Testing different forms of regulation of yolk thyroid hormone transfer in pied flycatchers

Tom Sarraude*, Bin-Yan Hsu, Ton G. G. Groothuis, Suvi Ruuskanen

*Corresponding author for this work

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Abstract

Hormones transferred from mothers to their offspring are considered a maternal tool to prepare progeny for expected environmental conditions, increasing maternal fitness. To flexibly influence offspring, mothers should be able to transmit the hormonal signals independent of their own hormonal status. However, the ability to regulate hormone transfer to the next generation is under debate. We studied the transfer of thyroid hormones (THs) to eggs in a bird model. We elevated thyroxine (T4, the prohormone for the biologically active triiodothyronine, T3) during egg-laying using T4 implants on females of a wild population of pied flycatchers (Ficedula hypoleuca), and measured plasma and yolk T4 and T3 as a response. We found an increase in plasma and yolk T4 and no changes in plasma or yolk T3 concentrations leading to a decrease in yolk T3/T4 ratio in response to the T4 treatment. The yolk T3/T4ratio was similar to the plasma ratio in females during the yolking phase. This suggests that mothers are not able to regulate TH transfer to yolk but may regulate the T4-to-T3 conversion to avoid potential costs of elevated exposure to the active hormone to herself and to her progeny. The absence of regulation in hormone transfer to eggs is in contrast to our predictions. Future studies on deiodinases activity that converts T4 to T3 in maternal and embryonic tissues may help understanding how mothers regulate circulating THs during breeding, as well as the embryos' role in converting maternal T4 to its biological active T3 form during development.

Original languageEnglish
Article numberjeb226688
Number of pages6
JournalThe Journal of Experimental Biology
Volume223
Issue number21
Early online date24-Sep-2020
DOIs
Publication statusPublished - 10-Nov-2020

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