The echinoderms are a phylum of marine deuterostomes characterized by the pentaradial (five fold) symmetry of their adult bodies. Due to this unusual body plan, adult echinoderms have long been excluded from comparative analyses aimed at understanding the origin and evolution of deuterostome nervous systems. Here, we investigated the neural anatomy of early juveniles of representatives of three of the five echinoderm classes: the echinoid Paracentrotus lividus, the asteroid Patiria miniata, and the holothuroid Parastichopus parvimensis. Using whole mount immunohistochemistry and confocal microscopy, we found that the nervous system of echinoid early juveniles is composed of three main structures: a basiepidermal nerve plexus, five radial nerve cords connected by a circumoral nerve ring, and peripheral nerves innervating the appendages. Our whole mount preparations further allowed us to obtain thorough descriptions of these structures and of several innervation patterns, in particular at the level of the appendages. Detailed comparisons of the echinoid juvenile nervous system with those of asteroid and holothuroid juveniles moreover supported a general conservation of the main neural structures in all three species, including at the level of the appendages. Our results support the previously proposed hypotheses for the existence of two neural units in echinoderms: one consisting of the basiepidermal nerve plexus to process sensory stimuli locally and one composed of the radial nerve cords and the peripheral nerves constituting a centralized control system. This study provides the basis for more in‐depth comparisons of the echinoderm adult nervous system with those of other animals, in particular hemichordates and chordates, to address the long‐standing controversies about deuterostome nervous system evolution.

The POU (Pit-Oct-Unc) genes encode a large transcription factor family comprising 6 classes (pou1f to pou6f ) involved in many developmental processes, such as cell commitment and differentiation. The pou3f class contains four members (pou3f1, pou3f2, pou3f3, pou3f4) characterized by expression in ectodermal tissue derivatives, such as nervous system and otic vesicle, during mammalian development. In order to obtain insights into the potential conservation of this class of transcription factors in vertebrates, we carried out a phylogenetic analysis and a comprehensive comparative study of pou3f expression in the frog Xenopus laevis. All vertebrates examined possessed members of the four pou3f subfamilies, excepting the zebrafish, which lacked a pou3f4 gene. Whole mount in situ hybridization and real-time quantitative polymerase chain reaction (RT-qPCR) analyses revealed that Xenopus pou3f genes were expressed in the forming neural tube and their expression was maintained in the brain, mostly in the dorsal part, at tailbud stages. The pou3f2, pou3f3, and pou3f4 genes were also expressed in the developing otic vesicle, and pou3f1 in some cells of the epidermis. Besides ectodermal derivatives, pou3f3 and pou3f4 were expressed in the developing kidney. Their expression started at the early tailbud stage in the pronephric anlage and partly overlapped. In the mature pronephric tubule, pou3f3 was restricted to the intermediate tubule, while pou3f4 was also expressed in the distal and connecting tubule. Together, our results highlight a significant conservation of pou3f gene expression in vertebrates and indicate that they may have distinct but also redundant functions during neural and renal development.

The origin of the pentaradial body plan of echinoderms from a bilateral ancestor is one of the most enduring zoological puzzles. Since echinoderms are defined by morphological novelty, even the most basic axial comparisons with their bilaterian relatives are problematic. Here, we used conserved antero-posterior (AP) axial molecular markers to determine whether the highly derived adult body plan of echinoderms masks underlying patterning similarities with other deuterostomes. To revisit this classical question, we used RNA tomography and in situ hybridizations in the sea star Patiria miniata to investigate the expression of a suite of conserved transcription factors with well-established roles in the establishment of AP polarity in bilaterians. We find that the relative spatial expression of these markers in P. miniata ambulacral ectoderm shows similarity with other deuterostomes, with the midline of each ray representing the most anterior territory and the most lateral parts exhibiting a more posterior identity. Interestingly, there is no ectodermal territory in the sea star that expresses the characteristic bilaterian trunk genetic patterning program. This suggests that from the perspective of ectoderm patterning, echinoderms are mostly head-like animals, and prompts a reinterpretation of the evolutionary trends that made echinoderms the most derived animal group.

The goal of comparative developmental biology is identifying mechanistic differences in embryonic development between different taxa and how these evolutionary changes have led to morphological and organizational differences in adult body plans. Much of this work has focused on direct-developing species in which the adult forms straight from the embryo and embryonic modifications have direct effects on the adult. However, most animal lineages are defined by indirect development, in which the embryo gives rise to a larval body plan and the adult forms by transformation of the larva. Historically, much of our understanding of complex life cycles is viewed through the lenses of ecology and zoology. In this review, we discuss the importance of establishing developmental rather than morphological or ecological criteria for defining developmental mode and explicitly considering the evolutionary implications of incorporating complex life cycles into broad developmental comparisons of embryos across metazoans. Expected final online publication date for the Annual Review of Genetics, Volume 57 is November 2023. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.