Yamakawa S et al. (2018), The role of retinoic acid signaling in starfish...

ECB-ART-46301

Evodevo 2018 Jan 22;9:10. doi: 10.1186/s13227-018-0098-x.

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The role of retinoic acid signaling in starfish metamorphosis.

Yamakawa S , Morino Y , Honda M , Wada H .

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Background: Although retinoic acid (RA) signaling plays a crucial role in the body patterning of chordates, its function in non-chordate invertebrates, other than its mediation of environmental cues triggering metamorphosis in cnidarians, is largely unknown. We investigated the role of RA signaling in the metamorphosis of starfish (Echinodermata). Results: We found that exogenous RA treatment induced metamorphosis in starfish larvae. In contrast, inhibitors of RA synthesis and RA receptors suppressed metamorphosis triggered by attachment to a substrate. Gene expressions of the RA signaling component were detected in competent larvae. Conclusions: This study provides insight into the ancestral function of RA signaling, which is conserved in the metamorphosis of cnidarians and starfish.

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Genes referenced: gfi1 LOC100893907 rarb

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	Fig. 1. Competent and metamorphosis larva of P. pectinifera. a, b Ventral (a) and left side (b) view of competent 20-dpf larva. c–e Metamorphosis larva in 24 h after settlement. c Larval ventral side of metamorphosis larva. d, e attend on aboral and oral side of juvenile rudiment, respectively. Dot lines indicate juvenile rudiment and asterisk marks do developed juvenile arms. Arrow heads point absorbed larval body and arrows do primary podia. Size of scale bar is 250 µm
	Fig. 2. Duration until P. pectinifera became competent. a Schematic experiment sets without or with substrate (left and right, respectively). Without external substrate, larva did not settle and complete metamorphosis. b Ratio of metamorphosis completion 24 h after adding substrate (n = 20 from each three batch)
	Fig. 3. Effects on metamorphosis by all-trans RA or DMSO treatments in P. pectinifera. a–e Metamorphosis induction of 14-dpf larvae by exogenous all-trans RA (0.1 or 1 µM) treatment (n = 50 from each three batch). a, b, d, e Induced juvenile of 96 h after RA 0.1, 1 µM treatment, respectively. c DMSO-treated larvae (96 h after treatment). Arrow heads indicate developed juvenile rudiment, white arrowheads point out absorbed larval body, and arrows show primary podia. The scale bars in a–c, d–e display 250, 50 µm, respectively. The number in figure show the ratio of metamorphosed larvae/treated larvae. f Temporal effects on metamorphosis induction of all-trans RA (1 µM). Numbers in figure show percentage of metamorphosed specimen (n = 20 from each three batch in each treatment) and background colors are depended on their percentage and divided into five darkness levels, meaning 0, 0–25, 25–50, 50–75 and 75–100%. Vertical axis indicates days after treatment and horizontal axis indicates days after fertilization
	Fig. 4. Effects on settlement or metamorphosis by RO or DEAB treatments in P. pectinifera. a–i RO 3 µM treatment inhibited metamorphosis of 14-dpf larvae induction by RA 1 µM treatments (n = 20 from each three batch). a–d Left side of specimen at 24 h after treatment and e–h does ventral or dorsal side of these. Each specimen is taken from following treatment, a, e RA 0.1 µM, b, f RA 0.1 µM with RO 3 µM, c, g RA 1 µM and d, h RA 1 µM with RO 3 µM. The scale bars in a–h display 125 µm. i indicates metamorphosis ratio of 24 h after treatments. j, k Effects of RO in settlement and metamorphosis (n = 24 from each three batch). j Schematic machinery of inhibition of RA signaling by RO. k Settlement and metamorphosis ratio of 14-dpf larvae in 24 h after RO 1 µM treatment with substrate (n = 72 from three batches). l, m Effects of DEAB in settlement and metamorphosis (n = 36 from each three batch). l Schematic machinery of inhibition of RA signaling by DEAB. m Settlement and metamorphosis ratio of 14-dpf larvae in 24 h after DEAB 100, 300 µM treatment with substrate
	Fig. 5. Spatial expression pattern of raldha, raldhb, raldhc, rar, and rxr in P. pectinifera.. a–o show results of whole mount in situ hybridization of raldha, raldhb, raldhc, rar and rxr in larval stages (a–e 6 dpf, f–j 9 dpf and k–o 14 dpf) and metamorphosing larvae just after settlement (p–t). Dot line means the boundary of before and after settlement. The inset figures of k, n and o show the expression in hydrolobes. L: left view, V: ventral view and D: dorsal view. Arrow heads indicate the expression in epidermis around brachiolar arms and white arrow heads point out the expression in hydrolobes. Arrows point expression in juvenile rudiment. Non-specific expressions are indicated by asterisk
	Fig. 6. Schematic RA signaling function as regulator of metamorphosis. RA signaling activation is induced by receiving environmental cue with adhesive papillae of brachiolar arms and commences metamorphosis process

References [+] :

Albalat, The retinoic acid machinery in invertebrates: ancestral elements and vertebrate innovations. 2009, Pubmed