Bone morphogenetic protein 15 and growth differentiation factor 9 expression in 1 the ovary of European sea bass ( Dicentrarchus labrax ) : Cellular localization , 2 developmental profiles , and

21 Vertebrate oocytes actively contribute to follicle development by secreting a variety of 22 growth factors, among which bone morphogenetic protein 15 (BMP15/Bmp15) and 23 growth differentiation factor 9 (GDF9/Gdf9) have been paid particular attention. In the 24 present study, we describe the cellular localization, the developmental profiles, and the 25 response to unilateral ovariectomy (a procedure implying the surgical removal of one of 26 the ovaries) of protein and mRNA steady-state levels of Bmp15 and Gdf9 in the ovary 27 of European sea bass, an important fish species for marine aquaculture industry. In situ 28 hybridization and immunohistochemistry demonstrated that the oocyte is the main 29 production site of Bmp15 and Gdf9 in European sea bass ovary. During oocyte 30 development, Bmp15 protein expression started to be detected only from the lipid 31 vesicle stage onwards but not in primary pre-vitellogenic (i.e. perinucleolar) oocytes as 32 the bmp15 mRNA already did. Gdf9 protein and gdf9 mRNA expression were both 33 detected in primary perinucleolar oocytes and followed similar decreasing patterns 34 thereafter. Unilateral ovariectomy induced a full compensatory growth of the remaining 35 ovary in the 2-month period following surgery (Á. Prat 2011). The compensatory growth elicited different changes in the 37 expression levels of mRNA and protein of both factors, although the involvement of 38 Bmp15 and Gdf9 in the regulatory network orchestrating such process remains unclear 39 at present. Altogether, our results establish a solid base for further studies focused on 40 elucidating the specific functions of Bmp15 and Gdf9 during primary and secondary 41 oocyte growth in European sea bass. 42 43