The rabbit as an efficient model for functional studies of gonad differentiation

Rabbit is a non-rodent species, which genome sequence is closer to the human than the mouse genome. It is now generally admitted that the rabbit is a more pertinent species than the mouse for the production of physiological model relevant to the human species. In the field of sex determination and gonadal development, the mouse offers a powerful and well- established model thanks to the multiple possibilities to perform functional genetics through genome modifications. However, the foetal development in this species is extremely short, leading to the difficulty to study overlapping events and to compare with other species. The rabbit is especially valuable because it fills an important niche between the laboratory mouse and larger farm mammals. This species has been extensively studied in the past to lay the foundations of sex determination. To date, it is possible to produce genetically modified rabbits by additive transgenesis using the classical DNA microinjection procedure, or through targeted deletion induced by specific nucleases (zinc finger nucleases (ZFN) or transcription activator-like effector nucleases (TALEN)). Thus, our present study was undertaken to investigate in the rabbit the molecular pathways already characterized in the mouse and involved in sex determination and gonad development. A series of developmental stages were selected from the early genital crest stage 13 days post coitum (dpc) until birth (30 dpc) and few months after birth until puberty (3-5 months). Gonad differentiation was analysed by histological methods. The expression of a series of more than 35 candidate-genes was determined by RT-qPCR, and whenever possible, the proteins were localised in the tissues by immunohistochemistry. Among the various expression patterns, some have retained our attention. As in other mammals that have been studied to date, and by opposition to what has been described in the mouse, there is a short peak of aromatase gene expression in the ovary at 20 dpc. The role of this peak is under investigation through invalidation of the aromatase gene by a specific ZFN. In both sexes, rabbit germ cells undergo a series of mitosis during foetal life before entering meiosis that occurs in perinatal life in the ovary (29-30 dpc), and 7-9 weeks after birth in the testis. Interestingly, we observed in males at 29-30 dpc also a small but significant peak of expression of a set of genes characteristic of the onset of meiosis (Stra8, Spo11) without any subsequent meiosis but followed by a peak of expression of the Nanos2 gene considered as a meiosis suppressor. The significance of these gene expression profiles is now under investigation with the aim to revisit the molecular mechanisms that regulate germ cell differentiation and meiosis.