Repeated ecological and life cycle transitions make salamanders an ideal model for evolution and development

Abstract

Observations on the ontogeny and diversity of salamanders provided some of the earliest evidence that shifts in developmental trajectories have made a substantial contribution to the evolution of animal forms. Since the dawn of evo-devo there have been major advances in understanding developmental mechanisms, phylogenetic relationships, evolutionary models, and an appreciation for the impact of ecology on patterns of development (eco-evo-devo). Molecular phylogenetic analyses have converged on strong support for the majority of branches in the Salamander Tree of Life, which includes 764 described species. Ancestral reconstructions reveal repeated transitions between life cycle modes and ecologies. The salamander fossil record is scant, but key Mesozoic species support the antiquity of life cycle transitions in some families. Colonization of diverse habitats has promoted phenotypic diversification and sometimes convergence when similar environments have been independently invaded. However, unrelated lineages may follow different developmental pathways to arrive at convergent phenotypes. This article summarizes ecological and endocrine based causes of life cycle transitions in salamanders, as well as consequences to body size, genome size, and skeletal structure. Salamanders offer a rich source of comparisons for understanding how the evolution of developmental patterns has led to phenotypic diversification following shifts to new adaptive zones.

Phylogenomic Analyses Show Repeated Evolution of Hypertrophied Lips Among Lake Malawi Cichlid Fishes

“…This is because traits might also have arisen once but be lost multiple times thereby appearing to have evolved repeatedly. The evolutionary loss of eyes in cave-adapted fishes ( Coghill et al 2014 ), flightlessness in island birds ( Wright et al 2016 ), and lack of terrestrial adult stages in neotenic salamanders ( Riley et al 2003 ; Bonett et al 2021 ) all have likely occurred multiple times and should not lead to the erroneous inference that distinctive and complex traits such as eyes, wings, or adult forms evolved multiple times. Therefore, even the best phylogenetic reconstructions of trait evolution make it difficult to ascertain how many times a trait was gained and lost among exceptionally closely related taxa.…”

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

“…[13][14][15][16] This is when aquatic larval traits and ecology are maintained into adulthood. Larval form paedomorphosis occurs in nine of the ten salamander families, [3][4][5]17 and is expressed as an alternative life cycle mode to metamorphosis (biphasic) in some species. 18,19 The primary characteristic used to classify larval form paedomorphs is their larval-shaped hyobranchial apparatus, which is adapted for aquatic living.…”

“…The development and activation of the hypothalamicpituitary-thyroid (HPT) axis appears to mirror amphibian life cycle variation. 11,17,[23][24][25][26][27][28][29] Alterations to a systemic regulator like thyroid hormone (TH) can simultaneously shift the developmental timing of many traits. 11,[23][24][25][26][27] A reduction in TH release or TH sensitivity can result in the juvenilization of a broad array of otherwise TH transformable structures.…”

“…The development and activation of the hypothalamic‐pituitary‐thyroid (HPT) axis appears to mirror amphibian life cycle variation 11,17,23‐29 . Alterations to a systemic regulator like thyroid hormone (TH) can simultaneously shift the developmental timing of many traits 11,23‐27 .…”

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.…”