Genomic insights of body plan transitions from bilateral to pentameral symmetry in Echinoderms.

Echinoderms are an exceptional group of bilaterians that develop pentameral adult symmetry from a bilaterally symmetric larva. However, the genetic basis in evolution and development of this unique transformation remains to be clarified. Here we report newly sequenced genomes, developmental transcriptomes, and proteomes of diverse echinoderms including the green sea urchin (L. variegatus), a sea cucumber (A. japonicus), and with particular emphasis on a sister group of the earliest-diverged echinoderms, the feather star (A. japonica). We learned that the last common ancestor of echinoderms retained a well-organized Hox cluster reminiscent of the hemichordate, and had gene sets involved in endoskeleton development. Further, unlike in other animal groups, the most conserved developmental stages were not at the body plan establishing phase, and genes normally involved in bilaterality appear to function in pentameric axis development. These results enhance our understanding of the divergence of protostomes and deuterostomes almost 500 Mya.

Direct relationship between osmotic and ionic conforming behavior and tissue water regulatory capacity in echinoids.

Echinoderms are considered marine osmoconforming invertebrates. However, many are intertidal or live next to estuaries, tolerating salinity changes and showing extracellular gradients to dilute seawater. Three species of echinoids - Lytechinus variegatus, which can occur next to estuarine areas, the rocky intertidal Echinometra lucunter, and the mostly subtidal Arbacia lixula - were submitted to a protocol of stepwise (rate of 2-3 psu/h) dilution, down to 15 psu, or concentration, up to 45 psu, of control seawater (35 psu). Coelomic fluid samples were obtained every hour. The seawater dilution experiment lasted 8h, while the seawater concentration experiment lasted 6h. Significant gradients (40-90% above value in 15 psu seawater) for osmolality, sodium, magnesium, and potassium were shown by L. variegatus and E. lucunter. A. lixula showed the smallest gradients, displaying the strongest conforming behavior. The esophagus of the three species was challenged in vitro with 20 and 50% osmotic shocks (hypo- and hyperosmotic). A. lixula, the most "conforming" species, showed the highest capacity to avoid swelling of its tissues upon the -50% hyposmotic shock, and was also the species less affected by salinity changes concerning the observation of spines and ambulacral feet movement in the whole-animal experiments. Thus, the most conforming species (A. lixula) displayed the highest capacity to regulate tissue water/volume, and was also the most euryhaline among the three studied species. In addition, tissues from all three species swelled much more than they shrank under osmotic shocks of same magnitude. This distinct trend to gain water, despite the capacity to hold some gradients upon seawater dilution, helps to explain why echinoderms cannot be fully estuarine, or ever enter fresh water.

Organic matrix-related mineralization of sea urchin spicules, spines, test and teeth.

The camarodont echinoderms have five distinct mineralized skeletal elements: embryonic spicules, mature test, spines, lantern stereom and teeth. The spicules are transient structural elements whereas the spines, and test plates are permanent. The teeth grow continuously. The mineral is a high magnesium calcite, but the magnesium content is different in each type of skeletal element, varying from 5 to 40 mole% Mg. The organic matrix creates the spaces and environments for crystal initiation and growth. The detailed mechanisms of crystal regulation are not known, but acidic and phosphorylated matrix proteins may be of special importance. Biochemical studies, sequencing of the complete genome, and high-throughput proteomic analysis have not yet provided insight into the mechanisms of crystallization, calcite composition, and orientation applicable to all skeletal elements. The embryonic spicules are not representative of the mature skeletal elements. The next phase of research will have to focus on the specific localization of the proteins and individual biochemistries of each system with regard to mineral content and placement.

Structure of first- and second-stage mineralized elements in teeth of the sea urchin Lytechinus variegatus.

Microstructure of the teeth of the sea urchin Lytechinus variegatus was investigated using optical microscopy, SEM (scanning electron microscopy) and SIMS (secondary ion mass spectroscopy). The study focused on the internal structure of the first-stage mineral structures of high Mg calcite (primary, secondary and carinar process plates, prisms) and on morphology of the columns of second-stage mineral (very high Mg calcite) that cement the first-stage material together. Optical micrographs under polarized light revealed contrast in the centers (midlines) of carinar process plates and in prisms in polished sections; staining of primary and carinar process plates revealed significant dye uptake at the plate centers. Demineralization with and without fixation revealed that the midlines of primary and carinar process plates (but not secondary plates) and the centers of prisms differed from the rest of the plate or prism, and SIMS showed proteins concentrated in these plate centers. SEM was used to study the morphology of columns, the fracture surfaces of mature teeth and the 3D morphology of prisms. These observations of internal structures in plates and prisms offer new insight into the mineralization process and suggest an important role for protein inclusions within the first-stage mineral. Some of the 3D structures not reported previously, such as twisted prisms and stacks of carinar process plates with nested wrinkles, may represent structural strengthening strategies.

Mapping of magnesium and of different protein fragments in sea urchin teeth via secondary ion mass spectroscopy.

Mature portions of sea urchin are comprised of a complex array of reinforcing elements yet are single crystals of high and very high Mg calcite. How a relatively poor structural material (calcite) can produce mechanically competent structures is of great interest. In teeth of the sea urchin Lytechinus variegatus, we recorded high-resolution secondary ion mass spectrometry (SIMS) maps of Mg, Ca ,and specific amino acid fragments of mineral-related proteins including aspartic acid (Asp). SIMS revealed strong colocalization of Asp residues with very high Mg. Demineralized specimens showed serine localization on membranes between crystal elements and reduced Mg and aspartic acid signals, further emphasizing colocalization of very high Mg with ready soluble Asp-rich protein(s). The association of Asp with nonequilibrium, very high magnesium calcite provides insight to the makeup of the macromolecules involved in the growth of two different composition calcites and the fundamental process of biomineralization.