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

Abstract

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.

Symmetry and Aesthetics in Dentistry

“…A large proportion of Eumetazoa shows a bilaterally symmetric body plan. Though adult echinoderms develop pentameral symmetry, they are bilaterally symmetric at least at their larval stage [1]. Exceptions, especially with regard to the head and face, are rare.…”

The genome of an apodid holothuroid (Chiridota heheva) provides insights into its adaptation to deep-sea reducing environment

“…Li et al . (2018) proposed that Hox6 was lost before the split of Echinozoa and Asterozoa (Li et al, 2018), while Li et al . (2020) suggested that the loss of Hox4 or Hox6 was a lineage-specific event (Li et al, 2020).…”

“…(2018) proposed that Hox6 was lost before the split of Echinozoa and Asterozoa (Li et al, 2018), while Li et al . (2020) suggested that the loss of Hox4 or Hox6 was a lineage-specific event (Li et al, 2020). We found that Hox4 and Hox 6 were missing in the genomes of both C. heheva and A. japonicus .…”

“…Compared with other echinoderms, holothurians have a unique body architecture and evolutionary history. The worm-like body of the holothurian preserves the pentameral symmetry structurally along the oral-aboral axis (Li et al, 2020). In addition, holothurians have a soft and stretchable body, in which the ossicles are greatly reduced in size (Pechenik, 2015).…”

Transcription Factors of the Alx Family: Evolutionarily Conserved Regulators of Deuterostome Skeletogenesis

“…With respect to echinoderms, considerable evidence supports the view that alx1 arose very early in echinoderm evolution through gene duplication, relatively quickly acquired a robust, biomineralization-related function, and was subsequently co-opted into the early embryo in echinoderm taxa that possess larval skeletons (echinoids and ophiuroids; Khor and Ettensohn, 2017;Shashikant et al, 2018). The biomineralizing cells of the ancestral echinoderm, which were likely of mesodermal origins, expressed alx1, ets1, erg, vegfr, and other components of a core skeletogenic program, as well as an assortment of more rapidly evolving biomineralization effector proteins (Gao and Davidson, 2008;Dylus et al, 2018;Erkenbrack and Thompson, 2019;Li et al, 2020). To draw inferences concerning the evolution of alx gene expression and function more deeply within Ambulacraria (echinoderms and hemichordates), it will be important to learn more about the single alx gene of hemichordates, including its pattern of expression, gene targets, and role in the formation of the small, calcareous skeletal elements of adult hemichordates (Cameron and Bishop, 2012) and to more precisely determine the embryological origins of the alx1-expressing cells of adult echinoderms, which are more relevant to the ancestral echinoderm condition than the more commonly studied larval forms.…”