The hypothalamic-pituitary-adrenal (HPA) axis, or stress axis, is a key physiological system that mediates the relationship of the organism with its environment. Because activation of the HPA axis mobilizes energy stores for immediate use, but sustained activation can have deleterious effects on survival, the HPA axis has been implicated in the tradeoff between reproduction and survival. In this thesis, I investigate whether there is an association between one life history trait, reproductive lifespan, and the functioning of the HPA axis as predicted by the “adaptive stress hypothesis”. The adaptive stress hypothesis predicts that species adopting life history strategies characterized by short lifespans and early reproduction should maximize the energy available for reproduction through high levels of circulating glucocorticoids caused by the dysregulation of the HPA axis in the breeding season, whereas those characterized by long lifespans and extended reproduction should maintain a functioning HPA axis with low levels of glucocorticoids throughout life. To test this hypothesis, I studied five species of ground squirrels that vary dramatically in male reproductive lifespan: arctic, Richardson’s, Columbian, thirteen-lined, and Franklin’s ground squirrels (Urocitellus parryii, U. richardsonii, U. columbianus, Ictidomys tridecemlineatus, and Poliocitellus franklinii). I used a stress profile to characterize the HPA axis of male ground squirrels immediately before and immediately after the breeding season. The stress profile included measures of plasma glucocorticoid concentrations, determinants of plasma glucocorticoid concentrations (corticosteroid binding globulin levels, adrenal sensitivity/capacity, negative feedback, and intrinsic restraint), and markers of the biological effects of glucocorticoids (energy mobilization, health, and immune function). Contrary to the adaptive stress hypothesis, I found no relationship between reproductive lifespan and postbreeding glucocorticoid levels. Species also varied significantly and unexpectedly in how determinants of glucocorticoid levels changed over the breeding season, and in how glucocorticoids levels translated into biological effects. I also observed unexpected patterns of individual variation within species. Thus, life history alone did not predict HPA axis functioning. My results suggest that the HPA axis is so flexible in its functioning, that we will need to adopt a much more detailed model of the HPA axis in order to fully understand the relationship between the HPA axis and life history variation.

The hypothalamic-pituitary-adrenal (HPA) axis, or stress axis, is a key physiological system that mediates the relationship of the organism with its environment. Because activation of the HPA axis mobilizes energy stores for immediate use, but sustained activation can have deleterious effects on survival, the HPA axis has been implicated in the tradeoff between reproduction and survival. In this thesis, I investigate whether there is an association between one life history trait, reproductive lifespan, and the functioning of the HPA axis as predicted by the “adaptive stress hypothesis”. The adaptive stress hypothesis predicts that species adopting life history strategies characterized by short lifespans and early reproduction should maximize the energy available for reproduction through high levels of circulating glucocorticoids caused by the dysregulation of the HPA axis in the breeding season, whereas those characterized by long lifespans and extended reproduction should maintain a functioning HPA axis with low levels of glucocorticoids throughout life. To test this hypothesis, I studied five species of ground squirrels that vary dramatically in male reproductive lifespan: arctic, Richardson’s, Columbian, thirteen-lined, and Franklin’s ground squirrels (Urocitellus parryii, U. richardsonii, U. columbianus, Ictidomys tridecemlineatus, and Poliocitellus franklinii). I used a stress profile to characterize the HPA axis of male ground squirrels immediately before and immediately after the breeding season. The stress profile included measures of plasma glucocorticoid concentrations, determinants of plasma glucocorticoid concentrations (corticosteroid binding globulin levels, adrenal sensitivity/capacity, negative feedback, and intrinsic restraint), and markers of the biological effects of glucocorticoids (energy mobilization, health, and immune function). Contrary to the adaptive stress hypothesis, I found no relationship between reproductive lifespan and postbreeding glucocorticoid levels. Species also varied significantly and unexpectedly in how determinants of glucocorticoid levels changed over the breeding season, and in how glucocorticoids levels translated into biological effects. I also observed unexpected patterns of individual variation within species. Thus, life history alone did not predict HPA axis functioning. My results suggest that the HPA axis is so flexible in its functioning, that we will need to adopt a much more detailed model of the HPA axis in order to fully understand the relationship between the HPA axis and life history variation.

It is generally thought that in amphibians, thyroid hormones (THs) regulate metamorphosis, while sex steroids (estrogens and androgens) regulate gonadal differentiation. However, inhibition of TH synthesis in frogs alters gonadal differentiation, suggesting instead that these two endocrine axes interact during development. Specifically, THs may be involved in male development, while estrogens may inhibit tadpole metamorphosis. However, we do not currently know the mechanisms that account for these interactions, let alone how such mechanisms may differ between species. To develop and test new hypotheses on the roles of sex steroids and THs, I first examined transcriptional profiles (mRNA) of enzymes and receptors related to sex steroids and THs during embryogenesis and metamorphosis in Silurana tropicalis. Tadpoles were exposed to either an estrogen synthesis inhibitor (fadrozole) or TH (triiodothyronine, T3) during early larval or tadpole development. Acute exposures of S. tropicalis to fadrozole or T3 during early development resulted in increased expression of androgen- and TH-related genes in whole body larvae, while chronic exposure to fadrozole during metamorphosis affected gonadal differentiation but did not affect tadpole development. On the other hand, acute exposure to T3 during metamorphosis increased the expression of androgen-related transcripts both in the brain and gonad. In S. tropicalis, the results suggested that cross-talk is primarily in one direction (i.e., effect of THs on the reproductive axis) with a strong relationship between TH and androgen status. Lastly, I established developmental transcript profiles and investigated T3 regulation of brain and gonad transcripts in Engystomops pustulosus. I then compared these results with S. tropicalis and an earlier study in Lithobates pipiens. While each species developed with similar profiles, they differed in their response to T3. Exposure to T3 resulted in either an increase in androgen-related genes (S. tropicalis) or a decrease in estrogen-related genes (E. pustulosus and L. pipiens). In conclusion, these data demonstrated that cross-talk mechanisms differ among these three evolutionary separate species, but in all cases, T3 appears to affect the balance of sex steroids, stimulating the androgen system and providing potential mechanisms of the masculinising effects of THs. These results will contribute to understanding the mechanisms of hormone interactions and their evolutionary basis in frogs.

It is generally thought that in amphibians, thyroid hormones (THs) regulate metamorphosis, while sex steroids (estrogens and androgens) regulate gonadal differentiation. However, inhibition of TH synthesis in frogs alters gonadal differentiation, suggesting instead that these two endocrine axes interact during development. Specifically, THs may be involved in male development, while estrogens may inhibit tadpole metamorphosis. However, we do not currently know the mechanisms that account for these interactions, let alone how such mechanisms may differ between species. To develop and test new hypotheses on the roles of sex steroids and THs, I first examined transcriptional profiles (mRNA) of enzymes and receptors related to sex steroids and THs during embryogenesis and metamorphosis in Silurana tropicalis. Tadpoles were exposed to either an estrogen synthesis inhibitor (fadrozole) or TH (triiodothyronine, T3) during early larval or tadpole development. Acute exposures of S. tropicalis to fadrozole or T3 during early development resulted in increased expression of androgen- and TH-related genes in whole body larvae, while chronic exposure to fadrozole during metamorphosis affected gonadal differentiation but did not affect tadpole development. On the other hand, acute exposure to T3 during metamorphosis increased the expression of androgen-related transcripts both in the brain and gonad. In S. tropicalis, the results suggested that cross-talk is primarily in one direction (i.e., effect of THs on the reproductive axis) with a strong relationship between TH and androgen status. Lastly, I established developmental transcript profiles and investigated T3 regulation of brain and gonad transcripts in Engystomops pustulosus. I then compared these results with S. tropicalis and an earlier study in Lithobates pipiens. While each species developed with similar profiles, they differed in their response to T3. Exposure to T3 resulted in either an increase in androgen-related genes (S. tropicalis) or a decrease in estrogen-related genes (E. pustulosus and L. pipiens). In conclusion, these data demonstrated that cross-talk mechanisms differ among these three evolutionary separate species, but in all cases, T3 appears to affect the balance of sex steroids, stimulating the androgen system and providing potential mechanisms of the masculinising effects of THs. These results will contribute to understanding the mechanisms of hormone interactions and their evolutionary basis in frogs.

It is generally thought that in amphibians, thyroid hormones (THs) regulate metamorphosis, while sex steroids (estrogens and androgens) regulate gonadal differentiation. However, inhibition of TH synthesis in frogs alters gonadal differentiation, suggesting instead that these two endocrine axes interact during development. Specifically, THs may be involved in male development, while estrogens may inhibit tadpole metamorphosis. However, we do not currently know the mechanisms that account for these interactions, let alone how such mechanisms may differ between species. To develop and test new hypotheses on the roles of sex steroids and THs, I first examined transcriptional profiles (mRNA) of enzymes and receptors related to sex steroids and THs during embryogenesis and metamorphosis in Silurana tropicalis. Tadpoles were exposed to either an estrogen synthesis inhibitor (fadrozole) or TH (triiodothyronine, T3) during early larval or tadpole development. Acute exposures of S. tropicalis to fadrozole or T3 during early development resulted in increased expression of androgen- and TH-related genes in whole body larvae, while chronic exposure to fadrozole during metamorphosis affected gonadal differentiation but did not affect tadpole development. On the other hand, acute exposure to T3 during metamorphosis increased the expression of androgen-related transcripts both in the brain and gonad. In S. tropicalis, the results suggested that cross-talk is primarily in one direction (i.e., effect of THs on the reproductive axis) with a strong relationship between TH and androgen status. Lastly, I established developmental transcript profiles and investigated T3 regulation of brain and gonad transcripts in Engystomops pustulosus. I then compared these results with S. tropicalis and an earlier study in Lithobates pipiens. While each species developed with similar profiles, they differed in their response to T3. Exposure to T3 resulted in either an increase in androgen-related genes (S. tropicalis) or a decrease in estrogen-related genes (E. pustulosus and L. pipiens). In conclusion, these data demonstrated that cross-talk mechanisms differ among these three evolutionary separate species, but in all cases, T3 appears to affect the balance of sex steroids, stimulating the androgen system and providing potential mechanisms of the masculinising effects of THs. These results will contribute to understanding the mechanisms of hormone interactions and their evolutionary basis in frogs.

It is generally thought that in amphibians, thyroid hormones (THs) regulate metamorphosis, while sex steroids (estrogens and androgens) regulate gonadal differentiation. However, inhibition of TH synthesis in frogs alters gonadal differentiation, suggesting instead that these two endocrine axes interact during development. Specifically, THs may be involved in male development, while estrogens may inhibit tadpole metamorphosis. However, we do not currently know the mechanisms that account for these interactions, let alone how such mechanisms may differ between species. To develop and test new hypotheses on the roles of sex steroids and THs, I first examined transcriptional profiles (mRNA) of enzymes and receptors related to sex steroids and THs during embryogenesis and metamorphosis in Silurana tropicalis. Tadpoles were exposed to either an estrogen synthesis inhibitor (fadrozole) or TH (triiodothyronine, T3) during early larval or tadpole development. Acute exposures of S. tropicalis to fadrozole or T3 during early development resulted in increased expression of androgen- and TH-related genes in whole body larvae, while chronic exposure to fadrozole during metamorphosis affected gonadal differentiation but did not affect tadpole development. On the other hand, acute exposure to T3 during metamorphosis increased the expression of androgen-related transcripts both in the brain and gonad. In S. tropicalis, the results suggested that cross-talk is primarily in one direction (i.e., effect of THs on the reproductive axis) with a strong relationship between TH and androgen status. Lastly, I established developmental transcript profiles and investigated T3 regulation of brain and gonad transcripts in Engystomops pustulosus. I then compared these results with S. tropicalis and an earlier study in Lithobates pipiens. While each species developed with similar profiles, they differed in their response to T3. Exposure to T3 resulted in either an increase in androgen-related genes (S. tropicalis) or a decrease in estrogen-related genes (E. pustulosus and L. pipiens). In conclusion, these data demonstrated that cross-talk mechanisms differ among these three evolutionary separate species, but in all cases, T3 appears to affect the balance of sex steroids, stimulating the androgen system and providing potential mechanisms of the masculinising effects of THs. These results will contribute to understanding the mechanisms of hormone interactions and their evolutionary basis in frogs.