Abstract
BackgroundIn birds as in mammals, a genetic switch determines whether the undifferentiated gonad develops into an ovary or a testis. However, understanding of the molecular pathway(s) involved in gonad differentiation is still incomplete.Methodology/Principal FindingsWith the aim of improving characterization of the molecular pathway(s) involved in gonad differentiation in the chicken embryo, we developed a large scale real time reverse transcription polymerase chain reaction approach on 110 selected genes for evaluation of their expression profiles during chicken gonad differentiation between days 5.5 and 19 of incubation. Hierarchical clustering analysis of the resulting datasets discriminated gene clusters expressed preferentially in the ovary or the testis, and/or at early or later periods of embryonic gonad development. Fitting a linear model and testing the comparisons of interest allowed the identification of new potential actors of gonad differentiation, such as Z-linked ADAMTS12, LOC427192 (corresponding to NIM1 protein) and CFC1, that are upregulated in the developing testis, and BMP3 and Z-linked ADAMTSL1, that are preferentially expressed in the developing ovary. Interestingly, the expression patterns of several members of the transforming growth factor β family were sexually dimorphic, with inhibin subunits upregulated in the testis, and bone morphogenetic protein subfamily members including BMP2, BMP3, BMP4 and BMP7, upregulated in the ovary. This study also highlighted several genes displaying asymmetric expression profiles such as GREM1 and BMP3 that are potentially involved in different aspects of gonad left-right asymmetry.Conclusion/SignificanceThis study supports the overall conservation of vertebrate sex differentiation pathways but also reveals some particular feature of gene expression patterns during gonad development in the chicken. In particular, our study revealed new candidate genes which may be potential actors of chicken gonad differentiation and provides evidence of the preferential expression of BMPs in the developing ovary and Inhibin/Activin subunits in the developing testis.
Highlights
Sex determination in birds is controlled by a genetic ZZ/ZW system in which the female is heterogametic
Among other model species in which the female is heterogametic, only the chicken is characterized by a high degree of differentiation of sex chromosomes Z and W, which may be considered as functional equivalents of the mammalian highly differentiated XY chromosome system where the specific small sex chromosome Y is shared by the male
Our results provide valuable information on the genes involved in chicken gonad differentiation
Summary
Sex determination in birds is controlled by a genetic ZZ/ZW system in which the female is heterogametic. The gonads of ZZ and ZW embryos remain indistinguishable until day 6.5–7 (stage 30 of the Hamburger and Hamilton classification [4]), when the first histological signs of sex differentiation become visible This process includes the thinning of the cortex and development of testicular cords enclosing germ cells in ZZ gonads. Inhibition of CYP19A1 causes female to male sex reversal [10], while estrogen treatment of ZZ embryos leads to feminization of left gonads and regression of right gonads [11,12] These experiments revealed a crucial role of estrogen production during avian ovary development and the fundamental role of regulation of CYP19A1 expression. The target and downstream pathways of these genes are poorly understood Another particular feature of chicken gonadal differentiation is its left-right (L-R) asymmetry in the female. Understanding of the molecular pathway(s) involved in gonad differentiation is still incomplete
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