Occurrence of intermediate hosts and structure of digenean communities of the black-headed gull, Larus ridibundus (L.)

The factors contributing to the similarity between digenean parasite assemblages parasitizing the black-headed gull were investigated. Thirteen different digenean species from the digestive tract and the bursa of Fabricus were found. We predicted that the structure of parasite communities is not random at either component community or infracommunity levels. We searched for nested patterns in the digenean community, and explored the possible factors contributing to nestedness. We found that digenean species which occupy a narrow range of intermediate hosts are placed out of order in the nested matrix. The influence of several variables related to the water reservoir, geographical distances, and the abundance of intermediate hosts on the species diversity and similarity of parasite communities were tested as well. Because of the complexity in bird digenean life-cycles we supposed that intermediate hosts and ecological characters of their environment could play a major role. We showed that the presence of intermediate hosts is the factor limiting the qualitative and quantitative similarity of parasite communities among different host populations as well as influencing the digenean species diversity in the definitive host. The similarity in abundance of intermediate hosts between different localities was facilitated by the presence of vegetation in water reservoir and more diversified type of water bottom. Digenean species diversity was higher when the water reservoir was exposed to temporal drying. We hypothesized that this factor could facilitate the inclusion of molluscs infected by digeneans in the diet of birds. Both species diversity and dominance were influenced by the bottom type of water reservoir and temporal drying up. Geographical distances between localities influenced the quantitative similarity of digenean communities as well as species richness.

Asexual multiplication of larval parasitic worms: a predictor of adult life-history traits in Taeniidae?

The hypothesis that asexual multiplication inside the intermediate host and adult life-history traits within the final host are independent is tested among Cestoda. Using phylogenetic relationships among the Cestoda species, we can show that asexual multiplication appears to have been lost and recovered several times throughout Taeniidae evolution; this allows a comparison of the adult life-history traits of species with and without asexual multiplication at the larval stage. The adult trait considered is the size of the parasite, since numerous life-history traits, such as fecundity and longevity, are correlated with size. If adult traits are independent of whether the larval stage reproduced asexually or not, we expect no difference in the adult size of the proliferative (i.e. with asexual multiplication) and non-proliferative species. The results are inconsistent with this hypothesis. In contrast, species with asexual multiplication in the intermediate host seem to have smaller adult size, reflecting a trade-off. We propose that ecological factors involving intraspecific and interspecific competition in the final host might be responsible for this trade-off. The role of these parameters in the evolution of life-history traits and more precisely in the acquisition of asexual multiplication is investigated with comparative analysis and discussed in the context of host-parasite interactions.

Order in ectoparasite communities of marine fish is explained by epidemiological processes

Two kinds of community structure referred to, nestedness and bimodal distribution, have been observed or were searched for in parasite communities. We investigate here the relation between these two kinds of organisation, using marine fishes as a model, in order to show that parasite population dynamics may parsimoniously explain the patterns of ectoparasite species distribution and abundance. Thirty six assemblages of metazoan ectoparasites on the gills and heads of marine fish showed the following patterns: a positive relationship between abundance and the variance of abundance; a positive relationship between abundance and prevalence of infection; a bimodal pattern of the frequency distribution of prevalence of infection; nestedness as indicated by Atmar and Patterson's thermodynamic measure (a mean of 7.9 degrees C); a unimodal distribution of prevalence in parasite assemblages with a temperature lower than the mean, and a bimodal distribution in assemblages with a temperature higher than the mean. We conclude that patterns are the result of characteristics of the parasite species themselves and that interspecific competition is not necessary to explain them. We emphasize that a holistic approach, taking all evidence jointly into account, is necessary to explain patterns of community structure. Ectoparasite assemblages of marine fish are among the animal groups that have been most thoroughly examined using many different methods, and all evidence supports the view that these animals live under non-equilibrium conditions, in largely non-saturated niche space in which interspecific competition occurs but is of little evolutionary importance.

Host patch selection induced by parasitism: basic reproduction ratio r(0) and optimal virulence

The effects of parasites on the behavior of their hosts are well documented. For example, parasites may affect the habitat selection of the host individual. We used variables aggregation methods to investigate the way in which parasites affect the spatial pattern of susceptible hosts. We developed a simple epidemiological model, taking into account both the reproduction processes of hosts (density-dependent birth and death) and infection, considered separately on two different patches, and the migration of susceptible hosts between these two patches. We used the complete model of three equations to generate an aggregated model describing the dynamics of the combined susceptible and infected host populations. We obtained the basic reproduction ratio (R(0)) from the aggregated model, and then studied the effect of the migratory behavior of susceptible hosts on the ability of the parasite to invade the system. We also used the basic reproduction ratio to investigate the evolution of parasite virulence in relation to the migration decisions of susceptible hosts. We found that host investment in avoidance of the infected patch leads to an increase in optimal virulence if host investment is costly.

Nonrandom association patterns in parasite infections caused by the host life cycle: empirical evidence from Kudoa camarguensis (Myxosporea) and Aphalloides coelomicola (Trematoda)

Pomatoschistus microps is 1 of the most common fish species inhabiting Mediterranean coastal lagoons. This small species uses an early maturation and a short life span to occupy sites subject to high variability in environmental factors. During monthly investigations on a P. microps population of the Vaccarès Lagoon from March 1993 to December 1997, 2 conspicuous parasites were noticed. The digenean Aphalloides coelomicola was harbored in the abdominal cavity of the fish, whereas the myxosporean Kudoa camarguensis was found in the muscle tissues. Monthly peaks of prevalence occurred at the same period for both parasites, and a positive correlation (permutation test and Janson and Vegelius correlation) was observed between their occurrences among sampled fish. These results suggest that this positive association is related to a third common factor, i.e., their host's life cycle.

A model to explain the replacement of Schistosoma intercalatum by Schistosoma haematobium and the hybrid S. intercalatum x S. haematobium in areas of sympatry

Numerous hypotheses have been postulated to explain the rapidly changing parasitological situation in Loum, Cameroon as a result of the interaction between Schistosoma haematobium and S. intercalatum. The aim of this study is to test the various hypotheses using a simple mathematical model, incorporating equal and unequal sex ratios of adult schistosomes, recombinations, and levels of compatibility with the intermediate molluscan hosts, B. forskalii and B. truncatus. The model assuming an equal sex ratio does not fit with the existing field data in that it predicts a continued presence of S. intercalatum, S. haematobium and the hybrids. The model assuming a sex bias in favour of males, which reflects the situation usually observed in schistosome populations, predicts the loss S. intercalatum which indeed concurs with the most recent data.

Usefulness of multilocus sequence typing for characterization of clinical isolates of Candida albicans

Molecular characterization of Candida albicans isolates is essential for understanding the epidemiology of nosocomial infections caused by this yeast. Here, we investigated the potential value of multilocus sequence typing (MLST) for characterizing epidemiologically related or unrelated C. albicans strains of various clinical origins. Accordingly, we sequenced the internal regions (loci) of six selected housekeeping genes of 40 C. albicans clinical isolates and 2 reference strains. In all, 68 polymorphic nucleotide sites were identified, of which 65 were found to be heterozygous in at least one isolate. Ten to 24 different genotypes were observed at the different loci, resulting, when combined, in 39 unique genotype combinations or diploid sequence types (DSTs). When MLST was applied to 26 epidemiologically unrelated isolates and the 2 reference strains, it allowed the identification of 27 independent DSTs, thus demonstrating a discriminatory power of 99.7. Using multidimensional scaling together with the minimum spanning tree method to analyze interstrain relationships, we identified six groups of genetically related isolates on the basis of bootstrap values of greater than 900. Application of MLST to 14 epidemiologically related isolates showed that those recovered from patients in the same hospital ward during the same 3 months had specific DSTs, although 73% of these isolates were genetically very close. This suggests that MLST can trace minute variations in the sequences of related isolates. Overall, MLST proved to be a highly discriminatory and stable method for unambiguous characterization of C. albicans.

Redescription of Mehdiella microstoma and description of Mehdiella petterae sp. n., with a new definition of the genus Mehdiella Seurat, 1918 (Nematoda: Pharyngodonidae)

The generic diagnosis of Mehdiella Seurat, 1918 is emended based on study and redescription of Mehdiella microstoma (Drasche, 1884) from the caecum of Testudo graeca Linnaeus, 1758 collected in Settat, Morocco and on study and description of a new species, Mehdiella petterae sp. n., from the large intestine of Testudo hermanni (Gmelin, 1789) collected in Catalonia, Spain. Scanning electron microscopy (SEM) studies revealed substantial differences in the structure of the mouth and the caudal end, and made possible to differentiate the new species from the others. SEM studies showed the real and sound characteristics of the genus Mehdiella, namely number of anal papillae 2 instead of 3, post-anal papillae pedunculate or sessile instead sessile.

Early cortical distinction between memories that pertain to ongoing reality and memories that don't

Patients with anterior limbic lesions, in particular of the posterior orbitofrontal cortex, often act on the basis of memories that do not relate to ongoing reality and justify their behavior with invented stories that can mostly be traced back to real events (spontaneous confabulation). Recent studies demonstrated that the patients fail to suppress activated memory traces that do not pertain to ongoing reality. In the present study, we used a similar paradigm and high-resolution event-related potentials to explore when this suppression happens. Healthy subjects made two runs of a continuous recognition task, composed of the same set of pictures, and were requested to indicate picture recurrences only within the ongoing run. Thus, performance in the first run depends on new learning, whereas the second run requires the ability to realize whether a picture is solely familiar from its occurrence in the previous run ('distracter') or whether it has already appeared in the ongoing second run ('target'). We find that correct rejection (suppression) of currently irrelevant pictures (distracters of run 2) is associated with absent negative deflection of a frontal potential and absence of a specific cortical potential map configuration after 220-300 ms. By contrast, learning and recognition of repeatedly presented information is associated with cortical amplitude modulation after 400-480 ms. These findings indicate that by the time the content of a mental association is recognized and consolidated, its cortical representation has already been adjusted according to whether it relates to ongoing reality or not. This sequence may also explain the ability to distinguish between the memory of a true event and the memory of a thought.

Electric source imaging of human brain functions

We review recent methodological advances in electromagnetic source imaging and present EEG data from our laboratory obtained by application of these methods. There are two principal steps in our analysis of multichannel electromagnetic recordings: (i) the determination of functionally relevant time periods in the ongoing electric activity and (ii) the localization of the sources in the brain that generate these activities recorded on the scalp. We propose a temporal segmentation of the time-varying activity, which is based on determination of changes in the topography of the electric fields, as an approach to the first step, and a distributed linear inverse solution based on realistic head models as an approach to the second step. Data from studies of visual motion perception, visuo-motor transfer, mental imagery, semantic decision, and cognitive interference illustrate that this analysis allows us to define the patterns of electric activity that are present at given time periods after stimulus presentation, as well as those time periods where significantly different patterns appear between different stimuli and tasks. The presented data show rapid and parallel activation of different areas within complex neuronal networks, including early activity of brain regions remote from the primary sensory areas. In addition, the data indicate information exchange between homologous areas of the two hemispheres in cases where unilateral stimulus presentation requires interhemispheric transfer.

Order and disorder in ectoparasite communities: the case of congeneric gill monogeneans (Dactylogyrus spp.)

The measure of order and disorder in the distribution of species in fragmented habitats proposed by Atmar and Patterson (Oecologia, 96 (1993) 373-82) was applied to investigate nested patterns of Dactylogyrus species parasitising the gills of roach. Organisation in dactylogyrid assemblages was investigated at three levels: (1) host populations between localities; (2) local host populations over seasons; and (3) individual hosts over one season within a local host population. Dactylogyrid assemblages showed nested patterns when analyses were conducted at the level of localities (among host populations) and at the level of seasons (among host populations within localities). The analysis at the level of hosts (infracommunities of parasites) revealed that nested pattern is not common. We suggest that nestedness may have a variety of causes and does not necessarily imply competition.

Sex discriminatory effect of the acanthocephalan Acanthocephaloides propinquus on a gobiid fish Gobius bucchichii

Most studies which aim at detecting effects of parasites on fish show that intermediate stages of parasites affect their host while adult parasites are usually less virulent in the final host. We studied the effect of the acanthocephalan Acanthocephaloides propinquus on one of its final hosts, the fish Gobius bucchichii. This study showed that the adult parasites affect host fitness. Moreover, our results showed that at the same level of infection in male and female gobiids, the female's reproductive success was negatively correlated to parasite abundance while the male's reproductive success was not. The negative effects on females includes reductions in gonado-somatic index and egg production. We hypothesized that it might be more difficult for females to compensate the cost of parasitism because of the very high cost of egg production. We discussed these results in terms of host population regulation.

Morphometric correlates of host specificity in Dactylogyrus species (Monogenea) parasites of European Cyprinid fish

We test the hypothesis that living on larger fish may impose constraints, i.e. the need to develop large attachment organs, related to the necessity to remain attached on large gills. For this, we compiled data on body size and morphometric measurements of attachment organs of 44 Dactylogyrus species (ectoparasites with direct life-cycle) from 19 cyprinid species. Nineteen dactylogyrid species were considered as specialists (infecting only 1 host species) and 25 as generalists (infecting more than 1 species). The lack of phylogenetic information lead us to perform comparative analyses using raw values and independent contrasts obtained by random phylogenies. Our results show that rich parasite communities are formed by specialists and generalists whereas poor communities are composed mainly of generalist parasites. Moreover, specialists are found on larger hosts, which may reflect a specialization on a predictable resource, as larger fish live longer and offer large gills for parasite colonization. Parasite specialization is shown to be linked with adaptation of attachment organs to their fish hosts. Two morphometric variables of the attachment organ, the total length of anchor and length of base of anchor, were positively correlated with host length for specialists.

Gill parasites of Cephalopholis argus (Teleostei: Serranidae) from Moorea (French Polynesia): site selection and coexistence

The distribution and coexistence of gill ectoparasites of 121 specimens of Cephalopholis argus Bloch et Schneider, caught between October 1994 and October 1995, were investigated. Adults of the monogenean Benedenia sp. and copepod Hatschekia sp., the larval caligid copepod Caligus sp. (copepodite and chalimus stages), and praniza larvae of the isopod Gnathia sp. were found. All species were aggregated within the host population. Infracommunities were poor, with only 40.5% of fish infected by two parasite species. Only two individual fish harboured all the parasite species observed at the component community level. Prevalences were less than 50% and mean intensities were low (less than 6 parasites/host). No dominant parasite species were observed in the host population. The spatial distribution of each parasite species was studied on different partitions of the gill arches. Adult parasite stages that are mobile showed much overlap in their distribution, whereas temporarily attached larvae of Caligidae were more site specific. Copepodite and chalimus larvae showed niche restriction that is probably due to gregarious behaviour. Positive associations between caligid larvae reflected intraspecific interaction for site and/or resources. Each of the Caligus sp. larval stages prefers specific sites, as do the adults, which occur exclusively in the buccal cavity of the host. Infracommunities were too poor and too few to induce processes of interspecific competition.

Do parasites live in extreme environments? Constructing hostile niches and living in them

We develop the hypothesis that parasites do not invade extreme environments, i.e. hostile hosts, but rather 'create' them. We argue that parasites may have driven the evolution of the constitutive and adaptive immune system. This leads to several implications. First, parasites respond to 'genes to kill' by 'genes to survive' and this triggers an indefinite selection of measures and counter-measures. Second, these coevolutionary arms races may lead to local adaptation, in which parasite populations perform better on local hosts. Third, the evolution of the immune system, whose responses are predictable, may allow parasites to specialize, to evade and even to manipulate. Finally we show that the correlations between the increase in the antibody repertoire, the expansion of MHC loci and parasite pressures support our hypothesis that both host complexity and parasite pressures can be invoked to explain the diversity of antibodies, T-receptors and MHC molecules.

Linking specialisation to diversification in the Diplectanidae Bychowsky 1957 (Monogenea, Platyhelminthes)

The hypothesis of a positive correlation between host specificity and taxonomic diversification was tested in a family of fish ectoparasites, the Diplectanidae Bychowsky 1957 (Monogenea). A comparative analysis of correlation of species richness with host specificity was performed using an adapted independent contrasts method. In order to control for phylogenetic effects, a phylogenetic tree of the genera in the Diplectanidae was reconstructed using morphological characters. The current taxonomy is retrieved in this phylogenetic hypothesis, except for the Murraytrematoidinae subfamily, which appears to be paraphyletic. There is no significant correlation between host specificity and taxonomic diversification in the Diplectanidae. The significance of this result is discussed, and different hypotheses which could have led to this observation are presented.

Oxyuroids of palearctic Testudinidae--new definition for Alaeuris Seurat, 1918 (Nematoda: Pharyngodonidae) and redescription of Alaeuris numidica (Seurat, 1918)

The generic diagnosis of Alaeuris is emended based on the study and redescription of Alaeuris numidica from the cecum of Testudo graeca collected in Settat, Morocco and of Testudo hermanni collected in Catalonia, Spain. Scanning electron microscopy studies revealed that the papillae previously described as adanal are simple lobes because of the lack of terminal nerves, and that both preanal and postanal papillae are pedunculate. These new findings allowed us to emend the diagnosis of the genus and the species. A list of species of Alaeuris with biogeographic regions and hosts is provided.

Acquisition of parasites correlated with social rank and behavioural changes in a fish species

A survey of parasites was conducted on 258 Symphodus ocellatus (Teleostei: Labridae) collected in Corsica National Park (west Mediterranean). In addition, the total length, sex and social status were recorded for each individual fish. Three species of trematodes were found in the digestive tract. One of the parasites, Genitocotyle mediterranea, was only present, with one exception, in males of large size, and principally in the individuals that had the highest status and that were involved in nest construction. Two hypotheses are suggested to explain this particular distribution of a parasite: the immunocompetence handicap and the changing trophic behaviour as the fish grows.

Spatial distribution and coexistence of monogenean gill parasites inhabiting two damselfishes from Moorea island in French Polynesia

The ectoparasite fauna of two damselfishes, Stegastes nigricans and Dascyllus aruanus, from Moorea Island in French Polynesia was investigated. Gills of these damselfishes were infected with congeneric Monopisthocotylea Monogenea belonging to the genus Haliotrema. Stegastes nigricans were found to harbour a guild of three Haliotrema species whereas only one species inhabited D. aruanus. Microhabitat distribution, inter- and intraspecific competition and interspecific associations on the gill were studied. Observations on site preference revealed no spatial segregation between the three congeneric species inhabiting the gills of S. nigricans. Juvenile and adult monogeneans of that guild occurred on the same microhabitat. The dominant species Haliotrema sp. 1 did not expand on the microhabitat when the intensity of infection increased. Interspecific association tests revealed positive and negative associations. Haliotrema sp. 4 expanded its distribution on the gills of Dascyllus aruanus when the intensity of infection increased suggesting the likelihood of intraspecific competition. Juvenile and adult monogeneans of Haliotrema sp. 4 appeared to segregate as a result of intraspecific competition. This competition may exist to enhance resource availability when the gill habitat is limited. Overlaps between niche breadth and species microhabitat were revealed for monogenean species inhabiting S. nigricans. Interspecific competition did not appear to play an important role in the distribution of S. nigricans congeneric ectoparasites. Reinforcement of reproductive barriers may have led to the avoidance of hybridization.