Paedomorphic salamanders are larval in form and patterns of limb emergence inform life cycle evolution

Abstract

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.

Salamander models for elucidating mechanisms of developmental biology, evolution, and regeneration: Part two

“…If life cycle transitions such as the ones described in Olejnickova et al 13 and Riquel‐Guzman et al 14 intrigue you, you will also enjoy two articles by Bonnet et al 15 and Bonnet and Ledbetter 16 in the current issue. Bonnet et al review explored the various causes of life cycle transitions in salamander species and the link between these transitions with body size, genome size, and skeletal evolution.…”