Expression patterns of wnt8 orthologs in two sand dollar species with different developmental modes

Gene expression profiling during the embryo-to-larva transition in the giant red sea urchin Mesocentrotus franciscanus

In echinoderms, major morphological transitions during early development are attributed to different genetic interactions and changes in global expression patterns that shape the regulatory program for the specification of embryonic territories. In order more thoroughly to understand these biological and molecular processes, we examined the transcriptome structure and expression profiles during the embryo‐to‐larva transition of a keystone species, the giant red sea urchin Mesocentrotus franciscanus. Using a de novo assembly approach, we obtained 176,885 transcripts from which 60,439 (34%) had significant alignments to known proteins. From these transcripts, ~80% were functionally annotated allowing the identification of ~2,600 functional, structural, and regulatory genes involved in developmental process. Analysis of expression profiles between gastrula and pluteus stages of M. franciscanus revealed 791 differentially expressed genes with 251 GO overrepresented terms. For gastrula, up‐regulated GO terms were mainly linked to cell differentiation and signal transduction involved in cell cycle checkpoints. In the pluteus stage, major GO terms were associated with phosphoprotein phosphatase activity, muscle contraction, and olfactory behavior, among others. Our evolutionary comparative analysis revealed that several of these genes and functional pathways are highly conserved among echinoids, holothuroids, and ophiuroids.

Selective accumulation of germ-line associated gene products in early development of the sea star and distinct differences from germ-line development in the sea urchin

BACKGROUND

Echinodermata is a diverse phylum, a sister group to chordates, and contains diverse organisms that may be useful to understand varied mechanisms of germ-line specification.

RESULTS

We tested 23 genes in development of the sea star Patiria miniata that fall into five categories: (1) Conserved germ-line factors; (2) Genes involved in the inductive mechanism of germ-line specification; (3) Germ-line associated genes; (4) Molecules involved in left-right asymmetry; and (5) Genes involved in regulation and maintenance of the genome during early embryogenesis. Overall, our results support the contention that the posterior enterocoel is a source of the germ line in the sea star P. miniata.

CONCLUSIONS

The germ line in this organism appears to be specified late in embryogenesis, and in a pattern more consistent with inductive interactions amongst cells. This is distinct from the mechanism seen in sea urchins, a close relative of the sea star clad. We propose that P. miniata may serve as a valuable model to study inductive mechanisms of germ-cell specification and when compared with germ-line formation in the sea urchin S. purpuratus may reveal developmental transitions that occur in the evolution of inherited and inductive mechanisms of germ-line specification.

A comprehensive survey of wnt and frizzled expression in the sea urchin Paracentrotus lividus

WNT signaling is, in all multicellular animals, an essential intercellular communication pathway that is critical for shaping the embryo. At the molecular level, WNT signals can be transmitted by several transduction cascades, all activated chiefly by the binding of WNT ligands to receptors of the FRIZZLED family. The first step in assessing the biological functions of WNT signaling during embryogenesis is thus the establishment of the spatiotemporal expression profiles of wnt and frizzled genes in the course of embryonic development. To this end, using quantitative polymerase chain reaction, Northern blot, and in situ hybridization assays, we report here the comprehensive expression patterns of all 11 wnt and 4 frizzled genes present in the genome of the sea urchin Paracentrotus lividus during its embryogenesis. Our findings indicate that the expression of these wnt ligands and frizzled receptors is highly dynamic in both time and space. We further establish that all wnt genes are chiefly transcribed in the vegetal hemisphere of the embryo, whereas expression of the frizzled genes is distributed more widely across the embryonic territories. Thus, in P. lividus, WNT ligands might act both as short- and long-range signaling molecules that may operate in all cell lineages and tissues to control various developmental processes during embryogenesis.

Evolutionary modification of T-brain (tbr) expression patterns in sand dollar

The sand dollars are a group of irregular echinoids that diverged from other regular sea urchins approximately 200 million years ago. We isolated two orthologs of T-brain (tbr), Smtbr and Pjtbr, from the indirect developing sand dollar Scaphechinus mirabilis and the direct developing sand dollar Peronella japonica, respectively. The expression patterns of Smtbr and Pjtbr during early development were examined by whole mount in situ hybridization. The expression of Smtbr was first detected in micromere descendants in early blastula stage, similar to tbr expression in regular sea urchins. However, unlike in regular sea urchin, Smtbr expression in middle blastula stage was detected in micromere-descendent cells and a subset of macromere-descendant cells. At gastrula stage, expression of Smtbr was detected in part of the archenteron as well as primary mesenchyme cells. A similar pattern of tbr expression was observed in early Peronella embryos. A comparison of tbr expression patterns between sand dollars and other echinoderm species suggested that broader expression in the endomesoderm is an ancestral character of echinoderms. In addition to the endomesoderm, Pjtbr expression was detected in the apical organ, the animal-most part of the ectoderm.