Wound repair in sea urchin larvae involves pigment cells and blastocoelar cells

An RNA interference approach for functional studies in the sea urchin and its use in analysis of nodal signaling gradients

Dicer substrate interfering RNAs (DsiRNAs) destroy targeted transcripts using the RNA-Induced Silencing Complex (RISC) through a process called RNA interference (RNAi). This process is ubiquitous among eukaryotes. Here we report the utility of DsiRNA in embryos of the sea urchin Lytechinus variegatus (Lv). Specific knockdowns phenocopy known morpholino and inhibitor knockdowns, and DsiRNA offers a useful alternative to morpholinos. Methods are described for the design of specific DsiRNAs that lead to destruction of targeted mRNA. DsiRNAs directed against pks1, an enzyme necessary for pigment production, show how successful DsiRNA perturbations are monitored by RNA in situ analysis and by qPCR to determine relative destruction of targeted mRNA. DsiRNA-based knockdowns phenocopy morpholino- and drug-based inhibition of nodal and lefty. Other knockdowns demonstrate that the RISC operates early in development as well as on genes that are first transcribed hours after gastrulation is completed. Thus, DsiRNAs effectively mediate destruction of targeted mRNA in the sea urchin embryo. The approach offers significant advantages over other widely used methods in the urchin in terms of cost, and ease of procurement, and offers sizeable experimental advantages in terms of ease of handling, injection, and knockdown validation.

A molecular basis for spine color morphs in the sea urchin Lytechinus variegatus

Animals of the phylum Echinodermata are characterized by a pentaradially symmetric endoskeleton in adults. Echinoids also have endoskeletal spines ranging in length from several millimeters (sand dollars e.g. Mellita quinquiesperforata of the order Clypeasteroida) to 30 cm (the black sea urchin, Diadema antillarum of the order Euechinoidea). Here we integrate an analysis of genetic, structural and molecular properties of spines from the variegated sea urchin, Lytechinus variegatus. Through genetic crosses we learned that white is dominant over red and green colors, and that pigmentation follows classic Mendelian genetics. The abundance of mRNAs encoding flavin mono-oxygenase variancts and polyketide synthase was predictive of the color of the adult and antibodies identified their histological location in the spine cells. By RNA in situ hybridization, candidate genes important for spine biomineralization and pigmentation were mapped onto the spine epithelia, and MicroCT scans of spines from different color morphs concluded that color morphs are entirely due to pigmentation and not to structural variations of the endoskeleton. By confocal microscopy we localized gene expression along and within the spines and learned that genes involved in pigment biosynthesis showed selective distribution along the spine. Spine epidermis is mitotically active and red spherule immunocytes are highly migratory within the spine. Overall the results provide a key foundation for examining the mechanisms of molecular diversity and patterning in the name sake of the phylum Echinodermata.

Interplay of geometry and mechanics in epithelial wound healing

Wound healing is a complex biological process critical for maintaining an organism's structural integrity and tissue repair following an infection or injury. Recent studies have unveiled the mechanisms involving the coordination of biochemical and mechanical responses in the tissue in wound healing. In this article, we focus on the healing property of an epithelial tissue as a material while the effects of biological mechanisms such as cell proliferation, tissue intercalation, cellular migration, cell crawling, and filopodia protrusion is minimal. We present a mathematical framework that predicts the fate of a wounded tissue based on the wound's geometrical features and the tissue's mechanical properties. Precisely, adapting the vertex model of tissue mechanics, we predict whether a wound of a specific size in an epithelial monolayer characterized by certain levels of actomyosin contractility and cell-cell adhesion will heal (i.e., close), shrink in size, or rupture the tissue further. Moreover, we show how tissue-mediated mechanisms such as purse-string tension at the wound boundary facilitate wound healing. Finally, we validate the predictions of our model by designing an experimental setup that enables us to create wounds of specific sizes in kidney epithelial cells (MDCK) monolayers. Altogether, this work sets up a basis for interpreting the interplay of mechanical and geometrical features of a tissue in the process of wound healing.

Localization and origins of juvenile skeletogenic cells in the sea urchin Lytechinuspictus

The development of the sea urchin larval body plan is well understood from extensive studies of embryonic patterning. However, fewer studies have investigated the late larval stages during which the unique pentaradial adult body plan develops. Previous work on late larval development highlights major tissue changes leading up to metamorphosis, but the location of specific cell types during juvenile development is less understood. Here, we improve on technical limitations by applying highly sensitive hybridization chain reaction fluorescent in situ hybridization (HCR-FISH) to the fast-developing and transparent sea urchin Lytechinus pictus, with a focus on skeletogenic cells. First, we show that HCR-FISH can be used in L. pictus to precisely localize skeletogenic cells in the rudiment. In doing so, we provide a detailed staging scheme for the appearance of skeletogenic cells around the rudiment prior to and during biomineralization and show that many skeletogenic cells unassociated with larval rods localize outside of the rudiment prior to localizing inside. Second, we show that downstream biomineralization genes have similar expression patterns during larval and juvenile skeletogenesis, suggesting some conservation of skeletogenic mechanisms during development between stages. Third, we find co-expression of blastocoelar and skeletogenic cell markers around juvenile skeleton located outside of the rudiment, which is consistent with data showing that cells from the non-skeletogenic mesoderm embryonic lineage contribute to the juvenile skeletogenic cell lineage. This work sets the foundation for subsequent studies of other cell types in the late larva of L. pictus to better understand juvenile body plan development, patterning, and evolution.

An RNA interference approach for functional studies in the sea urchin and its use in analysis of Nodal signaling gradients

Dicer substrate interfering RNAs (DsiRNAs) destroy targeted transcripts using the RNA-Induced Silencing Complex (RISC) through a process called RNA interference (RNAi). This process is ubiquitous among eukaryotes. Here we report the utility of DsiRNA in embryos of the sea urchin Lytechinus variagatus (Lv). Specific knockdowns phenocopy known morpholino and inhibitor knockdowns, and DsiRNA offers a useful alternative to morpholinos. Methods for designing and obtaining specific DsiRNAs that lead to destruction of targeted mRNA are described. DsiRNAs directed against pks1, an enzyme necessary for pigment production, show how successful DsiRNA perturbations are monitored by RNA in situ analysis and by qPCR to determine relative destruction of targeted mRNA. DsiRNA-based knockdowns phenocopy morpholino- and drug-based inhibition of nodal and lefty. Other knockdowns demonstrate that the RISC operates early in development as well as on genes that are first transcribed hours after gastrulation is completed. Thus, DsiRNAs effectively mediate destruction of targeted mRNA in the sea urchin embryo. The approach offers significant advantages over other widely used methods in the urchin in terms of cost, and ease of procurement, and offers sizeable experimental advantages in terms of ease of handling, injection, and knockdown validation.

More than a colour; how pigment influences colourblind microbes

Many animals have pigments when they themselves cannot see colour. Perhaps those pigments enable the animal to avoid predators, or to attract mates. Maybe even those pigmented surfaces are hosts for microbes, even when the microbes do not see colour. Do some pigments then serve as a chemical signal for a good or bad microbial substrate? Maybe pigments attract or repel various microbe types? Echinoderms serve as an important model to test the mechanisms of pigment-based microbial interactions. Echinoderms are marine benthic organisms, ranging from intertidal habitats to depths of thousands of metres and are exposed to large varieties of microbes. They are also highly pigmented, with a diverse variety of colours between and even within species. Here we focus on one type of pigment (naphthoquinones) made by polyketide synthase, modified by flavin-dependent monoxygenases, and on one type of function, microbial interaction. Recent successes in targeted gene inactivation by CRISPR/Cas9 in sea urchins supports the contention that colour is more than it seems. Here we dissect the players, and their interactions to better understand how such host factors influence a microbial colonization. This article is part of the theme issue 'Sculpting the microbiome: how host factors determine and respond to microbial colonization'.

Early expression onset of tissue-specific effector genes during the specification process in sea urchin embryos

BACKGROUND

In the course of animal developmental processes, various tissues are differentiated through complex interactions within the gene regulatory network. As a general concept, differentiation has been considered to be the endpoint of specification processes. Previous works followed this view and provided a genetic control scheme of differentiation in sea urchin embryos: early specification genes generate distinct regulatory territories in an embryo to express a small set of differentiation driver genes; these genes eventually stimulate the expression of tissue-specific effector genes, which provide biological identity to differentiated cells, in each region. However, some tissue-specific effector genes begin to be expressed in parallel with the expression onset of early specification genes, raising questions about the simplistic regulatory scheme of tissue-specific effector gene expression and the current concept of differentiation itself.

RESULTS

Here, we examined the dynamics of effector gene expression patterns during sea urchin embryogenesis. Our transcriptome-based analysis indicated that many tissue-specific effector genes begin to be expressed and accumulated along with the advancing specification GRN in the distinct cell lineages of embryos. Moreover, we found that the expression of some of the tissue-specific effector genes commences before cell lineage segregation occurs.

CONCLUSIONS

Based on this finding, we propose that the expression onset of tissue-specific effector genes is controlled more dynamically than suggested in the previously proposed simplistic regulation scheme. Thus, we suggest that differentiation should be conceptualized as a seamless process of accumulation of effector expression along with the advancing specification GRN. This pattern of effector gene expression may have interesting implications for the evolution of novel cell types.

Bald sea urchin disease shifts the surface microbiome on purple sea urchins in an aquarium

Bald sea urchin disease (BSUD) is most likely a bacterial infection that occurs in a wide range of sea urchin species and causes the loss of surface appendages. The disease has a variety of additional symptoms, which may be the result of the many bacteria that are associated with BSUD. Previous studies have investigated causative agents of BSUD, however, there are few reports on the surface microbiome associated with the infection. Here, we report changes to the surface microbiome on purple sea urchins in a closed marine aquarium that contracted and then recovered from BSUD in addition to the microbiome of healthy sea urchins in a separate aquarium. 16S rRNA gene sequencing shows that microhabitats of different aquaria are characterized by different microbial compositions, and that diseased, recovered, and healthy sea urchins have distinct microbial compositions, which indicates that there is a correlation between microbial shifts and recovery from disease.