Structure of photoreceptors ofpolystoma integerrimum (platyhelminths, monogenea)

Serial electron microscopic reconstruction of the drosophila larval eye: Photoreceptors with a rudimentary rhabdomere of microvillar-like processes

Photoreceptor cells (PRCs) across the animal kingdom are characterized by a stacking of apical membranes to accommodate the high abundance of photopigment. In arthropods and many other invertebrate phyla PRC membrane stacks adopt the shape of densely packed microvilli that form a structure called rhabdomere. PRCs and surrounding accessory cells, including pigment cells and lens-forming cells, are grouped in stereotyped units, the ommatidia. In larvae of holometabolan insects, eyes (called stemmata) are reduced in terms of number and composition of ommatidia. The stemma of Drosophila (Bolwig organ) is reduced to a bilateral cluster of subepidermal PRCs, lacking all other cell types. In the present paper we have analyzed the development and fine structure of the Drosophila larval PRCs. Shortly after their appearance in the embryonic head ectoderm, PRC precursors delaminate and lose expression of apical markers of epithelial cells, including Crumbs and several centrosome-associated proteins. In the early first instar larva, PRCs show an expanded, irregularly shaped apical surface that is folded into multiple horizontal microvillar-like processes (MLPs). Apical PRC membranes and MLPs are covered with a layer of extracellular matrix. MLPs are predominantly aligned along an axis that extends ventro-anteriorly to dorso-posteriorly, but vary in length, diameter, and spacing. Individual MLPs present a "beaded" shape, with thick segments (0.2-0.3 μm diameter) alternating with thin segments (>0.1 μm). We show that loss of the glycoprotein Chaoptin, which is absolutely essential for rhabdomere formation in the adult PRCs, does not lead to severe abnormalities in larval PRCs.

Phototaxis and the origin of visual eyes

Vision allows animals to detect spatial differences in environmental light levels. High-resolution image-forming eyes evolved from low-resolution eyes via increases in photoreceptor cell number, improvements in optics and changes in the neural circuits that process spatially resolved photoreceptor input. However, the evolutionary origins of the first low-resolution visual systems have been unclear. We propose that the lowest resolving (two-pixel) visual systems could initially have functioned in visual phototaxis. During visual phototaxis, such elementary visual systems compare light on either side of the body to regulate phototactic turns. Another, even simpler and non-visual strategy is characteristic of helical phototaxis, mediated by sensory-motor eyespots. The recent mapping of the complete neural circuitry (connectome) of an elementary visual system in the larva of the annelid Platynereis dumerilii sheds new light on the possible paths from non-visual to visual phototaxis and to image-forming vision. We outline an evolutionary scenario focusing on the neuronal circuitry to account for these transitions. We also present a comprehensive review of the structure of phototactic eyes in invertebrate larvae and assign them to the non-visual and visual categories. We propose that non-visual systems may have preceded visual phototactic systems in evolution that in turn may have repeatedly served as intermediates during the evolution of image-forming eyes.

Response of Neobenedenia girellae (Monogenea) oncomiracidia to brightness and black-and-white contrast

Neobenedenia girellae, a capsalid monogenean, is a significant pathogen due to both its ability to cause high mortality in fishes and its low host specificity. Established control methods have both advantages and disadvantages. Biological control measures with no unfavourable effects on the environment should be incorporated into the control strategy. The response of N. girellae oncomiracidia to brightness and black-and-white contrast was investigated to search for an alternative approach of disease prevention or control. Japanese flounder, Paralichthys olivaceus (Paralichthyidae), were exposed to oncomiracidia in an aquarium divided into areas of different brightness ( approximately 1.3, 41.3 and 138.0 lux). The number of parasites on the fish group reared in 138.0 lux was significantly higher than on those reared in the lower brightness levels. Thus, the fish tended to be more vulnerable to infection by N. girellae under brighter conditions. Challenge trials using host fish mucus and whole live fish were established to detect the response by oncomiracidia to black-and-white contrast on a white versus a black background. Markedly more N. girellae oncomiracidia attached to black-painted areas and dark-coloured fish (normal spotted halibut, Verasper variegatus (Pleuronectidae) compared with white-painted areas and light-coloured fish (mal-coloured V. variegatus) on a white-coloured background. On a black-coloured background, more N. girellae oncomiracidia tended to attach to white-painted areas and light-coloured fish. Thus, black-and-white contrast is considered important for host finding by N. girellae oncomiracidia. The simplicity of the positive phototactic behaviour and the response to black-and-white contrast may lead to the development of a simple, practical and inexpensive method to control N. girellae outbreaks.

Microscopy and the helminth parasite

Microscopy has a long and distinguished history in the study of helminth parasites and has made a singularly outstanding contribution to understanding how these complex animals organise their lives and relate to their hosts. Increasingly, the microscope has been used as a powerful investigative tool in multidisciplinary approaches to parasitological problems, placing emphasis on functional correlates rather than anatomical detail. In doing so, microscopy has also uncovered a number of attributes of parasites that are of wider significance in the field of biology. Parasite surfaces have understandably demanded most of the attention of microscopists, largely as a result of the pioneering studies using transmission electron microscopy. Their findings focused the attention of physiologists and immunologists on the tegument and cuticle of helminths and in doing so helped unravel the complex molecular exchanges that are fundamental to understanding host-parasite interactions. Scanning electron microscopy succeeded in augmenting these data by revealing novel microtopographical features of the host-parasite relationship, as well as proving invaluable in helminth taxonomy and in assessing the efficacy of test substances in drug screens. Control of helminth parasites has never been more critical: problems of drug resistance demand urgent action to identify exploitable targets for new generation anthelmintics. In this regard, the neuropeptide signalling system of helminths is envisioned as central to nerve-muscle function, and thereby a crucial regulatory influence on their motility, alimentation and reproduction. The use of immunocytochemistry interfaced with confocal scanning laser microscopy has not only been instrumental in discovering the peptidergic system of helminths and its potential for chemotherapeutic exploitation, but through increasingly sophisticated bio-imaging technologies has continued to help dissect and analyse the molecular dynamics of this and other cellular systems within these important parasites.

Ultrastructure of the eyes of the larva of Neoheterocotyle rhinobatidis (Platyhelminthes, Monopisthocotylea), and phylogenetic implications

The oncomiracidium of Neoheterocotyle rhinobatidis (Monogenea, Monopisthocotylea, Monocotylidae) has two pairs of eyes, each eye with a lens and pigment cup. The anterior eyes have a single rhadomere; the posterior ones, two rhadomeres. Lenses are part of the pigment cup cells, as indicated by cytoplasmic connections between them and the pigment cups, and they are of mitochondrial origin because mitochondrial cristae are present in the periphery of the lenses. This is the first time that mitochondrial lenses have been shown to exist in a neodermatan. Such lenses may be a synapomorphy of a large taxon comprising the Neodermata and its turbellarian sister groups, or they may have evolved convergently in several not closely related groups as a result of strong selection pressure to find a suitable habitat or host.

Parasitism and the platyhelminthes

by Graeme C. Kearn, Chapman & Hall, 1998. pound115.00 (xii +544 pages) ISBN 0 412 80460 3.

Non-monophyly of the monogeneans?

There is currently no congruence between phylogenies based on morphology, in which the Monogenea are considered a monophylum, and molecular phylogenies based on 18S or 28S rDNA, in which the Monogenea are never considered monophyletic. However, all analyses based on morphology and sperm characters or molecular data found constantly the two subgroups composing the Monogenea, namely the Monopisthocotylea and Polyopisthocotylea (or Polyonchoinea and Oligonchoinea) to be independently monophyletic. This conflict concerns not only monophyly of the Monogenea, but also the relationships of the Monopisthocotylea and Polyopisthocotylea with the Trematoda (Digenea and Aspidogastrea) and Cestoda, and therefore the phylogeny of the parasitic Platyhelminthes as a whole. It is concluded that a reappraisal of morphological synapomorphies should be undertaken.

The phylogenetic relationships of Kronborgia (Platyhelminthes, Fecampiida) based on comparison of 18S ribosomal DNA sequences

Approximately 580 bp at the 5' end of the small subunit RNA gene were amplified by PCR for 19 platyhelminth taxa, and Homo and Artemia were used as outgroups. These were analysed to test the hypothesis that fecampiids and Neodermata are sister groups. No evidence was found that the fecampiid Kronborgia isopodicola is closely related to the Neodermata or to the Rhabdocoela (in which the fecampiids are usually included). Morphological, including ultrastructural, characters and DNA data do not support a close relationship of fecampiids with any other platyhelminth taxon, although the DNA sequence analysis provides some evidence that the Acoela and Tricladida are closest. Fecampiids are sufficiently different from any other platyhelminth group to warrant establishment of a class, Fecampiida, for them. A diagnosis of the new class is given.

[Evolution of the tegument of Polystoma (Monogenea Polystomatidae) during the cycle. Persistence of the nucleated embryonic epidermis in oncomiracidia (author's transl)]

The evolution of tegument ultrastructures during development was studied in two Polystome species, Polystoma integerrimum and Polystoma pelobatis. It differs from Monogenea and other Platyhelminths in the presence of nuclei in the tegumentary syncytium of the oncomiracidium and their deferred elimination which occurs in the post-larva attached to the gills of the tadpole. This represents a delay in the loss of embryonic characteristics in Polystoma larvae which may be related to the possibility of neotenic development of these larvae. This delay allows us to follow naturally the considerable cytoplasmic changes which accompany the elimination of embryonic nuclei (disappearance of the ergastoplasm, golgi complexes and ribosomes, and of the vacuoles) and the transfer of control of this "enucleated" cytoplasm to nuclear information from tegumentary parenchymatic cells (appearance of new inclusions in the "annexed" cytoplasmic zone, maintenance of numerous organelles involved in the formation of these inclusions in the deep perinuclear region). The ultrastructual characteristics of ciliated cells and the tegumentary syncytium are discussed from the general point of view of the Platyhelminths and with respect to their adaptative function in the Polystomatidae. The originality of the Polystomatidae among the Monogenea is emphasized.