The Homoscleromorph sponge Oscarella lobularis,a promising sponge model in evolutionary and developmental biology

Sponges branch basally in the metazoan phylogenetic tree and are believed to be composed of four distinct lineages with still uncertain relationships. Indeed, some molecular studies propose that Homoscleromorpha may be a fourth Sponge lineage, distinct from Demospongiae in which they were traditionally classified. They harbour many features that distinguish them from other sponges and are more evocative of those of the eumetazoans. They are notably the only sponges to possess a basement membrane with collagen IV and specialized cell‐junctions, thus possessing true epithelia. Among Homoscleromorphs, we have chosen Oscarella lobularis as a model species. This common and easily accessible sponge is characterized by relatively simple histology and cell composition, absence of skeleton, and strongly pronounced epithelial structure. In this review, we explore the specific features that make O. lobularis a promising homoscleromorph sponge model for evolutionary and developmental researches.     

Locomotion and contraction in an asconoid calcareous sponge

Various large‐scale behaviors (e.g., locomotion, shape changes, contractions) have been documented numerous times in intact sponges of the class Demospongiae. However, little is known about such motile events in calcareous sponges (Class Calcarea). Here, we report on whole‐sponge behaviors of the calcareous asconoid sponge Leucosolenia botryoides, as revealed by time‐lapse videos. These behaviors included locomotion and contraction. Locomotion in these sponges appeared as an outward movement (25–130 μm h^?1) of the asconoid tubes away from the sponge's center; such translocations were always accompanied by extensive movements of protruding spicules, which appear to act as anchoring hooks for the sponge's translocations. This is the first report of whole‐sponge locomotion in the Calcarea. Contractile waves also were propagated in these sponges at speeds of 50–150 μm h^?1, and they involved systemic contraction, then re‐extension of the asconoid tubes. The observations suggest that, like the more complex demosponges, these simple calcareous sponges are capable of adaptive whole‐animal behaviors (changes in flow, shape, and location), which occur in response to environmental stimuli such as crawling intruders.     

Ultrastructure, development, and homology of insect embryonic cuticles

Ultrastructure and deposition of the cuticles secreted by embryos representing eight insect orders were examined by transmission and scanning electron microscopy. Embryos of the apterygote silverfish Thermobia domestica deposit two embryonic cuticles. Deposition of the first (EC1) is initiated at the beginning of appendage development when the intercalary segment and the neural groove are clearly visible. This cuticle lacks surface microsculpture and consists of an outer epicuticle and an underlying fibrous layer, thought to represent procuticle. At the time of dorsal closure, deposition of a second embryonic cuticle (EC2) begins; this bears sensilla and functions in the first instar larva. In representative embryos of seven pterygote orders (Ephemeroptera, Odonata, Plecoptera, Neuroptera, Coleoptera, Lepidoptera, and Mecoptera), three cuticles were found to be secreted. The first cuticle in pterygotes is homologous to EC1 of T. domestica, but consists solely of outer epicuticle. EC2, the "prolarval cuticle," bears a characteristic surface microsculpture in embryos of some species and egg-teeth and other hatching devices, and consists of outer and inner epicuticles and a more or less reduced procuticle. EC2 is reduced in the embryos of derived endopterygotes, where a procuticle is lacking and the inner epicuticle is reduced. After hatching, when EC2 is shed, the first instar larva is covered by a third embryonic cuticle (EC3), whose deposition was initiated while the insect was still within the egg. Presence of only two embryonic cuticles in cyclorrhaphous flies is due to the total loss of prolarval cuticle. Investigated exopterygote and endopterygote insects excluding flies thus deposit three embryonic cuticles, and their juveniles (exopterygote "nymphs"; endopterygote "larvae") seem to hatch at equivalent stages of development. Differences between the modes of cuticulogenesis in silverfish and pterygote embryos suggest that the apterygote first larval instar was embryonized and became a fully embryonic prolarva in pterygotes.     

Behavior and development of larvae in the sponge Haliclona amboinensis

Sponges play important roles in marine ecosystems by contributing to habitat complexity and benthopelagic coupling of nutrients. Yet, the reproduction and settlement behaviors of diverse sponge species are not well understood. Here, we examined the brooding demosponge Haliclona amboinensis, which is common on shallow reefs in Bolinao, northwestern Philippines. Gravid sponges were found between the months of May and August, coinciding with warmer sea surface temperature. Sponges released parenchymella larvae from brood chambers in the mid‐morning, suggesting that light and temperature may serve as cues to initiate hatching. Larvae immediately swam toward the surface upon emergence and migrated to the bottom of the tanks 1–2 hr after release. The presence of light and crustose coralline algae induced high larval settlement. Metamorphosis proceeded rapidly in vitro, with larval cells spreading laterally on the substrate. The osculum was first visible at 3 days after settlement. The short pelagic duration of larvae in H. amboinensis promotes local recruitment and may be important for the maintenance of sponge populations in the face of disturbances.     

The origin of metazoan development: a palaeobiological perspective

The rapid diversification of early Metazoa remains one of the most puzzling events in the fossil record. Several models have been proposed to explain a critical aspect of this event: the origin of Metazoan development. These include the origin of the eukaryotic cell, environmental triggers, key innovations or selection among cell lineages. Here, the first three hypotheses are evaulated within a phylogenetic framework using fossil, molecular and developmental evidence. Many elements of metazoan development are widely distributed among unicellular eukaryotes, yet only 3 of the 23 multicellular eukaryotic lineages evolved complex development. Molecular evidence indicates the lineage leading to the eukaryotic cell is nearly as old as the eubacterial and archaebacterial lineages, although the symbiotic events established that the eukaryotic cell probably occurred about 1.5 billion years ago. Yet Metazoa did not appear until 1000 to 600 million years ago (Myr), suggesting the origin of metazoan development must be linked to either an environmental trigger, perhaps an increase in atmospheric oxygen, or key innovations such as the development of collagen. Yet the first model fails to explain the unique appearance of complex development in Metazoa, while the latter fails to explain the simultaneous diversification of several ‘protist’ groups along with the Metazoa. A more complete model of the origin of metazoan development combines environmental triggering of a series of innovations, with successive innovations generating radiations of metazoan clades as lineages breached functional thresholds. The elaboration of new cell classes and the appearance of such developmental innovations as cell sheets may have been of particular importance. Evolutionary biologists often implicitly assume that evolution is a uniformitarian, time-homogeneous process without strong temporal asymmetries in evolutionary mechanisms, rate or context. Yet evolutionary patterns do exhibit such asymmetries, raising the possibility that such innovations as metazoan development impose non-uniformities of evolutionary process.     

Ubiquitins (polyubiquitin and ubiquitin extension protein) in marine sponges: cDNA sequence and phylogenetic analysis

The complete nucleotide sequences of two Suberites domuncula cDNAs and one Sycon raphanus cDNA, all encoding ubiquitin, have been determined. One cDNA from S. domuncula codes for polyubiquitin with four tandemly repeated monomeric units and the second cDNA encodes ubiquitin fused to a ribosomal protein of 78 amino acids (aa). S. domuncula possesses at least one additional polyubiquitin gene, from which the last two monomers were also sequenced. All analysed genes from S. domuncula encode identical ubiquitin proteins, with only one aa difference (Ala 19) to the human/higher animals ubiquitin (Pro 19). Ubiquitin in S. domuncula is identical with the ubiquitin found in another Demospongia, Geodia cydonium. The cDNA from S. raphanus encodes polyubiquitin with seven tandemly repeated units. All these gene monomers code for the same ubiquitin, which differs from the human/higher animals ubiquitin only at position 24 (Asp in Sycon, Glu in others). However, ubiquitin from S. raphanus (Calcarea) shows two aa differences (positions 19 and 24), when compared with the ubiquitin sequences from the two Demospongiae. In a phylogenetic tree constructed by multiple sequence alignment of all sponge ubiquitin gene monomers so far identified, all monomers from the same species cluster together, with the clear exception of the monomer from S. domuncula ribosomal protein fusion gene. This monomer branches off first from the tree and forms a separate line; this gives evidence for a very ancient split of ubiquitin-ribosomal-protein fusion genes from polyubiquitin encoding genes and their long separate coexistence in eukaryotes. The ubiquitin extension protein from S. domuncula is 78 aa long, displays all characteristics of 76–81 aa long ribosomal fusion proteins and shows 78% identity in the first 73 aa with the human S27a protein. However, its C-terminal sequence: 69-GLTYVYKKSD-78 is more similar to the plant consensus (69-GLTYVYQ/NK-76), than to the higher animal consensus (69-CLTYCFNK-76). This protein isolated from a sponge, belonging to the phylogenetically oldest multicellular animals, the Porifera, branches off first from the phylogenetic tree of metazoan ubiquitin extension proteins of the small ribosomal subunits.     

Phylogenetic signal in the evolution of body colour and spicule skeleton in calcareous sponges

Some of the morphological characters used in Porifera taxonomy have often been shown to be inconsistent. In the present study, we tested the phylogenetic coherence of currently used taxonomic characters of the calcarean genus Clathrina. For this, 20 species of Clathrina and three other calcinean genera (Ascandra, Guancha, and Leucetta) were sequenced for the ITS and D2 region of the 28S ribosomal DNA. Maximum‐likelihood and maximum‐parsimony algorithms were used to reconstruct phylogenetic trees. Deep divergences were observed in our tree and Clathrina was shown to be paraphyletic. The major split in our topology showed a clear‐cut distinction between sponges with and without tetractine spicules. Moreover, a group of yellow‐coloured Clathrina was clearly separated from the remaining white‐coloured species. Our results show that the presence of diactines, water‐collecting tubes, the degree of cormus anastomosis, and actine shapes do not correlate with the major clades of the calcinean phylogeny. On the other hand, the presence of tripods, the absence of tetractines, and the presence of spines in the apical actine of tetractines seem to be good synapomorphies for clades in our tree. Our results demonstrate that skeleton characters can be reliably used in higher level taxonomy in Clathrinida. © 2011 The Linnean Society of London, Zoological Journal of the Linnean Society, 2011, 163 , 1026–1034.     

Three‐dimensional fate mapping of larval tissues through metamorphosis in the glass sponge Oopsacas minuta

The tissue of glass sponges (Class Hexactinellida) is unique among metazoans in being largely syncytial, a state that arises during early embryogenesis when blastomeres fuse. In addition, hexactinellids are one of only two poriferan groups that already have clearly formed flagellated chambers as larvae. The fate of the larval chambers and of other tissues during metamorphosis is unknown. One species of hexactinellid, Oopsacas minuta, is found in submarine caves in the Mediterranean and is reproductive year round, which facilitates developmental studies; however, describing metamorphosis has been a challenge because the syncytial nature of the tissue makes it difficult to trace the fates using conventional cell tracking markers. We used three‐dimensional models to map the fate of larval tissues of O. minuta through metamorphosis and provide the first detailed account of larval tissue reorganization at metamorphosis of a glass sponge larva. Larvae settle on their anterior swimming pole or on one side. The multiciliated cells that formed a belt around the larva are discarded during the first stage of metamorphosis. We found that larval flagellated chambers are retained throughout metamorphosis and become the kernels of the first pumping chambers of the juvenile sponge. As larvae of O. minuta settle, larval chambers are enlarged by syncytial tissues containing yolk inclusions. Lipid inclusions at the basal attachment site gradually became smaller during the six weeks of our study. In O. minuta, the flagellated chambers that differentiate in the larva become the post‐metamorphic flagellated chambers, which corroborate the view that internalization of these chambers during embryogenesis is a process that resembles gastrulation processes in other animals.     

Post-larval development of the commercial sponge Spongia officinalis L. (Porifera, Demospongiae)

This study investigated the development of the larvae of Spongia officinalis in experimental conditions, after settlement on plastic substrates, using electron and light microscopy. The released larvae show a dark pigmented ring distinguishes the posterior larval pole. The youngest larvae, covered with a flagellate epithelium, move onwards by rotating on their longitudinal axis. Over time a creeping-like motion prevails, probably linked to the need for settlement. After a free-swimming period of 24-48 h, larvae settle on the artificial substrate by the anterior pole. At settlement, the flagellate epithelium is substituted by flattened cells, which delimit the outermost surface. Post-larvae were reared to about three months. The early phase of post-larval differentiation shows a solid interior mainly consisting of granular cells varying in shape and size. They are included in a dense collagen matrix that contains a conspicuous amount of bacteria. Lacunae are already evident in the initial phase of metamorphosis. In several of them, cell debris and nucleate cells are visible. This feature is consistent with a progressive reduction of the cell mass (autolysis). Neither choanocyte chambers nor canals differentiate. The morphogenetic process leads to a metamorph only consisting of vacuolated cells and collagen fibrils included in a thin fibrous coat.     

Mitochondrial diversity of early-branching metazoa is revealed by the complete mt genome of a haplosclerid demosponge

The first mitochondrial (mt) genomes of demosponges have recently been sequenced and appear to be markedly different from published eumetazoan mt genomes. Here we show that the mt genome of the haplosclerid demosponge Amphimedon queenslandica has features that it shares with both demosponges and eumetazoans. Although the A. queenslandica mt genome has typical demosponge features, including size, long noncoding regions, and bacterialike rRNA genes, it lacks atp9, which is found in the other demosponges sequenced to date. We found strong evidence of a recent transposon-mediated transfer of atp9 to the nuclear genome. In addition, A. queenslandica bears an incomplete tRNA set, unusual amino acid deletion patterns, and a putative control region. Furthermore, the arrangement of mt rRNA genes differs from that of other demosponges. These genes evolve at significantly higher rates than observed in other demosponges, similar to previously observed nuclear rRNA gene rates in other haplosclerid demosponges.     

Novel protein from larval sponge cells,ilborin, is related to energy turnover and calcium binding and is conserved among marine invertebrates

Sponges (phylum Porifera) are early-branching animals, whose outwardly simple body plan is underlain by a complex genetic repertoire. The transition from a mobile larva to an attached filter-feeding organism occurs by metamorphosis, a process accompanied by a radical change of the body plan and cell transdifferentiation. The continuity between larval cells and adult tissues is still obscure. In a previous study, we have produced polyclonal antibodies against the major protein of the flagellated cells covering the larva of the sponge Halisarca dujardini, used them to trace the fate of these cells and shown that the larval flagellated cells transdifferentiate into the choanocytes. In the present work, we identified the sequence of this novel protein, which we named ilborin. A search in the open databases showed that multiple orthologues of the newly identified protein are present in sponges, cnidarians, flatworms, ctenophores and echinoderms, but none of them has been described yet. Ilborin has two conserved domains: triosephosphate isomerase-barrel, which has enzymatic activity against macroergic compounds, and canonical EF-hand, which binds calcium. mRNA of ilborin is expressed in the larval flagellated cells. We suggest that the new protein is involved in the calcium-mediated regulation of energy metabolism, whose activation precedes metamorphosis.     

Altered apoptosis/autophagy and epigenetic modifications cause the impaired postimplantation octaploid embryonic development in mice

Polyploids are pervasive in plants and have large impacts on crop breeding, but natural polyploids are rare in animals. Mouse diploid embryos can be induced to become tetraploid by blastomere fusion at the 2-cell stage and tetraploid embryos can develop to the blastocyst stage in vitro. However, there is little information regarding mouse octaploid embryonic development and precise mechanisms contributing to octaploid embryonic developmental limitations are unknown. To investigate the genetic and epigenetic mechanisms underlying octaploid embryonic development, we generated mouse octaploid embryos and evaluated the in vitro/in vivo developmental potential. Here we show that octaploid embryos can develop to the blastocyst stage in vitro, but all fetus impaired immediately after implantation. Our results indicate that cell lineage specification of octaploid embryo was disorganized. Furthermore, these octaploid embryos showed increased apoptosis as well as alterations in epigenetic modifications when compared with diploid embryos. Thus, our cumulative data provide cues for why mouse octaploid embryonic development is limited and its failed postimplantation development.     

Role of N6-methyl-adenosine modification in mammalian embryonic development

N6-methyl-adenosine (m6A) methylation is one of the most common and abundant modifications of RNA molecules in eukaryotes. Although various biological roles of m6A methylation have been elucidated, its role in embryonic development is still unclear. In this review, we focused on the function and expression patterns of m6A-related genes in mammalian embryonic development and the role of m6A modification in the embryonic epigenetic reprogramming process. The modification of m6A is regulated by the combined activities of methyltransferases, demethylases, and m6A-binding proteins. m6A-related genes act synergistically to form a dynamic, reversible m6A pattern, which exists in several physiological processes in various stages of embryonic development. The lack of one of these enzymes affects embryonic m6A levels, leading to abnormal embryonic development and even death. Moreover, m6A is a positive regulator of reprogramming to pluripotency and can affect embryo reprogramming by affecting activation of the maternal-to-zygotic transition. In conclusion, m6A is involved in the regulation of gene expression during embryonic development and the metabolic processes of RNA and plays an important role in the epigenetic modification of embryos.     

First observations on egg release in the oviparous sponge Chondrilla nucula (Demospongiae, Chondrosida, Chondrillidae) in the Mediterranean Sea

Abstract. The reproduction of the demosponge Chondrilla nucula in Portofino (Ligurian Sea, Italy) was studied during August 2001. Eighteen individuals were sampled and examined with light microscopy for the presence of gametes, and 5 individuals carrying oocytes were found. In addition to microscopic observations, reproductive individuals could be easily identified as female even at the macroscopic level because of the presence of a grayish layer in the mesohyl where oocytes were concentrated. Oogenesis resulted in modifications of the external sponge morphology and of the aquiferous system. Approximately one‐third of the sponge body was filled with oocytes with the consequent disappearance of choanocyte chambers in the reproductive portion of the sponge. Under laboratory conditions, we obtained fertilized eggs from females and observed the first stages of embryonic development. Our observations suggest that fertilization in specimens of Chondrilla nucula occurs internally and not in the water. During the 2 years following these observations, no reproductive specimens were found among the same population during the reported reproductive period.     

Temperature and oxygen tension influence the development of muscle cellularity in embryonic rainbow trout

Muscle cellularity at a developmental stage around the time of hatching was examined in rainbow trout which had been reared from the eyed stage at three different temperature regimes (5, 10 and 15° C) and different O₂ tensions [70% of air saturation value (ASV) at 5° C, 100% of ASV at all temperatures, and 150% of ASV at 10 and 15° C]. It was found that, as has been shown for other species, there was a difference in muscle fibre numbers and fibre cross-sectional areas between some of the regimes. There was a decrease in fibre number at the intermediate and higher temperature, and a decrease in fibre size at the high temperature. The temperature effects observed were modified by the applied changes in O₂ tension. An increased O₂ tension at 10° C led to an increase in fibre size whereas a decrease in O₂ tension at the low temperature resulted in a decrease in fibre number. The largest total white muscle cross-sectional area was achieved at 10° C under high O₂ conditions. Temperature and O₂ tension therefore had a clear effect on muscle cellularity and there was a significant interaction between the two parameters.     

Effects of temperature during early life history on embryonic and larval development and growth in haddock

The effect of incubation temperature (2, 4, 6, 8 and 10° C) on haddock Melanogrammus aeglefinus development and growth during the embryonic period and in subsequent ontogeny in a common post‐hatch thermal environment (6° C) was investigated. Hatching times were inversely proportional to incubation temperature and ranged from 20·3 days at 2° C to 9·1 days at 10° C. Growth rates were directly proportional to incubation temperature during both the embryonic and larval periods. There was a significant decline in growth rates following hatch in all temperature groups. Compared to the endogenously feeding embryos, growth rates in the exogenous period declined by 4·4‐fold at 4° C to 3·9‐fold at 8° C, indicative of the demarcation between the endogenous and exogenous feeding periods. Yolk utilization varied from 17 days at 2° C to 6 days at 10° C and followed a three‐stage sigmoidal pattern with the initial lag period inversely proportional to incubation temperature. Time to 50% yolk depletion varied inversely with temperature but occurred 1–1·5 days post‐hatch at all temperatures. Additionally, the period between 10 and 90% yolk depletion also decreased with increased temperature. Overall developmental rate was sequential with and directly proportional (2·3‐fold increase) to incubation temperature while the time spent in each developmental stage was inversely proportional to temperature. Larger embryos tended to be produced at lower temperatures but this pattern reversed following hatch, as larvae from higher temperature groups grew more rapidly than those from other temperature groups. Larvae from all temperatures achieved a similar length (c.total length 4·5 mm) upon complete yolk absorption. The study demonstrated the significant impact that temperature has upon developmental and growth rates in both endogenous and exogenous feeding periods. It also illustrated that temperature changes during embryogenesis had significant and persistent effects on growth in subsequent ontogeny.     

Svenzea zeai, a Caribbean reef sponge with a giant larva, and Scopalina ruetzleri: a comparative fine-structural approach to classification (Demospongiae, Halichondrida, Dictyonellidae)

Abstract. Svenzea zeai , abundant on many deep Caribbean fore-reef habitats but of uncertain systematic position within the Demospongiae, is closely examined histologically and cytologically for evidence of its phylogenetic relationship beyond the traditional analysis of gross morphology and skeletal structure. We document that S. zeai is a bacteriosponge containing substantial quantities of unicellular photosynthetic and autotrophic microbes; that the most abundant cell type is an unusual cell with refractile granules that only few species share and whose composition and function are still enigmatic; and that it produces the largest—by a factor of 3—embryos and larvae recorded in the phylum Porifera. A combination of characters such as the granular cells, ciliary pattern, and aspects of larval shape and behavior are comparable with those of Scopalina ruetzleri , family Dictyonellidae, a prominent member of the Caribbean mangrove community. These results support our earlier decision to establish Svenzea as a new genus in Dictyonellidae to accommodate its unprecedented skeletal structure, styles in isodictyal reticulation.