny? Clearly, these vermiform taxa share interesting similarities. However, they also possess some striking differences, which have led many authors to choose not to include them in a single phylum, the Aschelminthes (e.g., Clark 1964; Anderson 1983; Inglis 1985; Pearse et al. 1987; Brusca & Brusca 1990; Kozloff 1990; Willmer 1990; Ruppert & Barnes 1994). In fact, for Inglis (1985) there is no common history at all, just completely independent solutions to similar evolutionary pressures. Further complicating the matter is Ruppert's (1991) view that calls into question the significance of 2 traditional characters that link the pseudocoelomates: pseudocoel and cuticle (see also Brusca & Brusca 1990). The alternative position, of course, argues for at least a loose relationship among some of these taxa (e.g., Hyman 1951; Boaden 1985; Lorenzen 1985; Sterrer et al. 1985; Bergstrom 1991; Meglitsch & Schram 1991; Malakhov 1994). The problem is to dissect possible relationships from what appears to be an intractable array of characteristics. However, there is no reason to abandon the logical search for phylogenetic relationships among these taxa. In fact, many workers have explored this field, with phylogenetic analyses that include most or all of the appropriate taxa (e.g., Lorenzen 1985; Meglitsch & Schram 1991; Schram 1991; Eernisse et al. 1992; Conway Morris 1994; Nielsen 1994; 1995; Neuhaus 1994; Winnepenninckx et al. 1995). Here we report our findings from a cladistic analysis of 45 morphological characters and 11 taxa (3 acoelomate, 7 pseudocoelomate, and 1 coelomate). We also compared trees yielded from this study to other published works. Thus, the purposes of this study were to assemble a useful set of light microscopical, ultrastructural, and biochemical characters on the pseudocoelomate taxa and to develop parsimonious cladograms (hypotheses) from those data. Such work may provide a useful foundation for the molecular data that are now forthcoming (e.g., Lake 1990; Brandl et al. 1992; This content downloaded from 157.55.39.30 on Wed, 20 Jul 2016 06:18:46 UTC All use subject to http://about.jstor.org/terms Phylogeny of pseudocoelomates Smothers et al. 1994; Garey et al. 1995; Winnepenninckx et al. 1995). A combination of morphological and molecular data may eventually lead to a comprehensive phylogeny of these taxa (cf. Hillis 1987; Patterson et al. 1993).

A Cladistic Analysis of Pseudocoelomate (Aschelminth) Morphology

Publisher Summary This chapter discusses lipid metabolism in parasitic helminthes. It describes lipid composition, the process of the absorption of lipids, lipid syntheses, lipid catabolism, lipid utilization, and lipid excretion and presents histochemical studies on lipid distribution in parasitic helminthes. Lipid catabolism is essential in higher animals, because it provides the most concentrated source of energy, yielding per gram over twice as many calories as do carbohydrates or proteins. Parasitic helminths contain appreciable quantities of lipids. Probably because of their anaerobic mode of life, most of the intestinal helminths studied do not utilize the significant amounts of lipids even during starvation and under aerobic conditions. Only a limited number of parasitic helminths are able to degrade lipids into CO 2 and hence, gain energy, and not all lipids are utilized for metabolic processes. It seems most unlikely and uneconomical in terms of energy preservation to consider lipids and, in particular, higher fatty acids, as end products of carbohydrate metabolism. The chapter provides a comparison between hosts and parasitic lipids. Studies on the comparison of lipids in parasites and their hosts have produced conflicting results and no distinct pattern has been discernible. Results from various experiments suggest that helminths are capable of manufacturing specific lipids from the host fatty acids.

Lipid metabolism in parasitic helminths.

The fine structure of the syncytial epidermis of Moniliformis dubius (Acanthocephala) was examined with emphasis on the Streifenzone. Numerous crypts elaborate the area of the absorptive surface. Surface architecture is supported by a tripartite filamentous terminal web consisting of a 100 A sheet underlying the plasma membrane, a more extensive and diffuse inner region surrounding the crypts and a differentiated cytoplasmic annulation encircling each crypt neck. Inclusions and organelles described include filaments, microtubules, glycogen, lipid droplets, vesicles, mitochondria, Golgi complexes, endoplasmic reticulum and five lysosomal types. The eutelic nuclei contain little beterochromatin and have unusual nucleolar features.

The absorptive surface of Moniliformis dubius (Acanthocephala). I. Fine structure.

The functional nature of the acanthocephalan absorptive surface was further elucidated by a series of morphological and cytochemical studies with the electron microscope. The use of lanthanum nitrate and ruthenium red in fixatives and Thorotrast staining en bloc and on epoxy thin sections provided information about the surface coat and the surface crypts. Acid phosphatase was localized in the lumina of the crypts and in the five lysosomal types. In vitro studies utilizing ferritin, Thorotrast and horseradish peroxidase implicated the surface crypts in the digestion and/or uptake of large molecules. A consideration of the evidence suggests that these crypts are highly selective and specific in their function.

J.E. Byram

Papers

The absorptive surface of Moniliformis dubius (Acanthocephala). I. Fine structure.

The absorptive surface of Moniliformis dubius (Acanthocephala). II. Functional aspects.