high-dose iodine effect Most thyroids removed surgically have "palpation thyroiditis" from all the physical exams,a mix of fibrosis and granulomas typically inside follicles.
Endogenous TH synthesis could be a preadaptation for the evolution of nonfeeding development among echinoids (, ). This hypothesis was built on the observation that different clades of echinoids seem to have evolved nonfeeding development at different frequencies. We predicted that one could find an overall enhanced capacity of endogenous hormone synthesis in clades that evolved nonfeeding development more frequently. Our new data provide support for this hypothesis in that both C. rosaceus (this study) and D. excentricus () have the capacity to synthesize TH endogenously and are part of a larger clade (Clypeasteroids) including species that synthesize TH and/or show phenotypic effects to TH treatment (L. sexiesperforata and P. japonica). This clade also evolved nonfeeding development multiple times. However, iodine incorporation and pharmacological data also show that a TPO-related peroxidase is likely involved in the process of TH synthesis in C. rosaceus. Recent data from our lab show similar results for the feeding larvae of the sea urchin L. variegatus and the mollusk A. californica (). These findings make endogenous TH synthesis via peroxidases a shared characteristic among deuterostomes and potentially among all metazoa. This has specific implications for the role of TH signaling as a mechanism underlying the evolution of nonfeeding development. Once selection has favored sufficiently large, yolk-rich eggs, upregulation of endogenous TH synthesis via peroxidases may result in accelerated development and differential shifts in investment from larval to juvenile structures. We predict that both feeding and nonfeeding larvae have the ability to synthesize TH endogenously; however, in lineages where selection has favored the evolution of increased maternal investment, an upregulation of endogenous TH synthesis via peroxidases may lead to the evolution of nonfeeding development.
Whereas several components of the TH signaling system are present in all chordates, increasing evidence suggests that urochordates (sea squirts) and cephalochordates (lancelets) share critical elements of this signaling pathway. Zone 7 of the urochordate endostyle expresses genes related to vertebrate thyroid function including Orthologs of TPO (; ). Furthermore, some evidence suggests that the endostyle is involved in the biosynthesis of TH (reviewed in ). Despite increasing indications that TH is synthesized and/or act as signaling molecules in various invertebrates (; ), the mechanisms of TH biosynthesis in invertebrates other than basal chordates are essentially unknown.
Finally, the synthesis enzyme itself is not the only component of the putative TH signaling module in echinoid larvae. Receptor expression and the role of activation enzymes such as deiodinases or sulfotransferases that can change the iodination state of the hormones are critical factors as well, and it should be the goal of future studies to identify them and their expression patterns in both feeding and nonfeeding larvae of echinoderms and other animal phyla as well (see also ).
Finally, experimentation with feeding larvae is inherently different from the experimentation with facultative planktotrophic larvae or nonfeeding larvae. Feeding larvae can never be reared to metamorphosis in the complete absence of food. Therefore, it is possible that feeding larvae can accumulate sufficient TH during the feeding period from algae (see also ; ) that allows them to develop to metamorphosis when larvae are exposed to thyroid hormone synthesis inhibitors without food (see also ).
A question that needs further investigation is whether the phenotypically plastic response of larval and juvenile structures to endogenous hormone levels is a general characteristic of nonfeeding larvae, or whether it is only present in C. rosaceus with its specialized feeding mode (facultative planktotrophy). Because these sea biscuit larvae have the ability to feed, a certain amount of plasticity is retained because it could give larvae an adaptive advantage: observed previously that egg size and egg energy in C. rosaceus can vary significantly between females. This natural variation could lead to a situation where insufficient maternal reserves are compensated for during the larval period, making feeding obligatory in order to reach metamorphosis. In such cases, it may be advantageous for larvae to have a phenotypically plastic response to food abundance. Unfortunately, there is not sufficient information about facultative planktotrophy and its functional significance with regard to food uptake and digestion. Experiments comparing particle ingestion rates in inhibitor-induced, long-arm larvae, and control would provide data to answer some of these questions, as initially proposed by . Such experiments will allow us to further understand the role that TH plays in the evolution of derived (nonfeeding) life histories in echinoderms.
Although hormonal effects are clearly reflected in the growth trajectories of larval and juvenile structures, we did not find significant effects of any of our treatments on developmental rates other than time to metamorphosis. Our heterochrony analysis shows no shifts in developmental timing between the control and any experimental treatment. These findings are in contrast to the effects we detected for the sand dollar D. excentricus where we found significant heterochronic shifts in development in the thyroxine treatment. The capacity for endogenous hormone synthesis in C. rosaceus might make larval and juvenile tissues less sensitive to exogenous hormone. Alternatively, the difference could be explained by the faster development of C. rosaceus larvae and the subsequent lack of resolution to detect such changes with the method we used.
We provide evidence for endogenous thyroxine (T4) synthesis in larvae of the sea biscuit C. rosaceus. Larvae incorporate iodine from the seawater to build T4, which plays a critical role in larval development and metamorphosis in several echinoderm species including C. rosaceus. Pharmacological and immunohistochemical evidence suggests that a peroxidase closely related to vertebrate TPO could be involved in the process of TH synthesis. Its expression in cell clusters at the base of juvenile spines during premetamorphic stages further emphasizes a role of TH signaling in echinoid metamorphosis. Comparing the ability to synthesize hormone endogenously between closely related species such as the sea biscuit C. rosaceus and the sand dollar D. excentricus, and the phenotypic plastic response of larval arms resulting from inhibition of endogenous hormone synthesis provides additional evidence for this conclusion. Finally, we propose that upregulation of endogenous TH synthesis in nonfeeding larvae may have been a critical factor for the evolution of nonfeeding development.
Larvae of the sand dollar D. excentricus use thyroxine as a signal for phenotypic plasticity (). When applied exogenously, the hormone simulates high food conditions in that it leads to a shift in investment from larval to juvenile structures. Larvae reared in low food environments show a shift in investment in the opposite direction (from juvenile to larval structures), resulting in relatively long larval arms (; ). For C. rosaceus, we show an increase in larval arm length after treatment with the TH synthesis inhibitor thiourea. The similarity in response of feeding larvae to low food conditions and C. rosaceus to inhibitor treatment (this study) suggests that facultatively feeding larvae have maintained vestiges of the phenotypically plastic response that is also induced by thyroxine. In feeding larvae, the cue is exogenous, whereas it appears to be endogenous for C. rosaceus. These findings further support the scenario that ancestral feeding larvae gained the ability to synthesize any hormones necessary for metamorphosis.
92: 3158, 2007) A problem with thyroxine synthesis or release -- the most common causeis peroxidase deficiency Non-functioning thyroid hormone receptors (J.
Exogenous TH effects on the development of facultative planktotrophic larvae of C. rosaceus are dramatically different from TH effects on the development of feeding larvae of D. excentricus. We previously reported a 42% acceleration of development in larvae of the sand dollar D. excentricus () when treated with TH at 10−9 M. The same treatment accelerated development in C. rosaceus (a species that produces significantly larger eggs than D. excentricus) by only 10–28%. Moreover, we were not able to find any significant difference in juvenile size and total energy content in these larvae, which is in stark contrast to the TH effects on juvenile size in D. excentricus where we reported a 56% decrease in size () upon thyroxine treatment. These findings suggest that developmental rate and juvenile size are much more plastic in feeding larvae than they are in facultative planktotrophic larvae. Moreover, we view these findings as evidence for a decreased dependence of facultative planktotrophic larvae from exogenous hormone sources. This hypothesis can be further tested using other closely related species from the Clypeasteroid clade. We previously published evidence for TH action in L. sexiesperforata () and have preliminary evidence on Melitta tenuis (A. Heyland, unpublished). Together, these species provide a range of maternal investments and feeding modes that will allow us to further test this hypothesis. Finally, TH has been measured in P. japonica (), another congeneric sand dollar with obligatorily nonfeeding larvae and high maternal investment. These findings show that P. japonica has the ability to synthesize hormones endogenously as well. Moreover, data by indicate that thiourea inhibits hormone synthesis.