The Relationship Between Seasonal Breeding and Deiodinase Expression in the Green Anole Lizard

Reproductive physiology and behavior is mainly regulated by the hypothalamus-pituitary-gonad (HPG) axis. Interestingly, abnormal levels of thyroid hormone (TH) results in the delayed onset of puberty and affects gonadal function of adults by altering HPG axis activity. Seasonally breeding animals undergo drastic hormonal and behavioral changes between breeding and non-breeding seasons. Green anole lizards (Anolis carolinensis), similar to other seasonally breeding animals, have increased sex steroid hormones, larger gonads, upregulated gonadal steroidogenic acute regulatory protein (StAR) mRNA and increased reproductive behavior during the breeding compared to non-breeding seasons. Relatively less is known regarding the regulation of gonadal TH in seasonal reproduction in reptiles. We examined whether the gonadal expression of enzymes involved in TH activation are altered in concert with seasonal regulation. Type 2 deiodinase (Dio2) mRNA, the TH activating enzyme, was upregulated in breeding male anole gonads compared to non-breeding males, while type 3 deiodinase (Dio3) mRNA, the TH deactivating enzyme, was upregulated in breeding female anole gonads. To study the association between the HPG axis and local activation of TH in regulating reproductive physiology, we manipulated the HPG axis during the non-breeding season by subcutaneously injecting luteinizing hormone (LH) and follicular stimulating hormone (FSH) in male and female green anoles. We examined mRNA expression of Dio2, Dio3 and StAR in gonads and measured plasma sex steroid hormone levels. LH and FSH injected males had significantly increased testes weight, StAR mRNA expression and testosterone levels, which indicates that gonadotropin injections were able to activate the HPG axis even during the non-breeding season. Surprisingly, Dio3 was upregulated in the testes after LH and FSH injections, while Dio2 mRNA levels were unchanged compared to the vehicle injected group. This result suggests that there might be different roles of local TH activation in developing and maintaining fully grown gonads. Additionally, as determined through a mirror test, gonadotropin injections did not induce aggressive behavior in males despite their increased testosterone levels. Our findings support the role for thyroid hormone in regulating reproduction and contribute to a growing body of work examining the evolution of puberty and reproductive development