Echinoderm conundrums: Hox genes, heterochrony, and an excess of mouths

Molecular evidence of anteroposterior patterning in adult echinoderms

The origin of the pentaradial body plan of echinoderms from a bilateral ancestor is one of the most enduring zoological puzzles1,2. Because echinoderms are defined by morphological novelty, even the most basic axial comparisons with their bilaterian relatives are problematic. To revisit this classical question, we used conserved anteroposterior axial molecular markers to determine whether the highly derived adult body plan of echinoderms masks underlying patterning similarities with other deuterostomes. We investigated the expression of a suite of conserved transcription factors with well-established roles in the establishment of anteroposterior polarity in deuterostomes3-5 and other bilaterians6-8 using RNA tomography and in situ hybridization in the sea star Patiria miniata. 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. Strikingly, there is no ectodermal territory in the sea star that expresses the characteristic bilaterian trunk genetic patterning programme. This finding suggests that from the perspective of ectoderm patterning, echinoderms are mostly head-like animals and provides a developmental rationale for the re-evaluation of the events that led to the evolution of the derived adult body plan of echinoderms.

Transcriptomic analysis of Nodal - and BMP- associated genes during development to the juvenile seastar in Parvulastra exigua (Asterinidae)

The molecular mechanisms underlying development of the pentameral body of adult echinoderms are poorly understood but are important to solve with respect to evolution of a unique body plan that contrasts with the bilateral body plan of other deuterostomes. As Nodal and BMP2/4 signalling is involved in axis formation in larvae and development of the echinoderm body plan, we used the developmental transcriptome generated for the asterinid seastar Parvulastra exigua to investigate the temporal expression patterns of Nodal and BMP2/4 genes from the embryo and across metamorphosis to the juvenile. For echinoderms, the Asteroidea represents the basal-type body architecture with a distinct (separated) ray structure. Parvulastra exigua has lecithotrophic development forming the juvenile soon after gastrulation providing ready access to the developing adult stage. We identified 39 genes associated with the Nodal and BMP2/4 network in the P. exigua developmental transcriptome. Clustering analysis of these genes resulted in 6 clusters with similar temporal expression patterns across development. A co-expression analysis revealed genes that have similar expression profiles as Nodal and BMP2/4. These results indicated genes that may have a regulatory relationship in patterning morphogenesis of the juvenile seastar. Developmental RNA-seq analyses of Parvulastra exigua show changes in Nodal and BMP2/4 signalling genes across the metamorphic transition. We provide the foundation for detailed analyses of this cascade in the evolution of the unusual pentameral echinoderm body and its deuterostome affinities.

Multiple origins of feeding head larvae by the Early Cambrian

In many animals the head develops early, most of the body axis later. A larva composed mostly of the developing front end therefore can attain mobility and feeding earlier in development. Fossils, functional morphology, and inferred homologies indicate that feeding head larvae existed by the Early Cambrian in members of three major clades of animals: ecdysozoans, lophotrochozoans, and deuterostomes. Some of these early larval feeding mechanisms were also those of juveniles and adults (the lophophore of brachiopod larvae and possibly the ciliary band of the dipleurula of hemichordates and echinoderms); some were derived from structures that previously had other functions (appendages of the nauplius). Trochophores that swim with a preoral band of cilia, the prototroch, originated before divergence of annelids and molluscs, but evidence of larval growth and thus a prototrochal role in feeding is lacking for molluscs until the Ordovician. Feeding larvae that definitely originated much later, as in insects, teleost fish, and amphibians, develop all or nearly all of what will become the adult body axis before they begin feeding. On present evidence, head larvae, including feeding head larvae, evolved multiple times early in the evolution of bilaterian animals and never since.

Conservation of BMP2/4 expression patterns within the clade Branchiostoma (amphioxus): Resolving interspecific discrepancies

In 2016, Kaji et al. concluded that the amphioxus mouth has the quality of a coelomoduct and is, therefore, not homologous to the oral opening of any other animal. They studied a Japanese population of Branchiostoma japonicum and based their conclusion, in part, on the larval expression of BMP2/4 in cells that reportedly joined the rim of the forming mouth. They did not detect transcription of that gene in any other tissues in the anterior region of the larva. Their results were almost the inverse of findings for B. floridae by Panopoulou et al. (1998), who detected BMP2/4 expression in several anterior tissues, but not in cells intimately associated with the nascent mouth. To resolve this discrepancy, we have studied BMP2/4 in a Chinese population of B. japonicum as well as in an additional species, the European B. lanceolatum. In both species, larval expression of BMP2/4 closely resembles the pattern previously reported for B. floridae-that is, transcription is undetectable in tissues juxtaposed to the forming mouth, but is seen in several other anterior structures (most conspicuously in the lining of the rostral coelom and the club-shaped gland). In sum, we could not repeat the BMP2/4 expression pattern of Kaji et al. (2016) even in the same species, and their findings for this gene, at least, cannot be counted as a support for their hypothesis for a coelomoduct mouth.

Early mesodermal expression of Hox genes in the polychaete Alitta virens (Annelida, Lophotrochozoa)

Hox genes are the key regulators of axial regionalization of bilaterian animals. However, their main function is fulfilled differently in the development of animals from different evolutionary branches. Early patterning of the developing embryos by Hox gene expression in the representatives of protostomes (arthropods, mollusks) starts in the ectodermal cells. On the contrary, the instructive role of the mesoderm in the axial patterning was demonstrated for vertebrates. This makes it difficult to understand if during the axial regionalization of ancestral bilaterians Hox genes first expressed in the developing mesoderm or the ectoderm. To resolve this question, it is necessary to expand the number of models for investigation of the early axial patterning. Here, we show that three Hox genes of the polychaete Alitta virens (formerly Nereis virens, Annelida, Lophotrochozoa)-Hox2, Hox4, and Lox5-are expressed in the mesodermal anlagen of the three future larval chaetigerous segments in spatially colinear manner before the initiation of Hox expression in the larval ectoderm. This is the first evidence of sequential Hox gene expression in the mesoderm of protostomes to date.

The deuterostome context of chordate origins

Our understanding of vertebrate origins is powerfully informed by comparative morphology, embryology and genomics of chordates, hemichordates and echinoderms, which together make up the deuterostome clade. Striking body-plan differences among these phyla have historically hindered the identification of ancestral morphological features, but recent progress in molecular genetics and embryology has revealed deep similarities in body-axis formation and organization across deuterostomes, at stages before morphological differences develop. These developmental genetic features, along with robust support of pharyngeal gill slits as a shared deuterostome character, provide the foundation for the emergence of chordates.