Cor Dijkstra

Thyroid hormones (THs) - triiodothyronine (T3) and thyroxine (T4) - are essential for embryonic development in vertebrates. All vertebrate embryos are exposed to THs from maternal origin. As maternal TH levels are known to be essential to embryonic development, the natural variation of maternal THs probably represents a pathway of maternal effects that can modify offspring phenotype. However, potential fitness consequences of variation of maternal TH exposure within the normal physiological range and without confounding effects of the mother have never been experimentally investigated. We experimentally manipulated the levels of yolk T3 and T4 within the physiological range in a species in which the embryo develops outside the mother's body, the Rock Pigeon (Columba livia) eggs. Making use of the natural difference of yolk testosterone between the two eggs of pigeon clutches, we were also able to investigate the potential interaction between THs and testosterone. Elevated yolk TH levels enhanced embryonic development and hatching success, and reduced body mass but not tarsus length between day 14 and fledging. The yolk hormones increased plasma T4 concentrations in females but reduced it in males, in line with the effect on metabolic rate at hatching. Plasma concentrations of T3 and testosterone were not significantly affected. The effects of treatment did not differ between eggs with high or low testosterone levels. Our data indicate that natural variation in maternal yolk TH levels affects offspring phenotype and embryonic survival, potentially influencing maternal and chick fitness.

Keywords: aggression birds courtship behaviour long-term effects maternal effects maternal hormones prenatal testosterone exposure reproduction In many animal species, mothers expose their embryos to maternal hormones. This provides a powerful pathway for affecting offspring phenotype and a potential mechanism for adaptive nongenomic inheritance. This has been studied extensively in birds because the embryo develops outside the mother's body, facilitating experimental studies. Some studies have demonstrated that maternal yolk testosterone (T) can have long-lasting effects into the period after fledging. However, these results are based on offspring in the juvenile stage or their first breeding season only. The lack of data further into adulthood limits a better assessment of the fitness consequences of prenatal T. In a series of previous experiments in the rock pigeon, Columba livia, we manipulated yolk T levels using oil-injected eggs as controls. In the present study, we examined the reproductive behaviour of birds from these eggs when they were 1 e3 years old and housed together in a large aviary. Our results showed that males from T-injected eggs were less aggressive and females from T-injected eggs laid lighter eggs than controls. Moreover, the experimental birds showed strongly disassortative mating with respect to hormone treatment. There were no indications of treatment effects on the process of sexual differentiation. Our results indicate that the effects of prenatal T do not diminish over age and may last far into adulthood, which should be taken into account in assessing the fitness consequences of maternal hormones. They may also stimulate new physiological and neurobiological studies on the mechanisms underlying such long-lasting effects of prenatal T exposure.

In oviparous species like birds, eggs provide the direct environment in which embryos are developing. Mothers may adjust different egg components in different ways in reaction to environmental cues either to adjust offspring development or because of constraints. In this study, we investigated the effects of food quality and quantity before and during egg laying on three different aspects of egg quality: macro-nutrients (egg and yolk mass), androgens (testos-terone and androstenedione), and thyroid hormones (3,5,3 0-triiodothyronine, T3 and L-thyroxine, T4), using the rock pigeon (Columba livia). As expected, egg and yolk mass were significantly reduced for the eggs laid under the poor-food condition, indicating a maternal trade-off between offspring and self in allocating important resources. We did not find any significant change in yolk testosterone or their within-clutch pattern over the laying sequence. This is consistent with the fact that, in contrast with nutrients, these hormones are not costly to produce, but does not support the hypothesis that they play a role in adjusting brood size to food conditions. In contrast, we found that T3 levels were higher in the egg yolks under the poor-food condition whereas the total T4 content was lower. This change could be related to the fact that iodine, the critical constituent of thyroid hormones, might be a limiting factor in the production of this hormone. Given the knowledge that food restriction usually lead to reduction of circulating T3 levels, our results suggested that avian mothers can independently regulate its concentrations in their eggs from their own circulation. The study demonstrates that environmentally induced maternal effects via the egg can be a result of a combination of constrained resources and unconstrained signals and that thyroid hormones might be an interesting case of both. Therefore, this hormone and the interplay of different maternal effects on the offspring phenotype deserve much more attention.

... a major part of the practical work, Ellis Mulder for extracting and sexing a large number of eggs and Sjoerd Veenstra, Roelie Veenstra-Wiegmann ... committee of the University of Groningen (license DEC 3001).This study was supported NWO grant No 810.67.024 to Cor Dijkstra. ...

... Serge Daan and Tim Fawcett for advice during the experiment and comments on the manuscript, Guido Meeuwissen for assistance with sexing birds, and Sjoerd Veenstra and Roelie Veenstra-Wiegman ... This study was supported by NWO grant No 810.67.024 to Cor Dijkstra. 46

... We tested the hypothesis that elevated yolk-testosterone levels in the zebra finch affect offspring devel-opment in a sex-specific manner. ... Eggs were injected with testosterone or sesame oil (controls) after two days of incubation. ...