Intra-epithelial spicules in a homosclerophorid sponge

Silica-associated proteins from hexactinellid sponges support an alternative evolutionary scenario for biomineralization in Porifera

Metazoans use silicon traces but rarely develop extensive silica skeletons, except for the early-diverging lineage of sponges. The mechanisms underlying metazoan silicification remain incompletely understood, despite significant biotechnological and evolutionary implications. Here, the characterization of two proteins identified from hexactinellid sponge silica, hexaxilin and perisilin, supports that the three classes of siliceous sponges (Hexactinellida, Demospongiae, and Homoscleromorpha) use independent protein machineries to build their skeletons, which become non-homologous structures. Hexaxilin forms the axial filament to intracellularly pattern the main symmetry of the skeletal parts, while perisilin appears to operate in their thickening, guiding extracellular deposition of peripheral silica, as does glassin, a previously characterized hexactinellid silicifying protein. Distant hexaxilin homologs occur in some bilaterians with siliceous parts, suggesting putative conserved silicifying activity along metazoan evolution. The findings also support that ancestral Porifera were non-skeletonized, acquiring silica skeletons only after diverging into major classes, what reconciles molecular-clock dating and the fossil record.

The terminology of sponge spicules

Sponges (Porifera) are a diverse and globally distributed clade of benthic organisms, with an evolutionary history reaching at least the Ediacaran-Cambrian (541 Ma) boundary interval. Throughout their research history, sponges have been subjects of intense studies in many fields, including paleontology, evolutionary biology, and even bioengineering and pharmacology. The skeletons of sponges are mostly characterized by the presence of mineral elements termed spicules, which structurally support the sponge bodies, though they also minimize the metabolic cost of water exchange and deter predators. The description of the spicules' shape and the skeleton organization represents the fundamental basis of sponge taxonomy and systematics. Here, we provide an illustrated catalogue of sponge spicules, which is based on previous works on sponge spicules, for example, and gathers and updates all terms that are currently used in sponge descriptions. Each spicule type is further illustrated through high quality scanning electron microscope micrographs. It is expected to be a valuable source that will facilitate spicule identification and, in certain cases, also enable sponge classification.

Produce, carry/position, and connect: morphogenesis using rigid materials

Animal morphogenesis can be summarized as a reconfiguration of a mass of cells. Although extracellular matrices that include rigid skeletal elements, such as cartilage/bones and exoskeletons, have important roles in morphogenesis, they are also secreted in situ by accumulated cells or epithelial cells. In contrast, recent studies of the skeleton construction of sponges (Porifera) illuminate a conceptually different mechanism of morphogenesis in which cells manipulate rather fine rigid materials (spicules) to form larger structures. Here, two different types of sponge skeleton formation using calcareous spicules or siliceous spicules are compared with regard to the concept of the production of rigid materials and their use in skeletons. The comparison highlights the advantages of their different strategies of forming sponge skeletons.

Spiculogenesis and biomineralization in early sponge animals

Most sponges have biomineralized spicules. Molecular clocks indicate sponge classes diverged in the Cryogenian, but the oldest spicules are Cambrian in age. Therefore, sponges either evolved spiculogenesis long after their divergences or Precambrian spicules were not amenable to fossilization. The former hypothesis predicts independent origins of spicules among sponge classes and presence of transitional forms with weakly biomineralized spicules, but this prediction has not been tested using paleontological data. Here, we report an early Cambrian sponge that, like several other early Paleozoic sponges, had weakly biomineralized and hexactine-based siliceous spicules with large axial filaments and high organic proportions. This material, along with Ediacaran microfossils containing putative non-biomineralized axial filaments, suggests that Precambrian sponges may have had weakly biomineralized spicules or lacked them altogether, hence their poor record. This work provides a new search image for Precambrian sponge fossils, which are critical to resolving the origin of sponge spiculogenesis and biomineralization.

Evolution of the main skeleton-forming genes in sponges (phylum Porifera) with special focus on the marine Haplosclerida (class Demospongiae)

The skeletons of sponges (Phylum Porifera) are comprised of collagen, often embedded with small siliceous structures (spicules) arranged in various forms to provide strength and flexibility. The main proteins responsible for the formation of the spicules in demosponges are the silicateins, which are related to the cathepsins L of other animals. While the silicatein active site, necessary for the formation of biosilica crystals, is characterized by the amino acids SHN, different variants of the silicatein genes have been found, some that retain SHN at the active site and some that don't. As part of an effort to further understand skeleton formation in marine sponges of the order Haplosclerida, a search for all silicatein variants were made in Irish species representing the main clades of this large sponge group. For this task, transcriptomes were sequenced and de novo assembled from Haliclona oculata, H. simulans and H. indistincta. Silicatein genes were identified from these and all available genomes and transcriptomes from Porifera. These were analysed along with all complete silicateins from GenBank. Silicateins were only found in species belonging to the class Demospongiae but excluding Keratosa and Verongimorpha and there was significant duplication and diversity of these genes. Silicateins showing SHN at the active site were polyphyletic. Indeed silicatein sequences were divided into six major clades (CHNI, CHNII, CHNIII, SHNI, SHNII and C/SQN). In those clades where haplosclerids were well represented the silicatein phylogeny reflected previous ribosomal and mitochondrial topologies. The most basal silicatein clade (CHNI) contained sequences only from marine haplosclerids and freshwater sponges while one silicatein from H. indistincta was more related to cathepsins L (outgroup) than to the overall silicatein clade indicating the presence of an old silicatein or an intermediary form. This data could suggest that marine haplosclerids were one of the first groups of extant demosponges to acquire silicatein genes. Furthermore, we suggest that the paucity of spicule types in this group may be due to their single copy of SHNI variants, and the lack of a silintaphin gene.

The physiology and molecular biology of sponge tissues

Sponges have become the focus of studies on molecular evolution and the evolution of animal body plans due to their ancient branching point in the metazoan lineage. Whereas our former understanding of sponge function was largely based on a morphological perspective, the recent availability of the first full genome of a sponge (Amphimedon queenslandica), and of the transcriptomes of other sponges, provides a new way of understanding sponges by their molecular components. This wealth of genetic information not only confirms some long-held ideas about sponge form and function but also poses new puzzles. For example, the Amphimedon sponge genome tells us that sponges possess a repertoire of genes involved in control of cell proliferation and in regulation of development. In vitro expression studies with genes involved in stem cell maintenance confirm that archaeocytes are the main stem cell population and are able to differentiate into many cell types in the sponge including pinacocytes and choanocytes. Therefore, the diverse roles of archaeocytes imply differential gene expression within a single cell ontogenetically, and gene expression is likely also different in different species; but what triggers cells to enter one pathway and not another and how each archaeocyte cell type can be identified based on this gene knowledge are new challenges. Whereas molecular data provide a powerful new tool for interpreting sponge form and function, because sponges are suspension feeders, their body plan and physiology are very much dependent on their physical environment, and in particular on flow. Therefore, in order to integrate new knowledge of molecular data into a better understanding the sponge body plan, it is important to use an organismal approach. In this chapter, we give an account of sponge body organization as it relates to the physiology of the sponge in light of new molecular data. We focus, in particular, on the structure of sponge tissues and review descriptive as well as experimental work on choanocyte morphology and function. Special attention is given to pinacocyte epithelia, cell junctions, and the molecules present in sponge epithelia. Studies describing the role of the pinacoderm in sensing, coordination, and secretion are reviewed. A wealth of recent work describes gene presence and expression patterns in sponge tissues during development, and we review this in the context of the previous descriptions of sponge morphology and physiology. A final section addresses recent findings of genes involved in the immune response. This review is far from exhaustive but intends rather to revisit for non-specialists key aspects of sponge morphology and physiology in light of new molecular data as a means to better understand and interpret sponge form and function today.

Nutrient fluxes through sponges: biology, budgets, and ecological implications

Marine sponges are able to process a variety of carbon (C), nitrogen (N), phosphorous (P), and silicon (Si) dissolved compounds, in addition to the particulate C, N, and P obtained through regular feeding. While Si fluxes through sponges are exclusively related to the elaboration of their skeleton of biogenic silica, C, N, and P fluxes derive from a complex combination of metabolic processes that include feeding, respiration, egestion, excretion, as well as hosting of large microbial populations within the sponge body. Because of the remarkable abundance of sponges in many benthic marine communities, they have the potential to impact the availability of the compounds they take up and release, affecting the benthic-pelagic coupling and cycling rates of chemical elements that are crucial to determine growth of bacterioplankton and primary producers at the ecosystem level. Unfortunately, our knowledge and understanding of the magnitude of the sponge-meditated nutrient fluxes and their ecological implications depends much on the compound type (i.e. C, N, P, or Si). Herein, we review the available knowledge on the subject with emphasis on recent developments.

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.

Complex structures - smart solutions: Formation of siliceous spicules

The siliceous skeletal elements of the sponges, the spicules, represent one of the very few examples from where the molecule toolkit required for the formation of an extracellular mineral-based skeleton, has been elucidated. The distinguished feature of the inorganic matrix, the bio-silica, is its enzymatic synthesis mediated by silicatein. Ortho-silicate undergoes in the presence of silicatein a polycondensation reaction and forms bio-silica under release of reaction water. The protein silicatein aggregates non-covalently to larger filaments, a process that is stabilized by the silicatein-associated protein, silintaphin-1. These structured clusters form the axial filament that is located in the center of the spicules, the axial canal. Surprisingly it has now been found that the initial axial orientation, in which the spicules grow, is guided by cell processes through evagination. The approximately two µm wide cell extensions release silicatein that forms the first organic axial filament, which then synthesizes the inner core of the siliceous spicule rods. In parallel, the radial growth of the spicules is controlled by a telescopic arrangement of organic layers, into which bio-silica and ortho-silicate are deposited. Hence, the formation of a mature siliceous spicule is completed by a centrifugal accretion of bio-silica mediated by the silicatein in the axial filament, and a centripetal bio-silica deposition catalyzed by the extra-spicular silicatein. Finally this contribution highlights that for the ultimate determination of the spicule shapes, their species-specific morphologies, bio-silica hardens during a process which removes reaction water. The data presented can also provide new blueprints for the fabrication of novel biomaterials for biomedical applications.