ITS-2 and 18S rRNA gene phylogeny of Aplysinidae (Verongida, Demospongiae)

Niche-dependent sponge hologenome expression profiles and the host-microbes interplay: a case of the hawaiian demosponge Mycale Grandis

BACKGROUND

Most researches on sponge holobionts focus primarily on symbiotic microbes, yet data at the level of the sponge hologenome are still relatively scarce. Understanding of the sponge host and its microbial gene expression profiles and the host-microbes interplay in different niches represents a key aspect of sponge hologenome. Using the Hawaiian demosponge Mycale grandis in different niches as a model, i.e. on rocks, on the surface of coral Porites compressa, under alga Gracilaria salicornia, we compared the bacterial and fungal community structure, functional gene diversity, expression pattern and the host transcriptome by integrating open-format (deep sequencing) and closed-format (GeoChip microarray) high-throughput techniques.

RESULTS

Little inter-niche variation in bacterial and fungal phylogenetic diversity was detected for M. grandis in different niches, but a clear niche-dependent variability in the functional gene diversity and expression pattern of M. grandis host and its symbiotic microbiota was uncovered by GeoChip microarray and transcriptome analyses. Particularly, sponge host genes related to innate immunity and microbial recognition showed a strong correlation with the microbial symbionts' functional gene diversity and transcriptional richness in different niches. The cross-niche variability with respect to the symbiont functional gene diversity and the transcriptional richness of M. grandis holobiont putatively reflects the interplay of niche-specific selective pressure and the symbiont functional diversity.

CONCLUSIONS

Niche-dependent gene expression profiles of M. grandis hologenome and the host-microbes interplay were suggested though little inter-niche variation in bacterial and fungal diversity was detected, particularly the sponge innate immunity was found to be closely related to the symbiotic microbes. Altogether, these findings provide novel insights into the black box of one sponge holobiont in different niches at the hologenome level.

Characterizing the bacterial communities associated with Mediterranean sponges: a metataxonomic analysis

The oceans cover over 70% of our planet, hosting a biodiversity of tremendous wealth. Sponges are one of the major ecosystem engineers on the seafloor, providing a habitat for a wide variety of species to be considered a good source of bioactive compounds. In this study, a metataxonomic approach was employed to describe the bacterial communities of the sponges collected from Faro Lake (Sicily) and Porto Paone (Gulf of Naples). Morphological analysis and amplification of the conserved molecular markers, including 18S and 28S (RNA ribosomal genes), CO1 (mitochondrial cytochrome oxidase subunit 1), and ITS (internal transcribed spacer), allowed the identification of four sponges. Metataxonomic analysis of sponges revealed a large number of amplicon sequence variants (ASVs) belonging to the phyla Proteobacteria, Cloroflexi, Dadabacteria, and Poribacteria. In particular, Myxilla (Myxilla) rosacea and Clathria (Clathria) toxivaria displayed several classes such as Alphaproteobacteria, Dehalococcoidia, Gammaproteobacteria, Cyanobacteria, and Bacteroidia. On the other hand, the sponges Ircinia oros and Cacospongia mollior hosted bacteria belonging to the classes Dadabacteriia, Anaerolineae, Acidimicrobiia, Nitrospiria, and Poribacteria. Moreover, for the first time, the presence of Rhizobiaceae bacteria was revealed in the sponge M. (Myxilla) rosacea, which was mainly associated with soil and plants and involved in biological nitrogen fixation.

First certain record of Demospongiae class (Porifera) alien species from the Mediterranean Sea

In this paper, we identify some sponge specimens collected in the Faro Lake in Sicily, and belonging to Haliclona (Halicoclona) by using morphological analysis accompanied by molecular analysis through amplification of several molecular markers (18S and 28S rRNA, CO1 and ITS). The samples are identified as. H. (Halichoclona) vansoesti de Weerdt, de Kluijver & Gómez, 1999, a species native to the Caribbean, and therefore this is the first record of an alien species of the Demospongiae class (Porifera) from the Mediterranean Sea. This presence can be ascribed as results of global change (mainly global warming) that are affecting marine environment.

Microbial diversity in Mediterranean sponges as revealed by metataxonomic analysis

Although the Mediterranean Sea covers approximately a 0.7% of the world's ocean area, it represents a major reservoir of marine and coastal biodiversity. Among marine organisms, sponges (Porifera) are a key component of the deep-sea benthos, widely recognized as the dominant taxon in terms of species richness, spatial coverage, and biomass. Sponges are evolutionarily ancient, sessile filter-feeders that harbor a largely diverse microbial community within their internal mesohyl matrix. In the present work, we firstly aimed at exploring the biodiversity of marine sponges from four different areas of the Mediterranean: Faro Lake in Sicily and "Porto Paone", "Secca delle fumose", "Punta San Pancrazio" in the Gulf of Naples. Eight sponge species were collected from these sites and identified by morphological analysis and amplification of several conserved molecular markers (18S and 28S RNA ribosomal genes, mitochondrial cytochrome oxidase subunit 1 and internal transcribed spacer). In order to analyze the bacterial diversity of symbiotic communities among these different sampling sites, we also performed a metataxonomic analysis through an Illumina MiSeq platform, identifying more than 1500 bacterial taxa. Amplicon Sequence Variants (ASVs) analysis revealed a great variability of the host-specific microbial communities. Our data highlight the occurrence of dominant and locally enriched microbes in the Mediterranean, together with the biotechnological potential of these sponges and their associated bacteria as sources of bioactive natural compounds.

A Metataxonomic Approach Reveals Diversified Bacterial Communities in Antarctic Sponges

Marine sponges commonly host a repertoire of bacterial-associated organisms, which significantly contribute to their health and survival by producing several anti-predatory molecules. Many of these compounds are produced by sponge-associated bacteria and represent an incredible source of novel bioactive metabolites with biotechnological relevance. Although most investigations are focused on tropical and temperate species, to date, few studies have described the composition of microbiota hosted by Antarctic sponges and the secondary metabolites that they produce. The investigation was conducted on four sponges collected from two different sites in the framework of the XXXIV Italian National Antarctic Research Program (PNRA) in November-December 2018. Collected species were characterized as Mycale (Oxymycale) acerata, Haliclona (Rhizoniera) dancoi, Hemigellius pilosus and Microxina sarai by morphological analysis of spicules and amplification of four molecular markers. Metataxonomic analysis of these four Antarctic sponges revealed a considerable abundance of Amplicon Sequence Variants (ASVs) belonging to the phyla Proteobacteria, Bacteroidetes, Actinobacteria and Verrucomicrobia. In particular, M. (Oxymycale) acerata, displayed several genera of great interest, such as Endozoicomonas, Rubritalea, Ulvibacter, Fulvivirga and Colwellia. On the other hand, the sponges H. pilosus and H. (Rhizoniera) dancoi hosted bacteria belonging to the genera Pseudhongella, Roseobacter and Bdellovibrio, whereas M. sarai was the sole species showing some strains affiliated to the genus Polaribacter. Considering that most of the bacteria identified in the present study are known to produce valuable secondary metabolites, the four Antarctic sponges could be proposed as potential tools for the discovery of novel pharmacologically active compounds.

Molecular and morphological assessment of tropical sponges in the subfamily Phyllospongiinae, with the descriptions of two new species

Sponges in the subfamily Phyllospongiinae are important components of coral reefs. However, significant taxonomic inconsistencies exist in this group due to the lack of useful morphological characters for species delineation. This study assesses the systematics of some common phyllospongiinids in the genera Carteriospongia, Phyllospongia and Strepsichordaia from tropical Australia and the Red Sea, by using a multigene approach that utilizes the Internal Transcribed Spacer 2, the complete ribosomal 18S rRNA and three 28s rRNA gene regions (D1‒D2, D3‒D5 and D6‒D8), which produced a phylogenetic framework in which complementary morphological taxonomic assessments were performed. Type specimens were included, where available, and six species clades were recovered, including the well-established Phyllospongia papyracea and Strepsichordaia lendenfeldi. Carteriospongia foliascens, the type species for the genus Carteriospongia, is transferred to the genus Phyllospongia, resulting in Carteriospongia becoming a synonym of Phyllospongia. Consequently, Carteriospongia flabellifera is removed from Carteriospongia and is reinstated to its original designation of Polyfibrospongia flabellifera. Two new species, Phyllospongia bergquistae sp. nov. and Polyfibrospongia kulit sp. nov., are described. With phyllospongiinid sponges increasingly used as models for assessing the effects of climate change and anthropogenic stressors, this study provides a reliable systematics framework for the accurate identification of common phyllospongiinids across the Indo-Pacific.

A consensus secondary structure of ITS2 in the chlorophyta identified by phylogenetic reconstruction

The definition of species plays a pivotal role in biology. It has been proposed that Compensatory Base Changes (CBCs) in the fast-evolving Internal Transcribed Spacer 2 (ITS2) correlate with speciation and thus can be used to distinguish species. The applicability of CBC - based species concepts using ITS2, however, rests on the homology of the investigated ITS2 positions. We studied the ITS2 molecule of 147 strains of Chlorophyceae (Chlorophyta, Viridiplantae) including 26 new sequences in the order Chaetophorales, and compared their secondary structures to ITS2 in the sister class Ulvophyceae, represented by the order Ulvales. Using a phylogenetic/comparative approach, it was possible to identify 1) the first consensus structure model of the ITS2 molecule that can be applied to two classes of green algae [Ulvophyceae (Ulvales), Chlorophyceae] and 2) landmarks (the spacer regions separating the ITS2 Helices) for more robust prediction of the secondary structures in green algae. Moreover, we found that CBCs in homologous positions in these 147 strains (representing 115 validly described species) are either completely absent or mostly associated with internal branches representing higher order taxonomic levels (genera, families, orders). As reported for the Ulvales, CBCs are not diagnostic at the species level in the dataset used.

Integrative taxonomy and molecular phylogeny of genus Aplysina (Demospongiae: Verongida) from Mexican Pacific

Integrative taxonomy provides a major approximation to species delimitation based on integration of different perspectives (e.g. morphology, biochemistry and DNA sequences). The aim of this study was to assess the relationships and boundaries among Eastern Pacific Aplysina species using morphological, biochemical and molecular data. For this, a collection of sponges of the genus Aplysina from the Mexican Pacific was studied on the basis of their morphological, chemical (chitin composition), and molecular markers (mitochondrial COI and nuclear ribosomal rDNA: ITS1-5.8-ITS2). Three morphological species were identified, two of which are new to science. A. clathrata sp. nov. is a yellow to yellow-reddish or -brownish sponge, characterized by external clathrate-like morphology; A. revillagigedi sp. nov. is a lemon yellow to green, cushion-shaped sometimes lobate sponge, characterized by conspicuous oscules, which are slightly elevated and usually linearly distributed on rims; and A. gerardogreeni a known species distributed along the Mexican Pacific coast. Chitin was identified as the main structural component within skeletons of the three species using FTIR, confirming that it is shared among Verongida sponges. Morphological differences were confirmed by DNA sequences from nuclear ITS1-5.8-ITS2. Mitochondrial COI sequences showed extremely low but diagnostic variability for Aplysina revillagigedi sp. nov., thus our results corroborate that COI has limited power for DNA-barcoding of sponges and should be complemented with other markers (e.g. rDNA). Phylogenetic analyses of Aplysina sequences from the Eastern Pacific and Caribbean, resolved two allopatric and reciprocally monophyletic groups for each region. Eastern Pacific species were grouped in general accordance with the taxonomic hypothesis based on morphological characters. An identification key of Eastern Pacific Aplysina species is presented. Our results constitute one of the first approximations to integrative taxonomy, phylogeny and evolutionary biogeography of Eastern Pacific marine sponges; an approach that will significantly contribute to our better understanding of their diversity and evolutionary history.

Sponge systematics facing new challenges

Systematics is nowadays facing new challenges with the introduction of new concepts and new techniques. Compared to most other phyla, phylogenetic relationships among sponges are still largely unresolved. In the past 10 years, the classical taxonomy has been completely overturned and a review of the state of the art appears necessary. The field of taxonomy remains a prominent discipline of sponge research and studies related to sponge systematics were in greater number in the Eighth World Sponge Conference (Girona, Spain, September 2010) than in any previous world sponge conferences. To understand the state of this rapidly growing field, this chapter proposes to review studies, mainly from the past decade, in sponge taxonomy, nomenclature and phylogeny. In a first part, we analyse the reasons of the current success of this field. In a second part, we establish the current sponge systematics theoretical framework, with the use of (1) cladistics, (2) different codes of nomenclature (PhyloCode vs. Linnaean system) and (3) integrative taxonomy. Sponges are infamous for their lack of characters. However, by listing and discussing in a third part all characters available to taxonomists, we show how diverse characters are and that new ones are being used and tested, while old ones should be revisited. We then review the systematics of the four main classes of sponges (Hexactinellida, Calcispongiae, Homoscleromorpha and Demospongiae), each time focusing on current issues and case studies. We present a review of the taxonomic changes since the publication of the Systema Porifera (2002), and point to problems a sponge taxonomist is still faced with nowadays. To conclude, we make a series of proposals for the future of sponge systematics. In the light of recent studies, we establish a series of taxonomic changes that the sponge community may be ready to accept. We also propose a series of sponge new names and definitions following the PhyloCode. The issue of phantom species (potential new species revealed by molecular studies) is raised, and we show how they could be dealt with. Finally, we present a general strategy to help us succeed in building a Porifera tree along with the corresponding revised Porifera classification.

Deep phylogeny and evolution of sponges (phylum Porifera)

Sponges (phylum Porifera) are a diverse taxon of benthic aquatic animals of great ecological, commercial, and biopharmaceutical importance. They are arguably the earliest-branching metazoan taxon, and therefore, they have great significance in the reconstruction of early metazoan evolution. Yet, the phylogeny and systematics of sponges are to some extent still unresolved, and there is an on-going debate about the exact branching pattern of their main clades and their relationships to the other non-bilaterian animals. Here, we review the current state of the deep phylogeny of sponges. Several studies have suggested that sponges are paraphyletic. However, based on recent phylogenomic analyses, we suggest that the phylum Porifera could well be monophyletic, in accordance with cladistic analyses based on morphology. This finding has many implications for the evolutionary interpretation of early animal traits and sponge development. We further review the contribution that mitochondrial genes and genomes have made to sponge phylogenetics and explore the current state of the molecular phylogenies of the four main sponge lineages (Classes), that is, Demospongiae, Hexactinellida, Calcarea, and Homoscleromorpha, in detail. While classical systematic systems are largely congruent with molecular phylogenies in the class Hexactinellida and in certain parts of Demospongiae and Homoscleromorpha, the high degree of incongruence in the class Calcarea still represents a challenge. We highlight future areas of research to fill existing gaps in our knowledge. By reviewing sponge development in an evolutionary and phylogenetic context, we support previous suggestions that sponge larvae share traits and complexity with eumetazoans and that the simple sedentary adult lifestyle of sponges probably reflects some degree of secondary simplification. In summary, while deep sponge phylogenetics has made many advances in the past years, considerable efforts are still required to achieve a comprehensive understanding of the relationships among and within the main sponge lineages to fully appreciate the evolution of this extraordinary metazoan phylum.

A close-up view on ITS2 evolution and speciation - a case study in the Ulvophyceae (Chlorophyta, Viridiplantae)

BACKGROUND

The second Internal Transcriber Spacer (ITS2) is a fast evolving part of the nuclear-encoded rRNA operon located between the 5.8S and 28S rRNA genes. Based on crossing experiments it has been proposed that even a single Compensatory Base Change (CBC) in helices 2 and 3 of the ITS2 indicates sexual incompatibility and thus separates biological species. Taxa without any CBC in these ITS2 regions were designated as a 'CBC clade'. However, in depth comparative analyses of ITS2 secondary structures, ITS2 phylogeny, the origin of CBCs, and their relationship to biological species have rarely been performed. To gain 'close-up' insights into ITS2 evolution, (1) 86 sequences of ITS2 including secondary structures have been investigated in the green algal order Ulvales (Chlorophyta, Viridiplantae), (2) after recording all existing substitutions, CBCs and hemi-CBCs (hCBCs) were mapped upon the ITS2 phylogeny, rather than merely comparing ITS2 characters among pairs of taxa, and (3) the relation between CBCs, hCBCs, CBC clades, and the taxonomic level of organisms was investigated in detail.

RESULTS

High sequence and length conservation allowed the generation of an ITS2 consensus secondary structure, and introduction of a novel numbering system of ITS2 nucleotides and base pairs. Alignments and analyses were based on this structural information, leading to the following results: (1) in the Ulvales, the presence of a CBC is not linked to any particular taxonomic level, (2) most CBC 'clades' sensu Coleman are paraphyletic, and should rather be termed CBC grades. (3) the phenetic approach of pairwise comparison of sequences can be misleading, and thus, CBCs/hCBCs must be investigated in their evolutionary context, including homoplasy events (4) CBCs and hCBCs in ITS2 helices evolved independently, and we found no evidence for a CBC that originated via a two-fold hCBC substitution.

CONCLUSIONS

Our case study revealed several discrepancies between ITS2 evolution in the Ulvales and generally accepted assumptions underlying ITS2 evolution as e.g. the CBC clade concept. Therefore, we developed a suite of methods providing a critical 'close-up' view into ITS2 evolution by directly tracing the evolutionary history of individual positions, and we caution against a non-critical use of the ITS2 CBC clade concept for species delimitation.

Widespread distribution of poribacteria in demospongiae

Poribacteria were found in nine sponge species belonging to six orders of Porifera from three oceans. Phylogenetic analysis revealed four distinct poribacterial clades, which contained organisms obtained from several different geographic regions, indicating that the distribution of poribacteria is cosmopolitan. Members of divergent poribacterial clades were also found in the same sponge species in three different sponge genera.

Molecular microbial diversity survey of sponge reproductive stages and mechanistic insights into vertical transmission of microbial symbionts

Many marine sponges, hereafter termed high-microbial-abundance (HMA) sponges, harbor large and complex microbial consortia, including bacteria and archaea, within their mesohyl matrices. To investigate vertical microbial transmission as a strategy to maintain these complex associations, an extensive phylogenetic analysis was carried out with the 16S rRNA gene sequences of reproductive (n = 136) and adult (n = 88) material from five different Caribbean species, as well as all published 16S rRNA gene sequences from sponge offspring (n = 116). The overall microbial diversity, including members of at least 13 bacterial phyla and one archaeal phylum, in sponge reproductive stages is high. In total, 28 vertical-transmission clusters, defined as clusters of phylotypes that are found both in adult sponges and their offspring, were identified. They are distributed among at least 10 bacterial phyla and one archaeal phylum, demonstrating that the complex adult microbial community is collectively transmitted through reproductive stages. Indications of host-species specificity and cospeciation were not observed. Mechanistic insights were provided using a combined electron microscopy and fluorescence in situ hybridization analysis, and an indirect mechanism of vertical transmission via nurse cells is proposed for the oviparous sponge Ectyoplasia ferox. Based on these phylogenetic and mechanistic results, we suggest the following symbiont transmission model: entire microbial consortia are vertically transmitted in sponges. While vertical transmission is clearly present, additional environmental transfer between adult individuals of the same and even different species might obscure possible signals of cospeciation. We propose that associations of HMA sponges with highly sponge-specific microbial communities are maintained by this combination of vertical and horizontal symbiont transmission.

Large expansion segments in 18S rDNA support a new sponge clade (Class Demospongiae, Order Haplosclerida)

Newly emerging molecular phylogenetic hypotheses involving the sponge Order Haplosclerida (Class Demospongiae) are far removed from traditional views on their classification using morphology. In the new grouping of marine haplosclerid taxa by molecular data all members of one highly supported clade were found to have three large indels in the 18S rRNA gene. These indels were not found in this gene in other marine haplosclerids or in any other demosponges analysed. These indels were found in the variable V4 and V7 region of the gene, had high GC contents and formed stable double stranded helices in the 18S rRNA secondary structure. These indels are very important synapomorphies, provide high support for an alternative taxonomic scheme and could help resolve the phylogeny of this order in conjunction with other phylogenetically informative characters.

Pan-eukaryote ITS2 homologies revealed by RNA secondary structure

For evolutionary comparisons, phylogenetics and evaluation of potential interbreeding taxa of a species, various loci have served for animals and plants and protistans. One [second internal transcribed spacer (ITS2) of the nuclear ribosomal DNA] is highly suitable for all. Its sequence is species specific. It has already been used extensively and very successfully for plants and some protistans, and a few animals (where historically, the mitochondrial genes have dominated species studies). Despite initial impressions that ITS2 is too variable, it has proven to provide useful biological information at higher taxonomic levels, even across all eukaryotes, thanks to the conserved aspects of its transcript secondary structure. The review of all eukaryote groups reveals that ITS2 is expandable, but always retains in its RNA transcript a common core structure of two helices with hallmark characteristics important for ribosomal RNA processing. This aspect of its RNA transcript secondary structure can rescue difficult alignment problems, making the ITS2 a more powerful tool for phylogenetics. Equally important, the recognition of eukaryote-wide homology regions provides extensive and detailed information to test experimental studies of ribosomal rRNA processing.

Phylogenetic Analyses Under Secondary Structure-Specific Substitution Models Outperform Traditional Approaches: Case Studies with Diploblast LSU

Many rDNA molecular phylogenetic studies result in trees that are incongruent to either alternative gene tree reconstructions and/or morphological assumptions. One reason for this outcome might be the application of suboptimal phylogenetic substitution models. While the most commonly implemented models describe the evolution of independently evolving characters fairly well, they do not account for character dependencies such as rRNA strands that form a helix in the ribosome. Such nonindependent sites require the use of models that take into account the coevolution of the complete nucleotide pair (doublet). We analyzed 28S rDNA (LSU) demosponge phylogenies using a "doublet" model for pairing sites (rRNA-helices) and compared our findings with the results of "standard" approaches using Bayes factors. We demonstrate that paired and unpaired sites of the same gene result in different reconstructions and that usage of a doublet model leads to more reliable demosponge trees. We show the influence of more sophisticated models on phylogenetic reconstructions of early-branching metazoans and the phylogenetic relationships of demosponge orders.