Host Species and Body Site Explain the Variation in the Microbiota Associated to Wild Sympatric Mediterranean Teleost Fishes.

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.

Interplay between the genetic clades of Micromonas and their viruses in the Western English Channel.

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.

Host specificity is linked to intraspecific variability in the genus Lamellodiscus (Monogenea).

SUMMARYWe investigated whether host specificity is linked to variability within species of Lamellodiscus monogeneans, which are gill ectoparasites of the Sparidae. We sampled fish parasites in the northeastern part of the Mediterranean Sea: 4 specialist species, using 1 single host species, and 3 generalist species, using 2 distinct host species. Intraspecific variability was assessed from 2 different datasets. Morphometric variability of the attachment organ, called the haptor, was estimated first from measurements of several sclerified haptoral parts on 102 individuals. Genetic variability was calculated based on comparisons of sequences derived from the first internal transcribed spacer (ITS-1) of nuclear ribosomal DNA of 62 individuals. Morphometric variances in the specialist versus generalist species were compared via principal component analysis and F-tests, and uncorrected genetic distances (p-distances) were estimated within each species. We showed that the inter-individual variance of morphometric characters, as well as p-distances, are clearly greater within generalist species than specialist ones. These findings suggest that a relative increase in morphological and molecular variability enhances the possibility to colonize new host species in monogeneans, and supports the hypothesis that intraspecific variability could be a potential determinant of host specificity.

Cophylogeny: insights from fish-parasite systems.

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.

Determinants of parasite species richness on small taxonomical and geographical scales: Lamellodiscus monogeneans of northwestern Mediterranean sparid fish.

Determinants of parasite species richness have been investigated in a host-parasite system comprising fish of the family Sparidae and their monogenean gill ectoparasites of the genus Lamellodiscus. This study was carried out on a small geographical scale in the northwestern Mediterranean Sea. Host phylogenetic relationships were taken into account by phylogenetic eigenvector regression which required the reconstruction of a phylogenetic tree for the sparid fish species using mtDNA sequences. Several ecological variables potentially acting on Lamellodiscus species richness were considered. Host body size and host migratory behaviour appeared to be the main determinants of parasite species richness in this system. It is concluded that structuring of monogenean communities is controlled more by ecological than evolutionary factors.

Phylogenetic relationships and evolution of cleaning behaviour in the family Labridae: importance of body colour pattern.

The family Labridae contains numerous fishes known to act as cleaners in the wild. Previous studies suggested that a small body size and specific colour patterns may be prerequisites for cleaning. We investigated whether cleaning behaviour is linked to particular fish phenotypes. We first present a phylogeny based on partial 12S rRNA gene sequences of 32 wrasses sampled from different localities in the Indo-Pacific and Atlantic oceans, and in the Mediterranean Sea. Secondly, descriptive data (fish body size, fish body shape and fish body colour patterns) were analyzed in a phylogenetic context using comparative methods. We found no relationship between fish cleaning behaviour and fish body size and shape, but instead a correlation between cleaning behaviour and the presence of a dark lateral stripe within wrasses. Our results suggest that the evolution of cleaning depends upon the presence of a dark median lateral stripe on the fish body surface.

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.

The phylogenetic position of Furnestinia echeneis (Monogenea, Diplectanidae) based on molecular data: a case of morphological adaptation?

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.

Co-existence of nine gill ectoparasites (Dactylogyrus: monogenea) parasitising the roach (Rutilus rutilus l.): history and present ecology.

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.

Comparison of ribosomal DNA sequences of Lamellodiscus spp. (monogenea, diplectanidae) parasitising Pagellus (sparidae, teleostei) in the North Mediterranean Sea: species divergence and coevolutionary interactions.

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.