Abstract
BackgroundIn chickens, as in most birds, female gonad morphogenesis is asymmetrical. Gonads appear first rather similarly, but only the left one undergoes full differentiation and gives rise to a functional ovary. The right gonad, in which the cortex does not develop, remains restricted to the medulla and finally regresses. Opportunity was taken of this left-right asymmetry to perform a suppression subtractive hybridization screening to select for transcripts preferentially expressed in the developing left ovary as compared to the right one, and thus identify genes that are potentially involved in the process of ovarian differentiation.ResultsOne of these transcripts, named Ovex1 according to its expression profile, corresponds to an endogenous retrovirus that has not been previously characterized. It is transcribed as full-length and singly spliced mRNAs and contains three uninterrupted open reading frames coding potentially for proteins with homology to Gag and Pro-Pol retroviral polyproteins and a third protein showing only a weak similarity with Env glycoproteins. Ovex1 is severely degenerated; it is devoid of typical long terminal repeats and displays some evidence of recombination. An orthologous Ovex1 locus was identified in the genome of zebra finch, a member of a different bird order, and similar sequences were detected in turkey, guinea fowl, and duck DNA. The relationship between these sequences follows the bird phylogeny, suggesting vertical transmission of the endogenous retrovirus for more than 100 million years.Ovex1 is transcribed in chicken gonads with a sex-dependent and left-right asymmetrical pattern. It is first expressed in the cortex of the left indifferent gonads of both sexes. Expression is transient in the left testis and absent in the right one. In developing ovaries, Ovex1 transcription increases sharply in the left cortex and is weakly detected in the medulla. After folliculogenesis, Ovex1-expressing cells constitute the follicular granulosa cell layer. Ovex1 expression highlights a striking desquamation process that leads to profound cortical remodeling associated with follicle morphogenesis.ConclusionEvidence for a selection pressure at the protein level suggests that this endogenous retrovirus, expressed in the ovarian supporting cell lineage, might play an active role in bird ovarian physiology.
Highlights
In chickens, as in most birds, female gonad morphogenesis is asymmetrical
Evidence for a selection pressure at the protein level suggests that this endogenous retrovirus, expressed in the ovarian supporting cell lineage, might play an active role in bird ovarian physiology
Identification of differentially expressed genes in chicken embryonic ovaries using Suppression Subtractive Hybridization (SSH) In order to identify unknown factors involved in early steps of the ovarian cortex differentiation, we performed a suppression subtractive hybridization screening (SSH) [37] to selected transcripts expressed in the chicken differentiating left ovary and underrepresented in the right gonad in which the cortex does not differentiate
Summary
As in most birds, female gonad morphogenesis is asymmetrical. Gonads appear first rather but only the left one undergoes full differentiation and gives rise to a functional ovary. The right gonad, in which the cortex does not develop, remains restricted to the medulla and regresses. As in most birds, gonad differentiation is characterized by left-right (L/R) asymmetry. Only the left gonad becomes a functional ovary. The right one fails to fully differentiate and disappears. By contrast, both male gonads, initially asymmetrical, become functional testes. Bird sex determination is not fully understood. The initial genetic sex trigger is not clearly identified (reviewed in [1,2])
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