Department of Biological Science, The University of Tulsa, Tulsa, Oklahoma, USA.
Department of Natural Science, Northwestern Oklahoma State University, Alva, Oklahoma, USA.
Dev Dyn. 2022 Jun;251(6):934-941. doi: 10.1002/dvdy.479. Epub 2022 May 6.
Amphibians undergo a variety of post-embryonic transitions (PETr) that are partly governed by thyroid hormone (TH). Transformation into a terrestrial form follows an aquatic larval stage (biphasic) or precedes hatching (direct development). Some salamanders maintain larval characteristics and an aquatic lifestyle into adulthood (paedomorphosis), which obscures the conclusion of their larval period. Paedomorphic axolotls exhibit elevated TH during early development that is concomitant with transcriptional reprogramming and limb emergence. A recent perspective suggested this cryptic TH-based PETr is uncoupled from metamorphosis in paedomorphs and concludes the larval period. This led to their question: "Are paedomorphs actual larvae?". To clarify, paedomorphs are only considered larval in form, even though they possess some actual larval characteristics. However, we strongly agree that events during larval development inform amphibian life cycle evolution. We build upon their perspective by considering the evolution of limb emergence and metamorphosis. Limbless hatchling larval salamanders are generally associated with ponds, while limbed larvae are common to streams and preceded the evolution of direct development. Permian amphibians had limbed larvae, so their PETr was likely uncoupled from metamorphosis, equivalent to most extant biphasic and paedomorphic salamanders. Coupling of these events was likely derived in frogs and direct developing salamanders.
两栖动物经历多种胚胎后发育转变（PETr），这些转变部分受甲状腺激素（TH）调控。从水生幼体阶段（双相）向陆地形态转变，或在孵化前转变（直接发育）。一些蝾螈在成年期仍保持幼体特征和水生生活方式（幼态持续），这掩盖了它们幼体期的结束。幼态持续的蝾螈在早期发育中表现出升高的 TH，这伴随着转录重编程和肢体出现。最近的观点认为，这种隐蔽的基于 TH 的 PETr 与幼态持续中的变态无关，并结束了幼体期。这导致了他们的问题：“幼态持续的动物是真正的幼虫吗？”。为了澄清这一点，幼态持续的动物仅在形态上被认为是幼虫，尽管它们具有一些真正的幼体特征。然而，我们强烈认为，幼体发育过程中的事件为两栖动物的生命周期进化提供了信息。我们通过考虑肢体出现和变态的进化来扩展他们的观点。无肢孵化的幼体蝾螈通常与池塘有关，而有肢的幼体则常见于溪流中，并先于直接发育的进化。二叠纪的两栖动物有有肢的幼体，因此它们的 PETr 可能与变态无关，这与大多数现生的双相和幼态持续的蝾螈相当。这些事件的耦合可能是在青蛙和直接发育的蝾螈中衍生出来的。
