Transcriptomes of the Premature and Mature Ovaries of an Ascidian, Ciona intestinalis

Oogenesis and folliculogenesis are key steps in reproduction leading to preservation of species. Oocytes are generated from primordial germ cells, mature in the ovaries, and full-grown oocytes are ovulated into oviducts. In vertebrates, follicle maturation is regulated by both the hypothalamus– pituitary–gonad (HPG) axis and the HPG axis-independent process. The former is initiated after puberty and induces oocyte maturation and ovulation (1–4). In brief, gonadotropin-releasing hormone, produced in the hypothalamus, is secreted into the pituitary and induces the release of gonadotropins to the circulatory system (1, 2). Subsequently, gonadotropins trigger oocyte maturation and ovulation via activation of multiple pathways in granulosa and theca cells (3, 4). By contrast, the HPG axis-independent reproductive system, which mainly functions in the premature ovaries, is responsible for the growth of early-stage follicles (e.g., primordial, primary, secondary, and preantral follicles in mammals) that are not regulated by gonadotropins. To date, however, the molecular mechanisms underlying HPG axis-independent oogenesis and folliculogenesis remain largely unknown, as most studies of reproductive biology have focused on mature ovaries. Indeed, little is known about the transcriptomes of premature ovaries of vertebrates, with most data of transcriptomes and conventional expressed sequence tags (ESTs) originating from adult ovaries. Furthermore, invertebrates are not endowed with the HPG axis (no hypothalamus, pituitary, or closed circulation system), suggesting that the HPG axis may have emerged along with the acquisition of the hypothalamus, pituitary, and closed circulation system during the evolution of chordates. In other words, it is presumed that HPG axis-independent reproductive systems are conserved in vertebrates and invertebrate chordates such as ascidians. Ascidians, or sea squirts, are marine invertebrate deuterostomes, belonging to the subphylum Tunicata or Urochordata within the phylum Chordata. Their phylogenetic position as protochordates has provided attractive and useful targets for wide-ranging biological research, including developmental biology, evolutionary biology, endocrinology, neuroendocrinology, and neuroscience. Ciona intestinalis is a cosmopolitan ascidian species and has outstanding advantages as a model organism (5–9). In particular, the whole-genome sequence, various ESTs, and microarray analysis data enable various gene model predictions, homology searches, and comprehensive comparisons with genomes and transcriptomes of other species1 (6–9). At present, approximately 17,000 ESTs of adult Ciona ovaries have been available, and microarray analysis comparing the Ciona ovary and central nervous system has detected several ovary-selective gene expressions (10). Furthermore, a Ciona tachykinin homolog, Ci-TK, was shown to specifically induce growth of vitellogenic oocytes by activating cathepsin D (11–13). These findings indicate that C. intestinalis has prominent potential as a model organism for research on HPG axis-independent folliculogenesis and the evolutionary process of folliculogenesis throughout chordates. By contrast, gene expression profiles for the Ciona premature ovaries have yet to be verified, which hampers investigation of the developmental process of the Ciona ovary. This report describes the gene expression profile of premature ovaries of C. intestinalis, which is expected to contribute a great deal not only to investigations of the matu ration