Ultrastructure of sperm and spermiogenesis of Kronborgia isopodicola(Platyhelminthes, Fecampiidae)

Evolutionary biology of parasitic platyhelminths: The role of molecular phylogenetics

As our appreciation of the diversity within the flatworms has grown, so too has our curiosity about the ways in which these varied creatures are related to one another. In particular, the parasitic groups (trematodes, cestodes and monogeneans have been the focus of enquiry. Until recently, morphology, anatomy and life histories have provided the raw data for building hypotheses on relationships. Now, ultrastructural evidence, and most recently, molecular data from nucleic acid sequences, have been brought to bear on the topic. Here, David Blair, Andrés Campos, Michael Cummings and Juan Pedro Laclette discuss the ways in which molecular data, in particular, are helping us recognize the various lineages of flatworms.

Spermatogenesis in Octomacrum lanceatum (Monogenoidea, oligonchoinea, mazocraeidea)

Ultrastructural aspects of spermatozoon development and morphology in Octomacrum lanceatum are presented. Spermiogenesis involves development of zones of differentiation from the surface of cytophores followed by formation of a middle cytoplasmic process and 2 free flagella whose axonemes form from basal bodies located within each zone of differentiation. The mitochondrion and nucleus penetrate through the zone of differentiation into the middle cytoplasmic process. An intercentriolar body is present. Initially the axes of the basal bodies are perpendicular to that of the intercentriolar body but subsequently rotate about 90 degrees to parallel positions as the flagella develop. Flagella are initially free, but eventually fuse with the middle cytoplasmic process from the proximal to distal end (proximodistal fusion). Subsurface microtubules occur within the zone of differentiation but are lacking from the lateral regions of the middle cytoplasmic process. The mature sperm possesses two axonemes, one mitochondrion, and a complete ring of cortical microtubules (sperm pattern 1). The finding of sperm pattern 1 in the Octomacridae suggests that many features of the spermatozoon of the Diplozoidae are autapomorphic.

Phylogeny of the Polystomatidae (Platyhelminthes, Monogenea), with particular reference to Polystoma integerrimum

Polystome phylogeny is examined, with emphasis on the dimorphism of the frog parasite Polystoma integerrimum, which exists in a fully differentiated and a neotenic form, and the evolutionary development of exclusively neotenic genera. Protopolystoma, which infects the aquatic toad Xenopus, has essentially the same morphology as the neotenic (branchial) adult of P. integerrimum and is interpreted as a neotenic genus; however, it inhabits the bladder of its host, the infection site of the normal adult of the dimorphic species. The sphyranurid Sphyranura, ectoparasitic on the external gills of the mud puppy Necturus, resembles the two-sucker larva of P. integerrimum in possessing a single pair of haptorial suckers in place of the 6 suckers of adult polystomes, and is probably a neotenic parasite associated with a neotenic host. The neotenic animals in general are parasites of aquatic hosts, and the uterus is lost or reduced in these genera; by contrast, uterine function is greatly enhanced among polystomes infecting amphibians best adapted to terrestrial life.

The origins of parasitism in the platyhelminthes

Symbiotic associations have arisen independently in several groups of the largely free-living turbellarians. Morphological adaptations of turbellarians to a symbiotic way of life include suckers and adhesive glands for attachment, elaborate systems of microvilli and other epidermal structures for absorption of food, glands for the formation of cysts, cocoons and cement material, and lack of a pharynx and intestine in some species. However, many species closely resemble their free-living relatives. Egg production is greatly increased at least in some species, and life cycles are always direct. Food of symbiotic turbellarians consists of host food and/or host tissue. Ectosymbiotes show fewer physiological adaptations than entosymbiotes. The major groups of parasitic Platyhelminthes (Trematoda Aspidogastrea, Trematoda Digenea, Monogenea, Udonellidea, Cestoda including Gyrocotylidea, Amphilinidea and Eucestoda), form one monophylum, the Neodermata, characterized by a neodermis (tegument) replacing the larval epidermis, epidermal cilia with a single horizontal rootlet, sensory receptors with electron-dense collars, spermatozoa with axonemes incorporated in the sperm body by proximodistal fusion, and protonephridial flame bulbs formed by two cells each contributing a row of longitudinal ribs to the filtration apparatus. The sister group of the Neodermata is unknown but is likely to be a large taxon including the Proseriata and some other turbellarian groups. Among the Neodermata, the Aspidogastrea is likely to be the most archaic group, as indicated by DNA studies, morphology, life cycles and physiology. Aspidogastreans can survive for many days or even weeks outside a host in simple media, they show little host specificity, and have an astonishingly complex nervous system and many types of sensory receptors, both in the larva and the adult. It is suggested that Aspidogastrea were originally parasites of molluscs (and possibly arthropods and other invertebrates) and that they are archaic forms which have remained at a stage where vertebrates represent facultative hosts or obligatory final hosts into which only the very last stages of the life cycle (maturation of the gonads) have been transferred. The complex life cycles of Digenea have evolved from the simple aspidogastrean ones by intercalation of multiplicative larval stages (sporocysts, rediae) in the mollusc host, and of cercarial stages ensuring dispersal to the now obligatory final host. Monogenea may have lost the molluscan host or evolved before the early neodermatans had acquired it. Cestoda either replaced the original molluscan with an arthropod host, retained an original arthropod host or evolved from an early neodermatan before molluscan hosts had been acquired, newly acquiring an arthropod host.(ABSTRACT TRUNCATED AT 400 WORDS)

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.