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Monogeneans are highly diverse fish ectoparasites with a direct life cycle, widely distributed, and are known to generally display strict host specificity. Factors related to the hosts and the parasite have been suggested to explain this high specificity. Monogeneans have also been observed to colonise fish species not in their natural host range under experimental conditions. We developed a specific metabarcoding protocol and applied it on the Sparidae-Lamellodiscus host-parasite system, to assess parasite diversity on skin and gills of several sparid host species. We first demonstrated that the use of a metabarcoding approach provided a better understanding of the diversity of monogeneans associated with teleost skin and gills than traditional approaches based on morphological identification. We identified a high diversity of both expected and unexpected (never observed on this host species) Lamellodiscus spp. on each host species and on skin and gills. No significant difference in parasite diversity was found between skin and gills. These results suggest that the establishment of the observed host specificity in monogeneans relies on multiple levels of regulation, involving the survival capacity of the larvae and host recognition mechanisms.

International audience; Despite considerable research effort, many aspects of the host-parasite relationships and parasite spatial distribution in invasive hosts remain poorly understood due to complex and context-dependent phenomena related to both the bioinvasions and the parasitism. Using macroecological patterns and theory is a useful approach to analyzing parasitological observations, but in practice parasite ecology and classical macroecology are disconnected. We propose a new framework that can use the conventional parasitology sampling data much more effectively. The innovative concept combines the data inferred from populations, infra- and component communities of parasites and the application of a macroecological approach in the analysis of complex and frequently hidden relationships in host-parasite systems. This comparative analysis draws on parasite data across regions and host species at different organizational (population vs. community) and hierarchical (infra vs. component community) levels of parasites. Our framework based on assessing and analysis of parasitological and ecological indexes, including descriptors of parasite species richness (individual and total), infection parameters, parasite aggregation (Taylor´s power law) and macroecological models (abundance-variance and abundance-occupancy relationships), can produce mechanistic explanations of the Enemy Release Hypothesis and unravel host-parasite relationships of an invasive host and its parasites. Moreover, abundance-variance and abundance-occupancy relationships, core-satellite species hypothesis, patterns on the aggregation and the frequency distribution of prevalence, infrapopulation size and individual parasite species richness provide useful tools to distinguish co-introduced and acquired parasites in communities of the invasive host based on quantitative descriptors. We hope that our framework becomes widely applied as it can potentially contribute to enhance future practice and research in biodiversity conservation and control of invasive species.

Medlin tested multiple outgroups with 18S rRNA dataset and found that haptophytes, ciliates, prasinophytes, and chlorophytes recovered monophyletic Coscinodiscophyceae, Mediophyceae, Bacillariophyc...

International audience; Almost sixty years ago, Bdellovibrio and like organisms (BALOs) were discovered as the first obligate bacterial predators of other bacteria known to science. Since then, they were shown to be diverse and ubiquitous in the environment, and to bear astonishing ecological, physiological, and metabolic capabilities. The last decade has seen important strides made in understanding the mechanistic basis of their life cycle, the dynamics of their interactions with prey, along with significant developments towards their use in medicine, agriculture, and industry. This review details these achievements, identify current understanding and knowledge gaps to encourage and guide future BALO research.

Background Animal-associated microbial communities appear to be key factors in host physiology, ecology, evolution and its interactions with the surrounding environment. Teleost fish have received relatively little attention in the study of surface-associated microbiota. Besides the important role of microbiota in homeostasis and infection prevention, a few recent studies have shown that fish mucus microbiota may interact with and attract some specific parasitic species. However, our understanding of external microbial assemblages, in particular regarding the factors that determine their composition and potential interactions with parasites, is still limited. This is the objective of the present study that focuses on a well-known fish-parasite interaction, involving the Sparidae (Teleostei), and their specific monogenean ectoparasites of the Lamellodiscus genus. We characterized the skin and gill mucus bacterial communities using a 16S rRNA amplicon sequencing, tested how fish ecological traits and host evolutionary history are related to external microbiota, and assessed if some microbial taxa are related to some Lamellodiscus species. Results Our results revealed significant differences between skin and gill microbiota in terms of diversity and structure, and that sparids establish and maintain tissue and species-specific bacterial communities despite continuous exposure to water. No phylosymbiosis pattern was detected for either gill or skin microbiota, suggesting that other host-related and environmental factors are a better regulator of host-microbiota interactions. Diversity and structure of external microbiota were explained by host traits: host species, diet and body part. Numerous correlations between the abundance of given bacterial genera and the abundance of given Lamellodiscus species have been found in gill mucus, including species-specific associations. We also found that the external microbiota of the only unparasitized sparid species in this study, Boops boops, harbored significantly more Fusobacteria and three genera, Shewenella, Cetobacterium and Vibrio, compared to the other sparid species, suggesting their potential involvement in preventing monogenean infection. Conclusions This study is the first to explore the diversity and structure of skin and gill microbiota from a wild fish family and present novel evidence on the links between gill microbiota and monogenean species in diversity and abundance, paving the way for further studies on understanding host-microbiota-parasite interactions.

The epicontinental fauna of the Iberian Peninsula is strongly influenced by its geographical history. As the possibilities for dispersion of organisms into and from this region were (and still are) limited, the local fauna consists almost exclusively of endemic species. Almost all Iberian freshwater fishes of the families Leuciscidae and Cyprinidae are endemic and on-going research on these taxa continually uncovers new species. Nevertheless, information on their host-specific parasites remains scarce. In this study, we investigate the diversity and phylogenetic relationships in monogeneans of the genus Dactylogyrus (gill ectoparasites specific to cyprinoid fish) in the Iberian Peninsula. Twenty-two species were collected and identified from 19 host species belonging to Cyprinidae and Leuciscidae. A high degree of endemism was observed, with 21 Dactylogyrus species reported from Iberia only and a single species, D. borealis, also reported from other European regions. Phylogenetic analysis split the endemic Iberian Dactylogyrus into two well-supported clades, the first encompassing Dactylogyrus parasitising endemic Luciobarbus spp. only, and the second including all Dactylogyrus species of endemic leuciscids and four species of endemic cyprinids. Species delimitation analysis suggests a remarkable diversity and existence of a multitude of cryptic Dactylogyrus species parasitising endemic leuciscids (Squalius spp. and representatives of Chondrostoma s.l.). These results suggest a rapid adaptive radiation of Dactylogyrus in this geographically isolated region, closely associated with their cyprinoid hosts. Moreover, phylogenetic analysis supports that Dactylogyrus parasites colonised the Iberian Peninsula through multiple dispersion events., This study was financially funded by the Czech Science Foundation (project number 15-1938S).

International audience; Microorganisms are an important component in shaping the evolution of hosts and as such, the study of bacterial communities with molecular techniques is shedding light on the complexity of symbioses between bacteria and vertebrates. Teleost fish are a heterogeneous group that live in a wide variety of habitats, and thus a good model group to investigate symbiotic interactions and their influence on host biology and ecology. Here we describe the microbiota of thirteen teleostean species sharing the same environment in the Mediterranean Sea and compare bacterial communities among different species and body sites (external mucus, skin, gills, and intestine). Our results show that Proteobacteria is the dominant phylum present in fish and water. However, the prevalence of other bacterial taxa differs between fish and the surrounding water. Significant differences in bacterial diversity are observed among fish species and body sites, with higher diversity found in the external mucus. No effect of sampling time nor species individual was found. The identification of indicator bacterial taxa further supports that each body site harbors its own characteristic bacterial community. These results improve current knowledge and understanding of symbiotic relationships among bacteria and their fish hosts in the wild since the majority of previous studies focused on captive individuals.

The study of host-parasite coevolution is one of the cornerstones of evolutionary biology. The majority of fish ectoparasites belonging to the genus Dactylogyrus (Monogenea) exhibit a high degree of host specificity. Therefore, it is expected that their evolutionary history is primarily linked with the evolutionary history of their cyprinoid fish hosts and the historical formation of the landmasses. In the present study, we used a cophylogenetic approach to investigate coevolutionary relationships between endemic Cyprinoidea (Cyprinidae and Leuciscidae) from selected regions in southern Europe and their respective Dactylogyrus species. A total of 49 Dactylogyrus species including endemic and non-endemic species were collected from 62 endemic cyprinoid species in the Balkan and Apennine Peninsulas. However, 21 morphologically identified Dactylogyrus species exhibited different genetic variants (ranging from two to 28 variants per species) and some of them were recognized as cryptic species on the basis of phylogenetic reconstruction. Phylogenetic analyses revealed several lineages of endemic and non-endemic Dactylogyrus species reflecting some morphological similarities or host affinities. Using distance-based and event-based cophylogenetic methods, we found a significant coevolutionary signal between the phylogenies of parasites and their hosts. In particular, statistically significant links were revealed between Dactylogyrus species of Barbini (Cyprinidae) and their hosts belonging to the genera Aulopyge, Barbus and Luciobarbus. Additionally, a strong coevolutionary link was found between the generalist parasites D. alatus, D. sphyrna, D. vistulae, and their hosts, and between Dactylogyrus species of Pachychilon (Leuciscidae) and their hosts. Cophylogenetic analyses suggest that host-switching played an important role in the evolutionary history of Dactylogyrus parasitizing endemic cyprinoids in southern Europe. We propose that the high diversification of phylogenetically related cyprinoid species in the Mediterranean area is a process facilitating the host switching of specific parasites among highly diverse congeneric cyprinoids.

Viruses display strong interactions with their hosts, from physiological and ecological point of views, often leading to strict patterns of host specificity. It is then tempting to consider that viruses evolve in the same way as their hosts, behaving more or less like hosts' characters. However, the cospeciation between viruses and their hosts, that is the degree to which their evolutionary trees are similar, has been the subject or relatively few studies, in a field otherwise very dynamic. The main concepts and methods to study the patterns of cospeciation, and more generally cophylogeny, are reviewed here. Their uses with host-virus systems suggest that, contrarily to a common belief, the joint evolutionary history of viruses and their hosts is often complex. Without a rigorous cophylogeny study, it is then very risky to consider that the evolutionary history of viruses mirrors that of their hosts.

11 pages, 5 figures, 1 table, supplementary materials http://advances.sciencemag.org/cgi/content/full/6/14/eaay2587/DC1.-- Data and materials availability: The genome sequence for O. mediterraneus RCC2590 and OmV2 can be found under GenBank accession numbers WMKK00000000 and MN688676, respectively. The complete gene annotations of RCC2590 can be accessed on ORCAE (https://bioinformatics.psb.ugent.be/orcae/). MMETSP sequence data can be accessed from www.imicrobe.us/#/projects/104. The genomes and their GenBank accession numbers used in this study are as follows: B. prasinos (GCA_002220235.1), M. commoda RCC299 (GCA_000090985.2), M. pusilla CCMP1545 (GCA_000151265.1), O. tauri RCC4221 (CAID01000001.2 to CAID01000020.2, CR954200.2, and CR954199.2), O. lucimarinus CCE9901 (GCA_000092065.1), BpV1 (HM004432.1), BpV2 (HM004430.1), MpV1 (HM004429.1), OtV1 (FN386611.1), OtV2 (FN600414.1), OtV5 (EU304328.2), OtV6 (JN225873.1), OlV1 (HM004431.1), OlV2 (KP874736.1), OlV3 (HQ633060.1), OlV4 (JF974316.1), OlV5 (HQ632827.1), OlV6 (HQ633059.1), OlV7 (KP874737.1), and OmV1 (KP874735.1). Ostreococcus sp. RCC809 is available from the JGI Genome Portal under project ID 16233, Virus-microbe interactions in the ocean are commonly described by “boom and bust”dynamics, whereby a numerically dominant microorganism is lysed and replaced by a virus-resistant one. Here, we isolated a microalga strain and its infective dsDNA virus whose dynamics are characterized instead by parallel growth of both the microalga and the virus. Experimental evolution of clonal lines revealed that this viral production originates from the lysis of a minority of virus-susceptible cells, which are regenerated from resistant cells. Whole-genome sequencing demonstrated that this resistant-susceptible switch involved a large deletion on one chromosome. Mathematical modeling explained how the switch maintains stable microalga-virus population dynamics consistent with their observed growth pattern. Comparative genomics confirmed an ancient origin of this “accordion” chromosome despite a lack of sequence conservation. Together, our results show how dynamic genomic rearrangements may account for a previously overlooked coexistence mechanism in microalgae-virus interactions., This research was funded by the ANR grants REVIREC ANR-12-BSV7-0006-01, DECOVIR ANR-12-BSV7-0009, and PHYTNESS ANR-13-JSV6-0005. E.V. is funded by BOF project GOA 01G01715, and L.F.B. is funded by the European Union’s Horizon 2020 research and innovation program under the Marie Skłodowska-Curie grant agreement no. H2020-MSCA-ITN-2015-675752, With the funding support of the ‘Severo Ochoa Centre of Excellence’ accreditation (CEX2019-000928-S), of the Spanish Research Agency (AEI)

International audience; The green picoalgal genus Micromonas is broadly distributed in estuaries, coastal marine habitats and open oceans, from the equator to the poles. Phylogenetic, ecological and genomic analyses of culture strains and natural populations have suggested that this cosmopolitan genus is composed of several cryptic species corresponding to genetic lineages. We performed a detailed analysis of variations in morphology, pigment content, and sequences of the nuclear-encoded small-subunit rRNA gene and the second internal transcribed spacer (ITS2) from strains isolated worldwide. A new morphological feature of the genus, the presence of tip hairs at the extremity of the hair point, was discovered and subtle differences in hair point length were detected between clades. Clear non-homoplasious synapomorphies were identified in the small-subunit rRNA gene and ITS2 spacer sequences of five genetic lineages. These findings lead us to provide emended descriptions of the genus Micromonas, of the type species M. pusilla, and of the recently described species M. commoda, as well as to describe 2 new species, M. bravo and M. polaris. By clarifying the status of the genetic lineages identified within Micromonas, these formal descriptions will facilitate further interpretations of large-scale analyses investigating ecological trends in time and space for this widespread picoplankter.

The epicontinental fauna of the Iberian Peninsula is strongly influenced by its geographical history. As the possibilities for dispersion of organisms into and from this region were (and still are) limited, the local fauna consists almost exclusively of endemic species. Almost all Iberian freshwater fishes of the families Leuciscidae and Cyprinidae are endemic and on-going research on these taxa continually uncovers new species. Nevertheless, information on their host-specific parasites remains scarce. In this study, we investigate the diversity and phylogenetic relationships in monogeneans of the genus Dactylogyrus (gill ectoparasites specific to cyprinoid fish) in the Iberian Peninsula. Twenty-two species were collected and identified from 19 host species belonging to Cyprinidae and Leuciscidae. A high degree of endemism was observed, with 21 Dactylogyrus species reported from Iberia only and a single species, D. borealis, also reported from other European regions. Phylogenetic analysis split the endemic Iberian Dactylogyrus into two well-supported clades, the first encompassing Dactylogyrus parasitizing endemic Luciobarbus spp. only, and the second including all Dactylogyrus species of endemic leuciscids and four species of endemic cyprinids. Species delimitation analysis suggests a remarkable diversity and existence of a multitude of cryptic Dactylogyrus species parasitizing endemic leuciscids (Squalius spp. and representatives of Chondrostoma s.l.). These results suggest a rapid adaptive radiation of Dactylogyrus in this geographically isolated region, closely associated with their cyprinoid hosts. Moreover, phylogenetic analysis supports that Dactylogyrus parasites colonized the Iberian Peninsula through multiple dispersion events.

Phytoplankton-virus interactions are major determinants of geochemical cycles in the oceans. Viruses are responsible for the redirection of carbon and nutrients away from larger organisms back towards microorganisms via the lysis of microalgae in a process coined the ‘viral shunt’. Virus-host interactions are generally expected to follow ‘boom and bust’ dynamics, whereby a numerically dominant strain is lysed and replaced by a virus resistant strain. Here, we isolated a microalga and its infective nucleo-cytoplasmic large DNA virus (NCLDV) concomitantly from the environment in the surface NW Mediterranean Sea,Ostreococcus mediterraneus, and show continuous growth in culture of both the microalga and the virus. Evolution experiments through single cell bottlenecks demonstrate that, in the absence of the virus, susceptible cells evolve from one ancestral resistant single cell, and vice–versa; that is that resistant cells evolve from one ancestral susceptible cell. This provides evidence that the observed sustained viral production is the consequence of a minority of virus-susceptible cells. The emergence of these cells is explained by low-level phase switching between virus-resistant and virus-susceptible phenotypes, akin to a bet hedging strategy. Whole genome sequencing and analysis of the ~14 Mb microalga and the ~200 kb virus points towards ancient speciation of the microalga within theOstreococcusspecies complex and frequent gene exchanges between prasinoviruses infectingOstreococcusspecies. Re-sequencing of one susceptible strain demonstrated that the phase switch involved a large 60 Kb deletion of one chromosome. This chromosome is an outlier chromosome compared to the streamlined, gene dense, GC-rich standard chromosomes, as it contains many repeats and few orthologous genes. While this chromosome has been described in three different genera, its size increments have been previously associated to antiviral immunity and resistance in another species from the same genus. Mathematical modelling of this mechanism predicts microalga–virus population dynamics consistent with the observation of continuous growth of both virus and microalga. Altogether, our results suggest a previously overlooked strategy in phytoplankton–virus interactions.

High-throughput sequencing of Prasinovirus DNA polymerase and host green algal (Mamiellophyceae) ribosomal RNA genes was used to analyse the diversity and distribution of these taxa over a ∼10 000 km latitudinal section of the Indian Ocean. New viral and host groups were identified among the different trophic conditions observed, and highlighted that although unknown prasinoviruses are diverse, the cosmopolitan algal genera Bathycoccus, Micromonas and Ostreococcus represent a large proportion of the host diversity. While Prasinovirus communities were correlated to both the geography and the environment, host communities were not, perhaps because the genetic marker used lacked sufficient resolution. Nevertheless, analysis of single environmental variables showed that eutrophic conditions strongly influence the distributions of both hosts and viruses. Moreover, these communities were not correlated, in their composition or specific richness. These observations could result from antagonistic dynamics, such as that illustrated in a prey–predator model, and/or because hosts might be under a complex set of selective pressures. Both of these reasons must be considered to interpret environmental surveys of viruses and hosts, because covariation does not always imply interaction.

The genus Micromonas comprises distinct genetic clades that commonly dominate eukaryotic phytoplankton community from polar to tropical waters. This phytoplankter is also recurrently infected by abundant and genetically diverse prasinoviruses. Here we report on the interplay between prasinoviruses and Micromonas with regard to the genetic diversity of this host. For 1 year, we monitored the abundance of three clades of Micromonas and their viruses in the Western English Channel, both in the environment using clade-specific probes and flow cytometry, and in the laboratory using clonal strains of Micromonas clades to assay for their viruses by plaque-forming units. We showed that the seasonal fluctuations of Micromonas clades were closely mirrored by the abundance of their corresponding viruses, indicating that the members of Micromonas genus are susceptible to viral infection, regardless of their genetic affiliation. The characterization of 45 viral isolates revealed that Micromonas clades are attacked by specific virus populations, which exhibit distinctive clade specificity, life strategies and genetic diversity. However, some viruses can also cross-infect different host clades, suggesting a mechanism of horizontal gene transfer within the Micromonas genus. This study provides novel insights into the impact of viral infection for the ecology and evolution of the prominent phytoplankter Micromonas.

Parasites exhibiting a high degree of host specificity are expected to be intimately associated with their hosts. Therefore, the evolution of host-specific parasites is at least partially shaped by the evolutionary history and distribution of such hosts. Gill ectoparasites of Dactylogyrus (Monogenea) are specific to cyprinid fish. In the present study, we investigated the evolutionary history of 47 Dactylogyrus species from the Balkan Peninsula, the Mediteranean region exhibiting the highest cyprinid diversity in Europe, and from central European cyprinids. Phylogenetic analyses revealed four well-supported clades of endemic and non-endemic Dactylogyrus spp. with four basal taxa. Endemic cyprinids with a limited distribution range were parasitized by endemic Dactylogyrus species, but some of them shared several Dactylogyrus species with central European cyprinids. Species delimitation analyses based on molecular data suggest that Dactylogyrus diversity is higher than that defined from morphology. Some endemic cyprinid species harboured Dactylogyrus species of different origins, this probably resulting from multiple host switching. Our results support the view that the evolution of Dactylogyrus in the Balkans has been influenced not only by the historical dispersion and distribution of their cyprinid hosts, but also by recent contacts of non-native cyprinid species with endemic cyprinid fauna in this region.

Phylogenetic diversity in the Phycodnaviridae (double-stranded DNA viruses infecting photosynthetic eukaryotes) is most often studied using their DNA polymerase gene (PolB). This gene and its translated protein product can harbor a selfish genetic element called an "intein" that disrupts the sequence of the host gene without affecting its activity. After translation, the intein peptide sequence self-excises precisely, producing a functional ligated host protein. In addition, inteins can encode homing endonuclease (HEN) domains that permit the possibility of lateral transfers to intein-free alleles. However, no clear evidence for their transfer between viruses has previously been shown. The objective of this paper was to determine whether recent transfers of inteins have occurred between prasinoviruses (Phycodnaviridae) that infect the Mamiellophyceae, an abundant and widespread class of unicellular green algae, by using DNA sequence analyses and cophylogenetic methods. Our results suggest that transfer among prasinoviruses is a dynamic ongoing process and, for the first time in the Phycodnaviridae family, we showed a recombination event within an intein.

Prasinoviruses infecting unicellular green algae in the order Mamiellales (class Mamiellophyceae) are commonly found in coastal marine waters where their host species frequently abound. We tested 40 Ostreococcus tauri viruses on 13 independently isolated wild-type O. tauri strains, 4 wild-type O. lucimarinus strains, 1 Ostreococcus sp. (“ Ostreococcus mediterraneus ”) clade D strain, and 1 representative species of each of two other related species of Mamiellales, Bathycoccus prasinos and Micromonas pusilla . Thirty-four out of 40 viruses infected only O. tauri , 5 could infect one other species of the Ostreococcus genus, and 1 infected two other Ostreococcus spp., but none of them infected the other genera. We observed that the overall susceptibility pattern of Ostreococcus strains to viruses was related to the size of two host chromosomes known to show intraspecific size variations, that genetically related viruses tended to infect the same host strains, and that viruses carrying inteins were strictly strain specific. Comparison of two complete O. tauri virus proteomes revealed at least three predicted proteins to be candidate viral specificity determinants.

In the last few decades, Aedes albopictus (Skuse) (Diptera: Culicidae) (= Stegomyia albopicta), the so-called "Asian tiger mosquito", has spread from its native range in southeast Asia to Africa, the Middle East, Europe, the Americas, and Pacific islands. The spread of this species poses a risk to human health as it is considered to be one of the main vectors of dengue and other arbovi- ruses. Aedes albopictus was reported in Croatia in 2004, thereafter it was discovered at several coastal localities in 2005 and to date it has spread to most coastal areas and islands in Croatia. Here we investigate the genetic variability of A. albopictus based on 39 individuals collected during the summer of 2009 along the East-Adriatic coast and islands of Croatia and Montenegro and using two mitochondrial molecular markers: cytochrome oxidase I (COI) and NADH dehydrogenase 5 (ND5). We identified a single ND5 hap- lotype, corresponding to the previously reported and worldwide-distributed haplotype H3. The COI marker was more variable and we identified four COI haplotypes. In order to identify the geographic origin of the populations that colonized Croatia, we performed phylogenetic analyses of ND5 and COI haplotypes in Croatian populations and other A. albopictus populations retrieved from the GenBank. The phylogenetic tree based on ND5 haplotypes revealed two well supported clades where the unique Croatian ND5 hap- lotype clustered with the majority of haplotypes originating from South-Asia, America, Europe, and Africa. Another smaller cluster consisted of only Brazilian haplotypes. The phylogenetic tree and haplotype network that resulted from the COI analysis also indi- cates that the three Croatian COI haplotypes cluster with European and American haplotypes. However the fourth Croatian COI hap- lotype was the only European haplotype that occurred in a separate clade (group) with Indian, South-Asian, and Brazilian haplotypes. This data suggests there have been several independent introduction events in Croatia.

Although marine picophytoplankton are at the base of the global food chain, accounting for half of the planetary primary production, they are outnumbered 10 to 1 and are largely controlled by hugely diverse populations of viruses. Eukaryotic microalgae form a ubiquitous and particularly dynamic fraction of such plankton, with environmental clone libraries from coastal regions sometimes being dominated by one or more of the three genera Bathycoccus , Micromonas , and Ostreococcus (class Prasinophyceae). The complete sequences of two double-stranded (dsDNA) Bathycoccus , one dsDNA Micromonas , and one new dsDNA Ostreococcus virus genomes are described. Genome comparison of these giant viruses revealed a high degree of conservation, both for orthologous genes and for synteny, except for one 36-kb inversion in the Ostreococcus lucimarinus virus and two very large predicted proteins in Bathycoccus prasinos viruses. These viruses encode a gene repertoire of certain amino acid biosynthesis pathways never previously observed in viruses that are likely to have been acquired from lateral gene transfer from their host or from bacteria. Pairwise comparisons of whole genomes using all coding sequences with homologous counterparts, either between viruses or between their corresponding hosts, revealed that the evolutionary divergences between viruses are lower than those between their hosts, suggesting either multiple recent host transfers or lower viral evolution rates.

*2 CNRS, UMR 7621, Observatoire oceanologique, 66650, Banyuls-sur-Mer, France. Summary Although large DNA viruses of eukaryotic algae represent a major force in shaping populations of plankton, knowledge about them is often limited to their overall diversity, abundance, and the flux of their constituent matter between ecosystem compartments. In order to gain insight about the genetics and structure of such populations, we used an easily cultivable model unicellular algal species, Ostreococcus tauri (Prasinophyceae), to monitor and compare populations of viruses in different marine environments. The abundance of O. tauri viruses showed very large temporal fluctuations, but remarkably was more than two orders of magnitude higher in lagoons than in coastal waters. We analysed 161 individual viruses found after plating out for lysis plaques on the host during a time series of water samplings. The haplotypes of viruses infecting our host strain were determined by sequence analysis of the partial DNA polymerase gene, permitting a spatiotemporal analysis of their population structure. We found 48 haplotypes, only the two most abundant ones being shared among all of the three study sites (lagoon, coastal and offshore), supporting the hypothesis that there is great diversity among the viruses infecting one host strain. However, our data suggest that the population structure differ between lagoons and coastal sea.

In the present study, we used morphometry as a proxy to study the microevolution of generalist Lamellodiscus (Monogenea, Diplectanidae) species, comprising gill parasites of sparid fish. We investigated 147 individuals, belonging to nine described species, regrouped in four morphotypes. Morphometric measurements were taken on sclerotized parts of the attachment organ. The formation of groups on the basis of the global morphometry within a host species, or between several host species, was assessed using both exploratory analyses (principal component analysis and clustering analysis) and statistical tests. We showed that: (1) for three out of four morphotypes, the global morphometry was significantly different according to host species used, and (2) the coexistence of two populations of Lamellodiscus elegans on Diplodus sargus could reflect an ongoing intra-host speciation event. We suggest that generalist Lamellodiscus are undergoing specialization on their different hosts, which may lead to speciation. © 2010 The Linnean Society of London, Biological Journal of the Linnean Society, 2010, 99, 559–569.

International audience; Following the pioneering work of Felsenstein and Garland, phylogeneticists have been using regression through the origin to analyze comparative data using independent contrasts. The reason why regression through the origin must be used with such data was revisited. The demonstration led to the formulation of a permutation test for the coefficient of determination and the regression coefficient estimates in regression through the origin. Simulations were carried out to measure type I error and power of the parametric and permutation tests under two models of data generation: regression model I and model II (correlation model). Although regression through the origin assumes model I data, in independent contrast data error is present in the explanatory as well as the response variables. Two forms of permutations were investigated to test the regression coefficients: permutation of the values of the response variable , and permutation of the residuals of the regression model. The simulations showed that the parametric tests or any of the permutation tests can be used when the error is normal, which is the usual assumption in independent contrast studies; only the test by permutation of should be used when the error is highly asymmetric; and the parametric tests should be used when extreme values are present in covariables. Two examples are presented. The first one concerns non-specificity in fish parasites of the genus , the second the richness in parasites in 78 species of mammals.

Summary Viruses play an important role in the regulation of phytoplankton populations. In the Mediterranean Sea, prasinophyte green algae are abundant and wide- spread, and within this group the genera Bathycoc- cus, Micromonas and Ostreococcus (Mamiellales) are the most common. Although these organisms consti- tute a significant part of the marine ecosystem, little is known about the viruses infecting them. We showed that double-stranded DNA viruses, likely members of the Phycodnaviridae family, can infect and grow in different host laboratory prasinophyte strains. Differ- ent pairs of degenerate primers were designed to PCR amplify a region of the conserved viral polymerase gene in order to characterize these viral strains. Twenty-seven new viral strains from five different host strains were thus analysed. We established phy- logenetic trees for the hosts (18S) and their associ- ated viruses (partial polymerase gene) and discuss the taxonomic significance of Phycodnaviridae. Within eukaryotic double-stranded DNA viruses, we showed that viruses from Bathycoccus, Micromonas and Ostreococcus form a monophyletic group that we refer to as 'Prasinovirus'.

The biological features observed in every living organism are the outcome of three sets of factors: historical (inherited by homology), functional (biological adaptation), and structural (properties inherent to the materials with which organs are constructed, and the morphogenetic rules by which they grow). Integrating them should bring satisfactory causal explanations of empirical data. However, little progress has been accomplished in practice toward this goal, because a methodologically efficient tool was lacking. Here we use a new statistical method of variation partitioning to analyze bone growth in amniotes. (1) Historical component. The variation of bone growth rates contains a significant phylogenetic signal, suggesting that the observed patterns are partly the outcome of shared ancestry. (2) Functional causation. High growth rates, although energy costly, may be adaptive (i.e., they may increase survival rates) in taxa showing short growth periods (e.g., birds). In ectothermic amniotes, low resting metabolic rates may limit the maximum possible growth rates. (3) Structural constraint. Whereas soft tissues grow through a multiplicative process, growth of mineralized tissues is accretionary (additive, i.e., mineralization fronts occur only at free surfaces). Bone growth of many amniotes partially circumvents this constraint: it is achieved not only at the external surface of the bone shaft, but also within cavities included in the bone cortex as it grows centrifugally. Our approach contributes to the unification of historicism, functionalism, and structuralism toward a more integrated evolutionary biology.

The effect of a marine protected area on the Lamellodiscus spp. monogenean community was tested by comparing the communities of parasites of Diplodus sargus inside and outside of the area. A total of 104 D. sargus were dissected harbouring 1280 monogeneans from the genus Lamellodiscus and belonging to 11 species. No modification in the global parasite community linked with the protection of the host populations was revealed. The most abundant and less specific parasite species, Lamellodiscus elegans, however, increased its abundance in the protected area. A significant relationship was found between parasite host range and the percentage of infected hosts. A significant relationship also occurred between epidemiological and genetic distances for the parasite species found. The results are discussed in term of parasite success and specificity and the importance of taking into account parasitism in the biological conservation of hosts.

89Makarenkov, V., Legendre, P. & Desdevises, Y. (2004). Modelling phylogenetic relationshipsusing reticulated networks. — Zoologica Scripta, 33, 89–96.Most traditional methods of phylogenetic analysis assume that species evolution can be rep-resented by means of a bifurcating tree model. In many phylogenetic situations, however,some of the evolutionary links between species are due to reticulate evolution. For instance,reticulate models can adequately describe such complicated mechanisms as lateral gene trans-fer in bacteria or species hybridization. The theoretical concepts of reticulate evolution devel-oped in the 1980s and 1990s need to be supported by appropriate analytical tools and software.In this paper, we present the main features of a new distance-based method for modelling phy-logenetic relationships among species by means of reticulated networks (RNs). The methoduses the least-squares model to build a RN by gradually improving upon the solution providedby a phylogenetic tree. A computer program facilitating the reconstruction and visualizationof reticulate phylogenies is made available to researchers. In the application section, we illus-trate the usefulness of the method by studying the evolution of honeybees (genus Apis). Themethod for reconstructing RNs has been included in the T-Rex (Tree and Reticulogram Recon-struction) package recently developed by the first-named author.Vladimir Makarenkov. Departement d’informatique, Universite du Quebec a Montreal, C.P. 8888,succursale Centre-Ville, Montreal (Quebec), Canada, H3C 3P8. E-mail:makarenkov.vladimir@uqam.caPierre Legendre, Departement de sciences biologiques, Universite de Montreal, C.P. 6128, succursaleCentre-Ville, Montreal (Quebec), Canada, H3C 3J7. E-mail: pierre.legendre@umontreal.caYves Desdevises, Laboratoire Arago, Universite Pierre et Marie Curie, UMR CNRS 7628, BP 44,66651 Banyuls-sur-Mer Cedex, France. E-mail: desdevises@obs-banyuls.fr

Comparative analysis methods control for the variation linked to phylogeny before attempting to correlate the remaining variation of a trait to present-day conditions (i.e., ecology and/or environment). A portion of the phylogenetic variation of the trait may be related to ecology, however; this portion is called "phylogenetic niche conservatism." We propose a method of variation partitioning that allows users to quantify this portion of the variation, called the "phylogenetically structured environmental variation." The new method is applied to published data to study, in a phylogenetic framework, the link between body mass and population density in 79 species of mammals. The results suggest that an important part of the variation of mammal body mass is related to the common influence of phylogeny and population density.

High-throughput sequencing of Prasinovirus DNA polymerase and host green algal (Mamiellophyceae) ribosomal RNA genes was used to analyse the diversity and distribution of these taxa over a ∼10 000 km latitudinal section of the Indian Ocean. New viral and host groups were identified among the different trophic conditions observed, and highlighted that although unknown prasinoviruses are diverse, the cosmopolitan algal genera Bathycoccus, Micromonas and Ostreococcus represent a large proportion of the host diversity. While Prasinovirus communities were correlated to both the geography and the environment, host communities were not, perhaps because the genetic marker used lacked sufficient resolution. Nevertheless, analysis of single environmental variables showed that eutrophic conditions strongly influence the distributions of both hosts and viruses. Moreover, these communities were not correlated, in their composition or specific richness. These observations could result from antagonistic dynamics, such as that illustrated in a prey-predator model, and/or because hosts might be under a complex set of selective pressures. Both of these reasons must be considered to interpret environmental surveys of viruses and hosts, because covariation does not always imply interaction.

The evolution and determinants of host specificity in Lamellodiscus species (Monogenea, Diplectanidae) were investigated. The 20 known Mediterranean species were studied, all parasites of fishes from the family Sparidae (Teleostei). An index of specificity, which takes into account the phylogenetic relationships of their fish host species, was defined. The link between specificity and its potential determinants was investigated in a phylogenetic context using the method of independent contrasts. Host specificity in Lamellodiscus species appeared to be highly constrained by phylogeny, but also linked to host size. Mapping specificity onto the parasite phylogenetic tree suggests that specialist species do not represent an evolutionary dead end, and that specialization is not a derived condition. It is hypothesized that the ability to be generalist or specialist in Lamellodiscus is controlled by intrinsic, phylogenetically-related characteristics, and that specialist species tend to use large hosts, which may be more predictable. © 2002 The Linnean Society of London, Biological Journal of the Linnean Society , 2002, 77, 431−443.

A new method, ParaFit, has been developed to test the significance of a global hypothesis of coevolution between parasites and their hosts. Individual host-parasite association links can also be tested. The test statistics are functions of the host and parasite phylogenetic trees and of the set of host-parasite association links. Numerical simulations are used to show that the method has correct rate of type I error and good power except under extreme error conditions. An application to real data (pocket gophers and chewing lice) is presented.

Numerous seawater lagoons punctuate the southern coastline of France. Exchanges of seawater between these lagoons and the open sea are limited by narrow channels connecting them. Lagoon salinities vary according to evaporation and to the volume of freshwater arriving from influent streams, whose nutrients also promote the growth of algae. We compared Prasinovirus communities, whose replication is supported by microscopic green algae, in four lagoons and at a coastal sampling site. Using high-throughput sequencing of DNA from a giant virus-specific marker gene, we show that the environmental conditions significantly affect the types of detectable viruses across samples. In spatial comparisons between 5 different sampling sites, higher levels of phosphates, nitrates, nitrites, ammonium and silicates tend to increase viral community richness independently of geographical distances between the sampling sites. Finally, comparisons of Prasinovirus communities at 2 sampling sites over a period of 10 months highlighted seasonal effects and the preponderant nature of phosphate concentrations in constraining viral distribution.

Viruses strongly influence the ecology and evolution of their eukaryotic hosts in the marine environment, but little is known about their diversity and distribution. Prasinoviruses infect an abundant and widespread class of phytoplankton, the Mamiellophyceae, and thereby exert a specific and important role in microbial ecosystems. However, molecular tools to specifically identify this viral genus in environmental samples are still lacking. We developed two primer sets, designed for use with polymerase chain reactions and 454 pyrosequencing technologies, to target two conserved genes, encoding the DNA polymerase (PolB gene) and the major capsid protein (MCP gene). While only one copy of the PolB gene is present in Prasinovirus genomes, there are at least seven paralogs for MCP, the copy we named number 6 being shared with other eukaryotic alga-infecting viruses. Primer sets for PolB and MCP6 were thus designed and tested on 6 samples from the Tara Oceans project. The results suggest that the MCP6 amplicons show greater richness but that PolB gave a wider coverage of Prasinovirus diversity. As a consequence, we recommend use of the PolB primer set, which will certainly reveal exciting new insights about the diversity and distribution of prasinoviruses at the community scale.

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.

The genus Furnestinia (Diplectanidae) contains only one species, Furnestinia echeneis. However, its close morphological similarity with the genus Lamellodiscus (Diplectanidae) raises doubt concerning its placement in a separate genus. These two genera differ only by their number of lamellodiscs: one for Furnestinia, two for Lamellodiscus. Here, the taxonomic position of F. echeneis is investigated via a phylogenetic reconstruction based on partial 18S rDNA for F. echeneis and several Lamellodiscus species. Furnestinia echeneis appears to be clearly nested into the Lamellodiscus genus, these two genera should then be synonymised. The hypertrophy of its unique lamellodisc is hypothesised to be a morphological adaptation for attachment to the host.

We sequenced DNA fragments from four monogenean species of the genus Lamellodiscus and their three fish host species from the genus Pagellus in the North Mediterranean Sea, in order to estimate the molecular divergence and the coevolutionary interactions in this association. By comparing the ITS1 sequences of the parasites, we assessed their level of interspecific differences and tested the phylogenetic status of Lamellodiscus virgula and Lamellodiscus obeliae, formerly described as two different species. Moreover, we wanted to know if closely related parasites used closely related hosts, to investigate the coevolutionary interactions in this complex. Phylogenetic relationships among Lamellodiscus species were estimated with partial 18S ribosomal DNA sequences while mitochondrial cytochrome-b DNA sequences were used for their fish hosts. The ITS1 sequences appear to be highly variable among Lamellodiscus species, except L.virgula and L.obeliae, suggesting an old divergence time or a rapid molecular evolution within this genus. This fish-parasite association seems to exhibit coevolutionary interactions. L.virgula and L.obeliae are proposed to be a single species on the basis of their almost identical ITS1 sequences.

Within ectoparasitic fish monogeneans, the genus Ligophorus contains a high number of species from which several were recently described. The precise determination of their taxonomic status requires robust diagnostic morphologic features that rely predominantly on a restricted set of sclerotized structures. In the present study, these morphological characters were used for the reconstruction of a phylogenetic tree, which was compared with a tree built from molecular data (28S and ITS1 DNA sequences). Thirty-eight morphological characters were used in 29 species of Ligophorus from the Atlantic and Pacific regions and 5 species within close genera of Dactylogyridae. The morphological and molecular phylogenetic trees are congruent and suggest that the genus Ligophorus is monophyletic, and that species parasitizing Liza spp. and Chelon labrosus occupy basal positions. The present study suggests that host switching is a common event in this host–parasite association, because about half of the species infecting the same host species are not close relatives. Following host switching, dispersal with vicariance is probably an important force shaping the present distribution and diversity of Ligophorus. The pattern of occurrence of Ligophorus spp. on Mugil cephalus supports that reproductive isolation and therefore parallel speciation are taking place among these parasitic organisms.

Phylogenetic diversity in the Phycodnaviridae (double-stranded DNA viruses infecting photosynthetic eukaryotes) is most often studied using their DNA polymerase gene (PolB). This gene and its translated protein product can harbor a selfish genetic element called an "intein" that disrupts the sequence of the host gene without affecting its activity. After translation, the intein peptide sequence self-excises precisely, producing a functional ligated host protein. In addition, inteins can encode homing endonuclease (HEN) domains that permit the possibility of lateral transfers to intein-free alleles. However, no clear evidence for their transfer between viruses has previously been shown. The objective of this paper was to determine whether recent transfers of inteins have occurred between prasinoviruses (Phycodnaviridae) that infect the Mamiellophyceae, an abundant and widespread class of unicellular green algae, by using DNA sequence analyses and cophylogenetic methods. Our results suggest that transfer among prasinoviruses is a dynamic ongoing process and, for the first time in the Phycodnaviridae family, we showed a recombination event within an intein.

The Clipperton lagoon in the North Pacific Ocean has been isolated from the surrounding sea for c. 160 years. It has a stratified water column that comprises an oxic and brackish upper water layer (mixolimnion) and a deep sulfuric anoxic saline layer (monimolimnion), separated by a steep pycnocline. Here, we test whether the Clipperton lagoon with its distinctive physico-chemical features, geographic isolation, recent water column stratification, and large nutrient input harbors original microbial communities. The combination of capillary electrophoresis single-strand polymorphism (CE-SSCP) fingerprinting and sequencing of cloned bacterial and archaeal 16S rRNA genes, and functional genes for methanogenesis (mcrA), methanotrophy (pmoA), and sulfate reduction (dsrAB), revealed that microbial communities and pathways were highly stratified down the water column. The mixolimnion contained ubiquitous freshwater clades of Alpha- and Betaproteobacteria, while the pycnocline contained mostly green sulfur bacteria (phylum Chlorobi). Sequences of the upper layers were closely related to sequences found in other aquatic ecosystems, suggesting that they have a strong potential for dispersal and colonization. In contrast, the monimolimnion contained new deeply branching bacterial divisions within the OP11 cluster and the Bacteroidetes, and was the most diverse of the layers. The unique environmental conditions characterizing the deep layers of the lagoon may explain the novelty of the microbial communities found at the Clipperton atoll.

We would like to thank Dag Atle Lysne and Pej Rohani for discussions and comments. The workshop was organized by Serge Morand (Universite de Perpignan, France), Robert Poulin (University of Otago, Dunedin, New Zealand) and Arne Skorping (University of Bergen, Norway), and was funded by the European Science Foundation, Centre National de la Recherche Scientifique, the Universite de Perpignan, the Pole Europeen, and the Region Languedoc-Rousillon.

Host-parasite cophylogeny is a topic that has grasped the attention of scientists since the end of the 19th century, but the development of dedicated analytical methods only arose in the last 30 years. Research on host-parasite systems and on the development of more and more sophisticated numerical methods to estimate the degree of cospeciation has thus progressed, permitting the elaboration of evolutionary scenarios. The main outcome of these studies is that the expected clear pattern of cospeciation between many hosts and parasites is often obscure. In practice, much attention has been devoted to few host-parasite systems. Particularly, aquatic host-parasite associations have not been so extensively studied, and, after briefly reviewing the main analytical methods, this paper focuses on host-monogenean systems, because this kind of interaction is expected to be an ideal model for cophylogeny studies. But is it? And what does it tell us about the evolutionary and ecological forces driving cospeciation in the open sea? Biogeography should also be considered when possible, and it has been useful for explaining some patterns of cospeciation. It should thus be more deeply exploited in the future. We need new methods and new biological models that better, if not fully, depict patterns and thereby permit deeper understanding of processes within cophylogenetic patterns.

The biological features observed in every living organism are the outcome of three sets of factors: historical (inherited by homology), functional (biological adaptation), and structural (properties inherent to the materials with which organs are constructed, and the morphogenetic rules by which they grow). Integrating them should bring satisfactory causal explanations of empirical data. However, little progress has been accomplished in practice toward this goal, because a methodologically efficient tool was lacking. Here we use a new statistical method of variation partitioning to analyze bone growth in amniotes. (1) Historical component. The variation of bone growth rates contains a significant phylogenetic signal, suggesting that the observed patterns are partly the outcome of shared ancestry. (2) Functional causation. High growth rates, although energy costly, may be adaptive (i.e., they may increase survival rates) in taxa showing short growth periods (e.g., birds). In ectothermic amniotes, low resting metabolic rates may limit the maximum possible growth rates. (3) Structural constraint. Whereas soft tissues grow through a multiplicative process, growth of mineralized tissues is accretionary (additive, i.e., mineralization fronts occur only at free surfaces). Bone growth of many amniotes partially circumvents this constraint: it is achieved not only at the external surface of the bone shaft, but also within cavities included in the bone cortex as it grows centrifugally. Our approach contributes to the unification of historicism, functionalism, and structuralism toward a more integrated evolutionary biology.

The aims of this study were to test the ability of CE-SSCP in describing the variability of the drgestive contents flora of SPF (Specified Pathogenic Free) pigs from our experimental husbandry and to reach, by mrxing individual samples , the concept of a digestive flora 's profile characteristic of a batch of pigs. The faeces of s1x SPF sows were sampled and extracted DNA were m1xed to constitute more and more composite samples. In addition , the caecal contents of 12 SPF piglets, 1ssued from a srngle sow, were collected after slaughter at 28, 56 and 84 days postpartum and rndividually or after pooling tested. The DNA of each sample was extracted using the QIAamp DNA Stool Min1kit. The PCR for amplifying the rONA 16S V3 region was carried out on the individual DNAs and the mrxed DNAs corresponding to each date of slaughtering. Then the PCR products were analysed by CE-SSCP The reproducibility of the method has been tested first, and then the analysis of the profiles obtarned from the faeces of the swine was conducted. It described a withinindividual variability of about 40% when comparing SPF sows. When we mixed samples, variability between the profiles decreases with increasing number of faeces constituting the pool of DNA. Concernrng the p1glets, for each date of sampling , the pool of ccecal contents DNA defined a characteristic group profile . Moreover, this profile vaned with the age of the prglets These results confirmed the use of CE SSCP as a tool for the description of the of digestive flora balances and their evolut1on at the batch level. Thrs will be of particular interest in both animal health or food hygrene contexts associated wrth drgestive flora perturbatron.

The arylalkylamine N-acetyltransferase (AANAT) is a key enzyme in the rhythmic production of melatonin. Two Aanats are expressed in Teleost fish (Aanat1 in the retina and Aanat2 in the pineal organ) but only Aanat1 is found in tetrapods. This study reports the cloning of Aanat1 from R. perezi. Transcripts were mainly expressed in the retina, diencephalon, intestine and testis. In the retina and pineal organ, Aanat1 expression was in the photoreceptor cells. Expression was also seen in ependymal cells of the 3rd ventricle and discrete cells of the suprachiasmatic area. The expression of Aanat1 in both the retina and pineal organ, and the absence of Aanat2 suggests that green frog resembles more to birds and mammals than to Teleost fish, as far as Aanat is concerned. The significance of Aanat1 in extra-pineal and extra-retinal tissues remains to be elucidated; in the diencephalon, it might be associated to the so-called deep brain photoreceptor cells.

We reviewed several published and ongoing studies concerning monogenean communities. Patterns of species richness, host specificity, community structure and host–parasite coevolutionary interaction were carefully analysed, and hypotheses of evolutionary processes are proposed. The structuring of monogenean communities seems to be related to both ecological and historical constraints. The database supports an absence of intra- and interspecific competition in monogeneans. Species richness seems to be more due to host characteristics than to parasite interactions. Monogeneans seem to specialise on large hosts, leading to greater species richness on those hosts. The morphometric evolution of attachment and copulatory organs support the hypothesis of a reproductive segregation among conspecifics parasitising the same host(s). It also suggests the existence of concurrent adaptive and non-adaptive processes. The general absence of a coevolutionary pattern between host and parasites also suggests the constraints of history without dismissing the influences of ecological factors in the structuring of the communities. More generally, we strengthen the need to study the structure of communities in a phylogenetic context.

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.

Co-existence among potentially competing species can be favoured by niche specialisation and/or by reducing the overall intensity of competition via aggregated utilisation of fragmented resources. We investigated the respective roles of niche specialisation and aggregation in the case of nine congeneric monogenean parasites on the gills of Roach ( Rutilus rutilus L.) belonging to the genus Dactylogyrus . The position of each individual parasite of the nine Dactylogyrus species was recorded. Niche breadth and niche overlap of parasite species were estimated. Comparative methods, which take into account phylogenetic information of the analysed species, were used. We reconstructed a phylogeny of the nine Dactylogyrus species based on morphological characters. We used the ‘aggregation model of co-existence’ in the model to test if species co-existence is facilitated when intraspecific aggregation exceeds interspecific aggregation. We observed a lack of negative correlation in abundance between pairs of parasites, and a negative correlation between niche size and parasite aggregation, for both intraspecific and interspecific aggregation. Our comparative analysis showed that parasite abundance is positively correlated with niche breadth. Then parasite abundance, and not interactions between Dactylogyrus species, seems to be the most important factor determining niche size This result gives some support to niche segregation by specialisation. Niche size was negatively correlated with both intraspecific and interspecific aggregation. No relationship was found between an increase of interspecific aggregation with an increase of niche overlapping, which suggests that competition may play little role. A lack of competition could be also confirmed by the lack of negative correlation in abundance between species pairs. A parsimony analysis of the evolution of gill distribution indicates a change in one parameter of the niche (arch, segment and/or area) at each branching event.