Iodine & Thyroid Hormone Signaling in Marine Ecosystems
Metamorphic life histories evolved in many animal taxa and diverse habitats. Many metamorphoses are coordinated by hormones. For example, thyroid hormones (THs) regulate virtually all aspects of amphibian metamorphosis and juvenile hormones and ecdysteroids are critical for regulating molting and metamorphosis in insects. Intriguingly, changes in the production, regulation and/or tissue-specific response of these hormones may lead to the evolution of alternative life histories. Since insect and amphibian metamorphoses likely evolved independently, this is a striking example of homoplasy (i.e. convergence, parallelism and reversal) at the level of regulatory mechanisms underlying complex developmental programs. It has recently been suggested that THs can also function in non-chordate animals with distinct body plans but otherwise comparable metamorphic transitions (Heyland et al. 2005). For example, in echinoid larvae (sea urchins, sand dollars), TH-like molecules can be endogenously synthesized, exogenously derived from ingested phytoplankton, or both (Heyland et al. 2006a,b; Heyland and Moroz 2006). When applied to developing echinoid larvae, THs induce changes in development, including alterations of morphogenesis and growth. Moreover, THs regulate metamorphosis via short-term changes in settlement (Bishop et al. 2006). Preliminary comparisons of related echinoid species with feeding larvae (i.e. that require food to develop to metamorphosis) and non-feeding larvae (i.e. those that do not require food to develop to metamorphosis) suggest that changes in endogenous hormone synthesis and signal transduction may have facilitated the evolution of non-feeding development from feeding development (Heyland et al. 2004; Heyland et al. 2005). As such, THs in echinoids perform functions analogous to those in chordates (and to juvenile hormones and ecdysteroids in insects). In addition, exogenous sources of THs (i.e. phytoplankton) may represent an important component of trophic connectivity among organisms in marine ecosystems. The majority of invertebrate phyla are marine (approximately 28 out of 32) and many of them evolved metamorphic life histories. This provides a broad comparative framework for studying the mechanistic bases of these complex life histories. My long-term goal is to gain insight into the signaling architecture underlying larval development and metamorphosis in selected marine invertebrate groups with particular emphasis on hormonal signaling. This will elucidate 1) the extent of homoplasy in mechanisms across phyla and 2) whether evolutionary changes in hormone signaling caused changes in life history modes, as appears to be the case for insects and vertebrates (3, 8). The short-term goal is to expand my work on the role of TH in larval development and metamorphosis of selected echinoid species and to test specific hypotheses regarding the role of these hormones in the evolution of non-feeding development.