Abstract
ABSTRACTMaternal hormones constitute a key signalling pathway for mothers to shape offspring phenotype and fitness. Thyroid hormones (THs; triiodothyronine, T3; and thyroxine, T4) are metabolic hormones known to play crucial roles in embryonic development and survival in all vertebrates. During early developmental stages, embryos exclusively rely on exposure to maternal THs, and maternal hypothyroidism can cause severe embryonic maldevelopment. The TH molecule includes iodine, an element that cannot be synthesised by the organism. Therefore, TH production may become costly when environmental iodine availability is low. This may yield a trade-off for breeding females between allocating the hormones to self or to their eggs, potentially to the extent that it even influences the number of laid eggs. In this study, we investigated whether low dietary iodine may limit TH production and transfer to the eggs in a captive population of rock pigeons (Columba livia). We provided breeding females with an iodine-restricted (I−) diet or iodine-supplemented (I+) diet and measured the resulting circulating and yolk iodine and TH concentrations and the number of eggs laid. Our iodine-restricted diet successfully decreased both circulating and yolk iodine concentrations compared with the supplemented diet, but not circulating or yolk THs. This indicates that mothers may not be able to independently regulate hormone exposure for self and their embryos. However, egg production was clearly reduced in the I− group, with fewer females laying eggs. This result shows that restricted availability of iodine does induce a cost in terms of egg production. Whether females reduced egg production to preserve THs for themselves or to prevent embryos from exposure to low iodine and/or THs is as yet unclear.
Highlights
We provided breeding pairs of rock pigeons (Columba livia) with a diet restricted in iodine, to create a potential trade-off between circulating and yolk Thyroid hormones (THs), and compared this group with a reference group provided with ample iodine, removing such a trade-off in the female
In this study we tested whether dietary iodine limits the mother’s circulating TH concentration, TH transfer to the yolk and egg production
Our study is the first to investigate the potential trade-off between circulating and yolk THs induced by low dietary iodine
Summary
Parental effects are included in this non-genetic inheritance and may be considered adaptive (Moore et al, 2019; Mousseau and Fox, 1998), the strength and ubiquity of adaptive parental effects is still under debate (e.g. Sánchez-Tójar et al, 2020; Yin et al, 2019; Zhang et al, 2020). Parental effects of maternal origin, i.e. maternal effects, have received increasing attention since the 1990s (Bernardo, 1996; Mousseau and Fox, 1998). Hormones of maternal origin can be transferred to the offspring and constitute a potential pathway for mothers to influence their offspring’s phenotype (Groothuis et al, 2019). Thyroid hormones (THs) may be considered costly, as their molecular structure includes iodine, a trace element that cannot be synthesised by organisms and must be found in the environment
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