Taylor Elias, Allen Jonathan D, Heyland Andreas
University of Guelph, Integrative Biology, 50 Stone Rd East, Guelph, ON, Canada, N1G 2W1.
William & Mary, 540 Landrum Drive, Williamsburg, VA 23187, USA.
J Exp Biol. 2025 Feb 1;228(3). doi: 10.1242/jeb.249351. Epub 2025 Feb 12.
The timing of metamorphosis and settlement is critical for the survival and reproductive success of marine animals with biphasic life cycles. Thyroid hormones (THs) regulate developmental timing in diverse groups of chordates, including the regulation of metamorphosis in amphibians, teleosts, lancelets, tunicates and lampreys. Recent evidence suggests a role for TH regulation of metamorphosis outside of the chordates, including echinoderms, annelids and molluscs. Among echinoderms, TH effects on development as well as underlying signaling mechanisms in early embryogenesis have been documented for echinoid (sea urchin) larvae, but we lack information on TH effects on metamorphic development in most other echinoderm groups, including the ophiuroids (brittle stars). Unexpectedly, we found that THs, principally 3,5,3',5'-tetraiodo-l-thyronine (T4), reversibly inhibit metamorphic development and settlement in the daisy brittle star (Ophiopholis aculeata). Exposure to thiourea, an inhibitor of TH synthesis, accelerated metamorphic development. We showed that these effects were highly stage specific, providing evidence for a developmental point-of-no-return in ophiuroid metamorphic development. Furthermore, starvation of O. aculeata accelerated juvenile morphogenesis and settlement. Starvation also prevented the inhibitory effect of thiourea on TH function, suggesting that TH synthesis may play a role in delaying metamorphosis under conditions of high food availability. These findings provide evidence for a function of TH signaling in ophiuroid metamorphic development and suggest that exogenous TH sources may be involved in the regulation of metamorphic timing in O. aculeata. Together with new evidence of TH involvement in metamorphic development in a range of invertebrates, these findings further emphasize the versatile and central role of endocrine signaling in metamorphosis.
变态和附着的时机对于具有双相生命周期的海洋动物的生存和繁殖成功至关重要。甲状腺激素（THs）调节多种脊索动物群体的发育时机，包括两栖动物、硬骨鱼、文昌鱼、被囊动物和七鳃鳗的变态。最近的证据表明，THs在脊索动物之外的变态调节中也发挥作用，包括棘皮动物、环节动物和软体动物。在棘皮动物中，已记录了THs对海胆幼虫早期胚胎发育的影响及其潜在的信号传导机制，但我们缺乏关于THs对大多数其他棘皮动物群体（包括蛇尾类（脆星））变态发育影响的信息。出乎意料的是，我们发现THs，主要是3,5,3',5'-四碘-L-甲状腺原氨酸（T4），可逆地抑制雏菊脆星（Ophiopholis aculeata）的变态发育和附着。暴露于硫脲（一种TH合成抑制剂）会加速变态发育。我们表明这些影响具有高度的阶段特异性，为蛇尾类变态发育中的发育不可逆点提供了证据。此外，饥饿的O. aculeata会加速幼体形态发生和附着。饥饿还阻止了硫脲对TH功能的抑制作用，这表明TH合成可能在食物供应充足的条件下延迟变态中发挥作用。这些发现为TH信号在蛇尾类变态发育中的功能提供了证据，并表明外源性TH来源可能参与了O. aculeata变态时间的调节。连同TH参与一系列无脊椎动物变态发育的新证据，这些发现进一步强调了内分泌信号在变态中的多功能和核心作用。
