(Eco)Physiology — Developmental Phenotypic Plasticity
I am interested in the evolution and environmental control of polyphenisms, which are an extreme form of phenotypic plasticity where multiple, environmentally-cued phenotypes stem from a single genotype. I am investigating the roles of both biotic and abiotic cues involved in eliciting alternative phenotypes, along with their underlying hormonal mechanisms. I use facultatively paedomorphic salamanders as a model system to investigate the roles that stressors play in affecting developmental trajectory and plastic traits. Facultative paedomorphosis is a retention of juvenile characteristics into adulthood.
Proposed model incorporating endocrine mechanisms regulating facultative paedomorphosis. Under the paedomorph advantage hypothesis, and above a minimum body size, larvae that are exposed to environmental stressors (e.g., crowding, pond drying, predator presence, starvation) and have at least reached the minimum body size required for metamorphosis (35 mm; (Semlitsch and Wilbur 1988)) are more likely to metamorphose than become paedomorphic. Stressors activate the HPI axis (blue arrow) releasing CRF from the hypothalamus and ACTH and TSH from the anterior pituitary. ACTH stimulates the adrenal gland to release CORT, while TSH stimulates the thyroid gland to release THs (T3/T4). CORT and THs act synergistically to affect gene expression in target tissues resulting in metamorphosis (left) (reviewed in Denver 2017). In the absence of stressors, the HPI axis is not activated and metamorphosis is delayed resulting in aquatic paedomorphs (right). Both metamorphs and paedomorphs attain sexual maturity, but paedomorphs reach sexual maturity earlier (Ryan and Semlitsch 1998), increasing lifetime expected fitness (Cole 1954, Roff 1992, Stearns 1992) (Bohenek et al 2021).
Typical Ambystoma talpoideum adult with terrestrial morphology. Most Ambystoma spp. have an aquatic larval phase and then metamorphose into terrestrial adults.
Paedomorphic adult Ambystoma talpoideum salamander with retention of gilled larval morphology. Paedomorphs are fully aquatic adults.
The complex life cycle of Notophthalmus viridescens. Eggs (1) are laid singly in aquatic vegetation and hatch into an aquatic larva (2). Larvae can either become paedomorphic or metamorphose. Paedomorphs are either complete neotenes (3), which have no degree of metamorphosis, or branchiates (4), which partially metamorphose. Both paedomorphic phenotypes retain gills, compressed tail fins and aquatic lifestyles. Metamorphosed phenotypes include aquatic juveniles (5), which are miniature adults, terrestrial efts (6) and semi-aquatic adults (7). Neotenes (3), branchiates (4) and semi-aquatic adults (7) are sexually mature while efts (6) and aquatic juveniles (5) are immature, but efts can take up to seven years longer than aquatic juveniles to reach sexual maturity (Bohenek & Resetarits, 2018). Illustration by Tatiana Tushyna.