Higher growth rate and gene expression in male zebra finch embryos are independent of manipulation of maternal steroids in the eggs.

Sexual dimorphism in prenatal development is widespread among vertebrates, including birds. Its mechanism remains unclear, although it has been attributed to the effect of maternal steroid hormones. The aim of this study was to investigate how increased levels of steroid hormones in the eggs influence early embryonic development of male and female offspring. We also asked whether maternal hormones take part in the control of sex-specific expression of the genes involved in prenatal development. We experimentally manipulated hormones' concentrations in the egg yolk by injecting zebra finch females prior to ovulation with testosterone or corticosterone. We assessed growth rate and expression levels of CDK7, FBP1 and GHR genes in 37h-old embryos. We found faster growth and higher expression of two studied genes in male compared to female embryos. Hormonal treatment, despite clearly differentiating egg steroid levels, had no effect on the sex-specific pattern of the embryonic gene expression, even though we confirmed expression of receptors of androgens and glucocorticoids at such an early stage of development. Thus, our study shows high stability of the early sex differences in the embryonic development before the onset of sexual differentiation and indicates their independence of maternal hormones in the egg.

Sexual dimorphism in the early embryogenesis of the chicken (Gallus Gallus domesticus).

Studies of dioecious animals suggest that sex-specific development occurs from the onset of embryogenesis. This must be accounted for when addressing issues involving sex-ratio regulation in domestic animals and conservation biology. We investigated the occurrence of growth-rate sexual dimorphism in 84 chicken embryos incubated for 30 hr and nucleic-acid abundance in 99 embryos incubated for 4 hr. Comparative expression of the genes engaged in cell-cycle regulation (16 genes), embryo growth (10 genes), metabolic activity (2 genes), and epigenetic regulation (4 genes) in 4-hr male and female embryos were further analyzed by reverse-trancriptase quantitative PCR. At the stage when somite structure commences, males are growing faster than females. DNA and RNA yields at 4 hr are elevated in males compared to females, and most cell-proliferation-promoting genes are overexpressed in males. Expression of key metabolic genes (G6PD and HPRT) and the principal genes responsible for DNA methylation (DNMTs), however, does not differ between the sexes. These data suggest that the faster growth of early male embryos is conserved among mammalian and bird phyla, and may have an evolutionary importance.

Sexual Dimorphism in the Early Embryogenesis in Zebra Finches.

Sex-specific gene expression before the onset of gonadogensis has been documented in embryos of mammals and chickens. In several mammalian species, differences in gene expression are accompanied by faster growth of pre-implantation male embryos. Here we asked whether avian embryos before gonadal differentiation are also sex-dimorphic in size and what genes regulate their growth. We used captive zebra finches (Taeniopygia guttata) whose freshly laid eggs were artificially incubated for 36-40 hours. Analyses controlling for the exact time of incubation of 81 embryos revealed that males were larger than females in terms of Hamburger and Hamilton stage and number of somites. Expression of 15 genes involved in cell cycle regulation, growth, metabolic activity, steroidogenic pathway and stress modulation were measured using RT-PCR in 5 male and 5 female embryos incubated for exactly 36 h. We found that in the presence of equal levels of the growth hormone itself, the faster growth of male embryos is most likely achieved by the overexpression of the growth hormone receptor gene and three other genes responsible for cell cycle regulation and metabolism, all of them located on the Z chromosome. Autosomal genes did not show sex-specific expression, except for the steroidogenic factor 1 which was expressed only in female embryos. To our knowledge this is the first report of sexual size dimorphism before gonadogenesis in birds. The finding suggests that faster growth of early male embryos is conserved through the mammalian and bird phyla, irrespective of their differential sex chromosome systems.