Meeting Abstract

22.6  Thursday, Jan. 3  Evolutionary Implications of Maternally-derived Thyroid Hormone on Development CROCKFORD, S.J.; Univ. of Victoria, BC scrock@uvic.ca

Many hormones influence embryonic growth and development but thyroid hormone (TH) regulates more additional functions and processes, at all life stages, than any other: TH is also required for post-natal growth and brain development, stress responses, reproduction, brain function, and metabolism of cells and tissues. During embryonic development, maternally-derived TH is necessary, in a time- and dose-dependent manner, from the moment of conception onward. Maternally-derived TH is required for appropriate expression of many critical developmental genes that control timely movement, proliferation and maturation of essential cell types (such as sonic hedgehog, fibroblast growth factor and bone morphogenic protein). Maternal TH is secreted in a rhythmic fashion and these patterns likely vary within and between species, as such rhythms do for other hormones. I contend that species-specific rhythms of maternal TH could be responsible for controlling and coordinating critical aspects of individually-variable species-specific embryonic growth. In non-placental vertebrates, I suggest that a layered concentration gradient of TH must exist (as shown for steroid hormones) to allow the yolk to reflect maternal TH rhythms. I propose that disruption of maternal TH rhythms, due to stress and founder effects associated with colonization of new habitats, could implement (via heterochrony) the rapid transformations in morphological and physiological life history traits that characterize new species, in a manner essentially independent of genetic change. This unifying concept, which has been fully developed as a comprehensive theoretical model and extensively peer-reviewed, not only fits available facts exceedingly well but is eminently testable. The theory therefore provides a robust theoretical framework for future evo-devo research.