Biomedical subjects

Jason Hodin

Publications and source records attributed to Jason Hodin.

3 recordsLinked to original sources

PubMed2005

Hormone signaling in evolution and development: a non-model system approach.

Cooption and modularity are informative concepts in evolutionary developmental biology. Genes function within complex networks that act as modules in development. These modules can then be coopted in various functional and evolutionary contexts. Hormonal signaling, the main focus of this review, has a modular character. By regulating the activities of genes, proteins and other cellular molecules, a hormonal signal can have major effects on physiological and ontogenetic processes within and across tissues over a wide spatial and temporal scale. Because of this property, we argue that hormones are frequently involved in the coordination of life history transitions (LHTs) and their evolution (LHE). Finally, we promote the usefulness of a comparative, non-model system approach towards understanding how hormones function and guide development and evolution, highlighting thyroid hormone function in echinoids as an example.

Animals↗

PubMed2004

Heterochronic developmental shift caused by thyroid hormone in larval sand dollars and its implications for phenotypic plasticity and the evolution of nonfeeding development.

Recent work on a diverse array of echinoderm species has demonstrated, as is true in amphibians, that thyroid hormone (TH) accelerates development to metamorphosis. Interestingly, the feeding larvae of several species of sea urchins seem to obtain TH through their diet of planktonic algae (exogenous source), whereas nonfeeding larvae of the sand dollar Peronella japonica produce TH themselves (endogenous source). Here we examine the effects of TH (thyroxine) and a TH synthesis inhibitor (thiourea) on the development of Dendraster excentricus, a sand dollar with a feeding larva. We report reduced larval skeleton lengths and more rapid development of the juvenile rudiment in the exogenous TH treatments when compared to controls. Also, larvae treated with exogenous TH reached metamorphic competence faster at a significantly reduced juvenile size, representing the greatest reduction in juvenile size ever reported for an echinoid species with feeding larvae. These effects of TH on D. excentricus larval development are strikingly similar to the phenotypically plastic response of D. excentricus larvae reared under high food conditions. We hypothesize that exogenous (algae-derived) TH is the plasticity cue in echinoid larvae, and that the larvae use ingested TH levels as an indicator for larval nutrition, ultimately signaling the attainment of metamorphic competence. Furthermore, our experiments with the TH synthesis inhibitor thiourea indicate that D. excentricus larvae can produce some TH endogenously. Endogenous TH production might, therefore, be a shared feature among sand dollars, facilitating the evolution of nonfeeding larval development in that group. Mounting evidence on the effects of thyroid hormones in echinoderm development suggests life-history models need to incorporate metamorphic hormone effects and the evolution of metamorphic hormone production.

Analysis of Variance↗

PubMedDate not supplied

Thyroid hormones determine developmental mode in sand dollars (Echinodermata: Echinoidea).

Evolutionary transitions in larval nutritional mode have occurred on numerous occasions independently in many marine invertebrate phyla. Although the evolutionary transition from feeding to nonfeeding development has received considerable attention through both experimental and theoretical studies, mechanisms underlying the change in life history remain poorly understood. Facultative feeding larvae (larvae that can feed but will complete metamorphosis without food) presumably represent an intermediate developmental mode between obligate feeding and nonfeeding. Here we show that an obligatorily feeding larva can be transformed into a facultative feeding larva when exposed to the thyroid hormone thyroxine. We report that larvae of the subtropical sand dollar Leodia sexiesperforata (Echinodermata: Echinoidea) completed metamorphosis without exogenous food when treated with thyroxine, whereas the starved controls (no thyroxine added) did not. Leodia sexiesperforata juveniles from the thyroxine treatment were viable after metamorphosis but were significantly smaller and contained less energy than sibling juveniles reared with exogenous food. In a second starvation experiment, using an L. sexiesperforata female whose eggs were substantially larger than in the first experiment (202+/-5 vs. 187+/-5 microm), a small percentage of starved L. sexiesperforata larvae completed metamorphosis in the absence of food. Still, thyroxine-treated larvae in this experiment completed metamorphosis faster and in much higher numbers than in the starved controls. Furthermore, starved larvae of the sand dollar Mellita tenuis, which developed from much smaller eggs (100+/-2 microm), did not complete metamorphosis either with or without excess thyroxine. Based on these data, and from recent experiments with other echinoids, we hypothesize that thyroxine plays a major role in echinoderm metamorphosis and the evolution of life history transitions in this group. We discuss our results in the context of current life history models for marine invertebrates, emphasizing the role of egg size, juvenile size, and endogenous hormone production for the evolution of nonfeeding larval development.

Animals↗