The Biology and Evolution of Trematodes

Fasciolosis is an economically important disease of domestic livestock, in particular cattle and sheep, and occasionally man. The disease is caused by digenean trematodes of the genus Fasciola, commonly referred to as liver flukes. The two species most commonly implicated as the aetiological agents of fasciolosis are F. hepatica and F. gigantica (family Fasciolidae). F. hepatica has a worldwide distribution but predominates in temperate zones while F. gigantica is found on most continents, primarily in tropical regions. The class Trematoda belongs to the phylum Platyhelminthes, and can be divided into two subclasses, the Digenea and Aspidogastrea. The Digenea are characterized by a complex life cycle in which one or more intermediate hosts are involved. Many variations on the life cycle exist, but each typically includes a molluscan primary or intermediate host in which larval multiplication occurs, and a vertebrate final or definitive host in which sexual reproduction occurs. Members of the family Fasciolidae are hermaphroditic and self-fertilization can occur, although sexual reproduction is normally by cross-fertilization in the final host. The story of the discovery of the life cycle of F. hepatica has been recounted before; therefore only a short account will be given here. A list of the principal discoveries is shown in Table 1.1. Integral to the successful completion of the life cycle, the biological factors involved will be discussed in the second part of the chapter. Additional details on many of the aspects of the life cycle will be covered in more depth in subsequent chapters.

The Life Cycle of Fasciola hepatica

We have investigated the development of the schistosome egg and its secretions in order to understand how it migrates through gut tissues and also initiates pathology in the liver. We show by electron microscopy that the subshell envelope is absent in the newly deposited egg, but appears very early and differentiates as development progresses. In the mature egg, this nucleated envelope contains extensive endoplasmic reticulum, suggestive of a protein synthetic capacity. Furthermore, Reynolds' layer only appears between the envelope and the egg-shell in the mature egg and may represent its accumulated secretions. We have biosynthetically labelled and collected the secretions (ESP) released by mature but not immature eggs during culture. Their fractionation by SDS–PAGE reveals a simple pattern of 6 bands, differing markedly in composition from soluble egg antigen preparations. Electrophoresis in casein substrate gels demonstrates the presence of 2 distinct proteases in the egg secretions. By immunocytochemistry, ESP localized predominantly to the envelope of the mature egg, suggesting that this layer rather than the miracidium is the source of egg secretions.

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The schistosome egg: development and secretions

Gyrodactylid developmental biology: historical review, current status and future trends.

Publisher Summary This chapter illustrates the role of the eggs in the biology of monogenean parasites. The life cycles of monogeneans are relatively simple compared with those of other parasitic platyhelminths (cestodes and digeneans). Most monogeneans inhabit the skin and gills of fishes, and these sites are accessible to free-swimming larvae (oncomiracidia) hatching from freely deposited aquatic eggs. This simple life cycle has sufficient evolutionary plasticity to enable monogeneans to adapt to fishes living in a range of marine and freshwater habitats and with strikingly different ways of life, such as bottom-living flat-fishes and fast-moving pelagic species. Greater evolutionary potential is revealed by the remarkable survival of monogeneans on those fish-like ancestors of the tetrapods that colonized the terrestrial environment, leading to monogenean life cycles of sometimes surprising complexity. The chapter focuses on hatching rhythms and hatching factors, which in many monogeneans are of fundamental importance in infection of new hosts, and the intriguing changes in reproductive biology that have accompanied the progressive attenuation of the ties between tetrapod hosts and the aquatic environment. Monogeneans deserve special attention, because they provide opportunities for direct observation of the egg assembly process and display a surprising variety of egg shapes and egg appendages, which have attracted little functional interpretation. Monogeneans show a remarkable diversity in the shape and size of their eggs and in their egg appendages. The possible significance of the shapes and sizes of monogenean eggs is discussed in the chapter.

The Eggs of Monogeneans

The hatching mechanism of the liver fluke egg was investigated and a new interpretation put forward. Light stimulates the miracidium and, when fully activated, the miracidium is responsible for altering the permeability of the membrane on the internal concave surface of the viscous cushion. Prior to expansion the cushion (a fibrillar mucoprotein complex) is in a dehydrated or semihydrated state. The change in permeability of the enclosing membrane allows the fluid egg contents to permeate into the cushion. The fibrillar matrix becomes hydrated and an increase in volume occurs. This compresses the miracidium and sacs and an internal pressure is set up which gradually increases until the operculum finally ruptures and flies back violently. The cushion and miracidium are then expelled by the hypertonicity of the egg contents. I would like to thank Professor O. W. Richards, in whose department this work was carried out, also Dr F. Call and Professor B. G. Peters for their many helpful criticisms. I am also grateful to Miss Mary Morris for assistance with the electron microscopy.

The hatching mechanism of the egg of Fasciola hepatica L.

The egg shell of Fasciola was investigated by electron microscopy and found to consist of a fine reticulum of fibrils. No evidence for concentric lamellae was discovered. The amino acids of the shell were identified using chromatography and it was suggested that the original protein was of a simple fibrous type. An unidentified spot was thought to represent a hydrolysis product of the cross-linkage. The effect of sodium hypochlorite on the shell, was also studied. The ‘vitelline membrane’ of the fully developed egg was found to consist of a layer of peri-vitelline material beneath which were two closely opposed unit membranes. The origin of these layers was discussed. The permeability of the shell and vitelline membrane was studied using simple physical techniques and radiotracers. The shell was found to be freely permeable to small molecules. The vitelline membrane complex was thought to represent the main barrier to permeability but an uptake of phosphate ions amounting to 8% of total phosphate, was recorded. This may have been more apparent than real as most of the activity was probably associated with egg shells rather than contents. I would like to thank Professor O. W. Richards in whose department this work was carried out, Dr F. Call for supervision of the work and Professor B. G. Peters for his many helpful criticisms. Lastly, I am indebted to Miss M. Morris without whose expert assistance the electron microscope studies would not have been possible.

https://www.cambridge.org/core/services/aop-cambridge-core/content/view/C831DCE0F941C025231A01DCC2BF5522/S0031182000071882a.pdf/div-class-title-the-structure-and-permeability-of-the-shell-and-vitelline-membrane-of-the-egg-of-span-class-italic-fasciola-hepatica-span-a-href-fn01-ref-type-fn-a-div.pdf

R. A. Wilson

Papers

The hatching mechanism of the egg of Fasciola hepatica L.

The structure and permeability of the shell and vitelline membrane of the egg of Fasciola hepatica