The role of prezygotic isolation mechanisms in the divergence of two parasite species.

BACKGROUND: The formation of reproductive barriers in diverging lineages is a prerequisite to complete speciation according to the biological species concept. In parasites with complex life cycles, speciation may be driven by adaptation to different intermediate hosts, yet diverging lineages can still share the same definitive host where reproduction takes place. In these cases, prezygotic isolation mechanisms should evolve very early and be particularly strong, preventing costly unfavourable matings. In this study, we investigated the importance of prezygotic barriers to reproduction in two cestode species that diverged 20-25mya and show an extraordinary degree of specificity to different intermediate hosts. Both species share the same definitive hosts and hybridize in the laboratory. Yet, natural hybrids have so far not been detected. METHODS: We used a combination of different experiments to investigate the role of prezygotic barriers to reproduction in the speciation of these parasites. First, we investigated whether hybridization is possible under natural conditions by exposing lab-reared herring gulls (Larus argentatus, the definitive hosts) to both parasites of either sympatric or allopatric combinations. In a second experiment, we tested whether the parasites prefer conspecifics over parasites from a different species in dichotomous mate choice trials. RESULTS: Our results show that the two species hybridize under natural conditions with parasites originating either from sympatric or allopatric populations producing hybrid offspring. Surprisingly, the mate choice experiment indicated that both parasite species prefer mates of the different species to conspecifics. CONCLUSIONS: Neither fundamental constraints against hybridization in a natural host nor assortative mate choice sufficiently explain the persistent segregation of the two tapeworm species in nature. Hence, postzygotic ecological selection against hybrids is presumably the more important driving force limiting gene flow between the two parasite sister species.

Sex allocation in a polyembryonic parasitoid with female soldiers: an evolutionary simulation and an experimental test.

Parasitoid wasps are convenient subjects for testing sex allocation theory. However, their intricate life histories are often insufficiently captured in simple analytical models. In the polyembryonic wasp Copidosoma koehleri, a clone of genetically identical offspring develops from each egg. Male clones contain fewer individuals than female clones. Some female larvae develop into soldiers that kill within-host competitors, while males do not form soldiers. These features complicate the prediction of Copidosoma's sex allocation. We developed an individual-based simulation model, where numerous random starting strategies compete and recombine until a single stable sex allocation evolves. Life-history parameter values (e.g., fecundity, clone-sizes, larval survival) are estimated from experimental data. The model predicts a male-biased sex allocation, which becomes more extreme as the probability of superparasitism (hosts parasitized more than once) increases. To test this prediction, we reared adult parasitoids at either low or high density, mated them, and presented them with unlimited hosts. As predicted, wasps produced more sons than daughters in all treatments. Males reared at high density (a potential cue for superparasitism) produced a higher male bias in their offspring than low-density males. Unexpectedly, female density did not affect offspring sex ratios. We discuss possible mechanisms for paternal control over offspring sex.

Early intrauterine embryonic development of the bothriocephalidean cestode Clestobothrium crassiceps (Rudolphi, 1819), a parasite of the teleost Merluccius merluccius (L., 1758) (Gadiformes: Merlucciidae).

The early intrauterine embryonic development of the bothriocephalidean cestode Clestobothrium crassiceps (Rudolphi, 1819), a parasite of the teleost Merluccius merluccius (L., 1758), was studied by means of light (LM) and transmission electron microscopy (TEM). Contrary to the generic diagnosis given in the CABI Keys to the cestode parasites of vertebrates, the eggs of C. crassiceps, the type of species of Clestobothrium Lühe, 1899, are operculate and embryonated. Our LM and TEM results provide direct evidence that an operculum is present and that the eggs exhibit various stages of intrauterine embryonic development, and in fact represent a good example of early ovoviviparity. The intrauterine eggs of this species are polylecithal and contain numerous vitellocytes, generally ∼30, which are pushed to the periphery and remain close to the eggshell, whereas the dividing zygote and later the early embryo remain in the egg centre. During early intrauterine embryonic development, several cleavage divisions take place, which result in the formation of three types of blastomeres, i.e. macro-, meso- and micromeres. These can be readily differentiated at the TEM level, not only by their size, but also by the ultrastructural characteristics of their nuclei and cytoplasmic organelles. The total number of blastomeres in these early embryos, enclosed within the electron-dense eggshells, can be up to ∼20 cells of various sizes and characteristics. Mitotic divisions of early blastomeres were frequently observed at both LM and TEM levels. Simultaneously with the mitotic cleavage divisions leading to blastomere multiplication and their rapid differentiation, there is also a deterioration of some blastomeres, mainly micromeres. A similar degeneration of vitellocytes begins even earlier. Both processes show a progressive degeneration of both vitellocytes and micromeres, and are good examples of apoptosis, a process that provides nutritive substances, including lipids, for the developing embryo.

Hatching strategies in monogenean (platyhelminth) parasites that facilitate host infection.

In parasites, environmental cues may influence hatching of eggs and enhance the success of infections. The two major endoparasitic groups of parasitic platyhelminths, cestodes (tapeworms) and digeneans (flukes), typically have high fecundity, infect more than one host species, and transmit trophically. Monogeneans are parasitic flatworms that are among the most host specific of all parasites. Most are ectoparasites with relatively low fecundity and direct life cycles tied to water. They infect a single host species, usually a fish, although some are endoparasites of amphibians and aquatic chelonian reptiles. Monogenean eggs have strong shells and mostly release ciliated larvae, which, against all odds, must find, identify, and infect a suitable specific host. Some monogeneans increase their chances of finding a host by greatly extending the hatching period (possible bet-hedging). Others respond to cues for hatching such as shadows, chemicals, mechanical disturbance, and osmotic changes, most of which may be generated by the host. Hatching may be rhythmical, larvae emerging at times when the host is more vulnerable to invasion, and this may be combined with responses to other environmental cues. Different monogenean species that infect the same host species may adopt different strategies of hatching, indicating that tactics may be more complex than first thought. Control of egg assembly and egg-laying, possibly by host hormones, has permitted colonization of frogs and toads by polystomatid monogeneans. Some monogeneans further improve the chances of infection by attaching eggs to the host or by retaining eggs on, or in, the body of the parasite. The latter adaptation has led ultimately to viviparity in gyrodactylid monogeneans.

Differentiation of somatic mitochondria and the structural changes in mtDNA during development of the dicyemid Dicyema japonicum (Mesozoa).

Dicyemids (Mesozoa) are extremely simple multicellular parasites found in the kidneys of cephalopods. Their mitochondria are known to contain single-gene minicircle DNAs. However, it is not known if the minicircles represent the sole form of mitochondrial genome in these organisms. Here we demonstrate that high-molecular-weight (HMW) mtDNA is present in dicyemids. This form of mtDNA is probably limited to germ cells, and has been analyzed by PCR and Southern hybridization. In situ hybridization revealed that mtDNA is initially amplified during early embryogenesis, and then gradually decreases in copy number as larval development proceeds. Furthermore, we demonstrated using BrdU as a tracer that many of the mitochondria in terminally differentiated somatic cells no longer support DNA synthesis. Taking these observations into account, we propose an "amplification-dilution" model for mesozoan mtDNA. "Stem" mitochondria in the germ cells (1) amplify the HMW form of mtDNA in early embryos, followed by minicircle formation via DNA rearrangement, or (2) selectively replicate minicircles from the HMW DNA, concomitantly with the differentiation of the soma. Minicircle formation may itself lead to the loss of replication origins. Thereafter, the minicircles are simply distributed to daughter mitochondria without replication, resulting in the "somatic" mitochondria, which have lost the replicative form of the HMW mtDNA. The change in mtDNA configuration is discussed in relation to mitochondrial differentiation.

Avaliaçäo in vitro da atividade ovicida e larvicida da ivermectina sobre Lagochilascaris minor / In vitro evaluation of the ovicide and larvicide activity of ivermectin over Lagochilaschares minor

Através de ensaios terapêuticos in vitro avaliou-se a açäo da ivermectina(IVM) sobre ovos de Lagochilascaris minor. Foram utilizados ovos recém-eliminados (Experimento I - açäo sobre ovos näo embrionados)e ovos com 40 dias de cultura (Experimento II - açäo sobre ovos embrionados). Em ambos os experimentos, as suspensöes de ovos dos grupos teste foram colocadas em soluçäo de IVM na concentraçäo de 200 ug/l de formalina a 1 por cento, por 28 dias, à temperatura ambiente. Para o controle dos experimentos, utilizou-se uma suspensäo de ovos do parasito mantida em soluçäo de formalina a 1 por cento. A avaliaçäo de eficácia terapêutica da IVM foi feita através do cálculo do percentual de embriogênese do teste de infectividade. Observou-se que a IVM, na concentraçäo de 200 ug por litro de formalina a 1 por cento durante 28 dias, näo impediu o processo de embriogênese e nem desvitalizou larvas do interior de ovos de L. minor.

Structure of the dermal-epidermal junction and potential mechanisms for its degradation: the possible role of inflammatory cells.

Evidence was presented indicating that the DEJ as a basement membrane is highly susceptible to degradation by a variety of neutral proteolytic enzymes with different specificities. The effect of endoglycosidases which degrade heparan sulfate was also discussed. The latter enzymes are capable of removing heparan sulfate from the DEJ, but little gross alteration of structure, such as tissue detachment, appears to result from the loss of this component. Of the proteinases discussed, PMN elastase and probably type IV collagenase are the most destructive. This is likely related to their ability to degrade the type IV collagen network. Even though proteinases with chymotrypsinlike and trypsinlike specificity were not efficient at degrading the lamina densa or removing type IV collagen from intact basement membranes, these proteinases were capable of producing epidermal detachment from the lamina densa. Many inflammatory cells of the immune system contain proteinases and endoglycosidases with the potential to degrade the DEJ and other basement membrane zones, suggesting that these cells may have a significant pathologic role in basement membrane-related diseases. PMNs and mast cells may be of particular interest because they have stored within their secretory granules high concentrations of neutral serine proteinases which have been demonstrated to degrade the DEJ.