Identification of ovarian gene expression patterns during vitellogenesis in Atlantic cod (Gadus morhua)

Abstract

Follicular maturational competence and ovulatory competence in teleost fish refer to the ability of the ovarian follicle to undergo final oocyte maturation and ovulation, respectively, in response to gonadotropin stimulation and other external cues. Some gene products related to competence acquisition are likely synthesized during vitellogenic growth, as these follicles gain in vivo responsiveness to exogenous gonadotropin stimulation and can be induced to undergo maturation and ovulation. In Atlantic cod (Gadus morhua), gonadotropin responsiveness has been shown to be oocyte size-dependent, and only ovaries containing late-stage vitellogenic follicles can be induced to ovulate. The purpose of the present study was to compare gene expression patterns between mid (unresponsive) and late (responsive) vitellogenic ovaries to identify genes involved in gonadotropin responsiveness and the acquisition of maturational and ovulatory competencies. Representational difference analysis was conducted in two reciprocal comparisons using intact ovarian fragments and follicle wall-enriched tissues, and genes of interest were used in real time quantitative PCR to confirm differential expression. Few differences were detected in intact ovarian fragments, but type IV ice-structuring protein and gephyrin were upregulated later in development and may be involved in lipid and sulfur metabolism, respectively. Candidate gene assays for luteinizing hormone receptor and aromatase also exhibited significant upregulation during vitellogenesis. Many genes were differentially expressed in follicle wall-enriched tissues, including endocrine maturational regulators and smooth muscle genes. Overall, maturational and ovulatory competencies during vitellogenesis in Atlantic cod are associated with up- and downregulation of many genes involved in lipid metabolism, endocrine regulation, and ovulatory preparation.

Global transcriptome analysis identifies regulated transcripts and pathways activated during oogenesis and early embryogenesis in atlantic cod

“…Similarly, we observed that ucp2 and star expression increases from early‐ to late‐vitellogenic oocytes in cod. The associated cyp11a1 and cyp19a1 , which are essential for estradiol‐17β synthesis (reviewed by Uno et al, ), show similar expression profiles to star , partly in agreement with the findings of Breton et al (). Increased expression of star , cyp19a1 , and cyp11a1 increases the steroid synthesis capacity, including the production of estradiol‐17β, which is increased in parallel with follicle‐stimulating hormone (FSH) in coho salmon plasma during cortical alveoli synthesis.…”

“…Apart from the biological results, our results may differ from previous studies of the cod transcriptome in follicles and early embryos due to different methods applied, sample types, and number of genes assayed per sample. For example, we measured gene expression in isolated follicle cells with a certain size range, whereas other studies used heterogenous ovary tissue (Goetz et al, ; Breton et al, ; Breton and Berlinsky ), which in addition to follicle cells of different size ranges contains connective cells. Hence, using whole‐ovary samples from cod may introduce heterogeneity in data since a mix of stage‐specific processes during oogenesis are being monitored.…”

Type-IV Antifreeze Proteins are Essential for Epiboly and Convergence in Gastrulation of Zebrafish Embryos

“…Along with AFP4s have been identified in temperate, subtropical and tropical fishes that have no need to prevent freezing 20, 24, 25, their biological roles have been speculated to bind to lipid or ligand other than ice because of the helix bundle structure similarity to certain apolipoproteins (Apos) 24, 26, 28, but the exact physiological functions have been unknown up to the present. Our current study has revealed the biological roles of AFP4a and AFP4b in embryonic development, and has confirmed that AFP4a and AFP4b contribute to epiboly progression and convergence movement during early zebrafish embryogenesis.…”

“…The complete sequences of other functionally characterized AFP4s in Myoxocephalus octodecemspinosus ( Mo ) 23, 26, Pleuragramma antarcticum ( Pa ), Notothenia coriiceps ( Nc ) 25 and Gadus morhua ( Gm ) 27, 28 were downloaded from GenBank non-redundant protein database. An unrooted maximum likelihood (ML) phylogenetic tree of AFP4s was constructed by MEGA 6 34 with 1000 bootstrap replications, in which their evolutionary distances were computed using the JTT+F model, which was selected by ProtTest 2.4 35 based on the corrected Akaike information criterion.…”

“…As a new type of AFP, their molecular structures have been characterized by the conserved four-helix bundle 24-26, and the antifreeze activity has been demonstrated in several fishes 23-26. Significantly, along with wide discoveries of afp4 homologues, their abundant expression has been detected recently in oocytes 27, 28 and in embryos 20 from some teleost fishes. So far, AFPs have been studied for more than 30 years, but almost all the previous studies have focused on the structural and biochemical properties in freezing avoidance 29-31, and little is known about the biological functions in embryonic development.…”