Amphibians display very diverse life cycles and development can be direct, where it occurs in ovo and a juvenile hatches directly, or biphasic, where an aquatic larva hatches and later undergoes metamorphosis followed by sexual maturation. In both cases, metamorphosis, corresponds to the post embryonic transition (PETr). A third strategy, only found in Urodeles, is more complex as larvae reach sexual maturity before metamorphosis, which can become accessory. The resulting paedomorphs retain their larval characters and keep their aquatic habitat. Does it mean that paedomorphs do not undergo PETr? Recent work using high throughput technologies coupled to system biology and developmental endocrinology revisited this question and provided novel datasets indicating that a paedomorph's "larval" tissue undergoes a proper developmental transition. Together with historical data, we propose that this transition is a marker of the PETr, which would be distinct from metamorphosis. This implies that (a) complex life cycles would result from the uncoupling of PETr and metamorphosis, and (b) biphasic life cycles would be a special cases where they occur simultaneously.
Keywords: metamorphosis; paedomorphosis; post embryonic transition; thyroid hormone signaling; urodeles.
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