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Title Life-long indeterminate growth in brittle stars is driven by a subterminal growth zone at arm tips. Abstract Background: Echinoderms are a phylum of marine invertebrates with close phylogenetic relationships to chordates. Many members of the phylum Echinodermata are capable of extensive post-traumatic regeneration and life-long indeterminate growth. Different from regeneration, the life-long elongation of the main body axis in adult echinoderms has received little attention. The anatomical location and the nature of the dividing progenitor cells contributing to adults' growth is unknown. Results: We show that the proliferating cells that drive the life-long growth of adult brittle star arms are mostly localized to the subterminal (second from the tip) arm segment. Each of the major anatomical structures contains dividing progenitors. These structures include: the radial nerve, water-vascular canal, and arm coelomic wall. Some of those proliferating progenitor cells are capable of multiple rounds of cell division. Within the nervous system, the progenitor cells were identified as a subset of radial glial cells that do not express Brn1/2/4, a transcription factor with a conserved role in the neuronal fate specification. In addition to characterizing the growth zone and the nature of the precursor cells, we provide a description of the microanatomy of the four distal-most arm segments contrasting the distal with the proximal segments, which are more mature. Conclusions: The growth of the adult brittle star arms occurs via proliferation of progenitor cells in the distal segments, which are most abundant in the second segment from the tip. At least some of the progenitors are capable of multiple rounds of cell division. Within the nervous system the dividing cells were identified as Brn1/2/4-negative radial glial cells. Supplementary materials Files in here (Zenodo.org, DOI: 10.5281/zenodo.5762494)are the supplementary digital materials corresponding to the manuscript "Life-long indeterminate growth in brittle stars is driven by a subterminal growth zone at arm tips," written by Mashanov et al. Files include: "Morphology.avi": Video illustration of the three-dimensional reconstruction of the arm tip of Amphipholis kochii. This video is also available on YouTube: https://youtu.be/eMpDZeKXQJE. "Model.blend": Blender file containing the 3D model of the A. kochii arm tip morphology. "FileS1_Statistics.Rmd" and "FileS1_Statistics.pdf": R Markdown file (R version 4.0.4). This filecontains the data and statistical analysis about proliferating cells counted in the four terminal arm segments of five different individuals, using only one arm per individual.

This Zenodo upload contains data from the draft genome of the brittle star Ophioderma brevispinum (Say, 1859). Please read our READ-ME.mdfile for additional details. You can citethe paper: Mashanov, V., Machado, D.J., Reid, R.et al.Twinkle twinkle brittle star: the draft genome ofOphioderma brevispinum(Echinodermata: Ophiuroidea) as a resource for regeneration research.BMC Genomics23, 574 (2022). https://doi.org/10.1186/s12864-022-08750-y. Abstract: Background—Echinoderms are established models in experimental and developmental biology, however genomic resources are still lacking for many species. Here, we present the draft genome ofOphioderma brevispinum, an emerging model organism in the field of regenerative biology. This new genomic resource provides a reference for experimental studies of regenerative mechanisms. Results—We report a de novo nuclear genome assembly for the brittle starO. brevispinumand annotation facilitated by the transcriptome assembly. The final assembly is 2.68 Gb in length and contains 146,703 predicted protein-coding gene models. We also report a mitochondrial genome for this species, which is 15,831 bp in length, and contains 13 protein-coding, 22 tRNAs, and 2 rRNAs genes, respectively. In addition, 29 genes of the Notch signaling pathway are identified to illustrate the practical utility of the assembly for studies of regeneration. Conclusions—The sequenced and annotated genome ofO. brevispinumpresented here provides the first such resource for an ophiuroid model species. Considering the remarkable regenerative capacity of this species, this genome will be an essential resource in future research efforts on molecular mechanisms regulating regeneration. &nbsp

Additional file 7 File S7. Protocol for de novo DNA repeat assembly from shotgun sequence reads.

Additional file 9 File S9. Template scripts for gene prediction and annotation.

Additional file 8 File S8. Protocol for DNA repeat identification.

Additional file 3 File S3. Bioinformatics protocols for quality control of raw sequence reads, subsequent genome assembly, and the methodology and main results for BUSCO v4.0.6 analyses.

Additional file 5 File S5. Protocols for Feulgen image analysis densitometry and sequence-based genome size estimation.

Additional file 6 File S6. Expected variation of haploid genome sizes in echinoderms.