Evaluation of viability, developmental competence, and apoptosis-related transcripts duringpost-ovulatory oocyte aging in zebrafish(Hamilton, 1822).

Abstract

INTRODUCTION: Post-ovulatory aging is a time-dependent deterioration of ovulated oocytes and a major limiting factor reducing the fitness of offspring. This process may lead to the activation of cell death pathways like apoptosis in oocytes. METHODOLOGY: We evaluated oocyte membrane integrity, egg developmental competency, and mRNA abundance of apoptosis-related genes by RT-qPCR. Oocytes from zebrafishwere retainedat 28.5°C for 24 h post-ovulation (HPO). Viability was assessed using trypan blue (TB) staining. The consequences of in vivo oocyte aging on the developmental competence of progeny were determined by the embryo survival at 24 h post fertilization, hatching, and larval malformation rates. RESULTS: The fertilization, oocyte viability, and hatching rates were 91, 97, and 65% at 0 HPO and dropped to 62, 90, and 22% at 4 HPO, respectively. The fertilizing ability was reduced to 2% at 8 HPO, while 72% of oocytes had still intact plasma membranes. Among the apoptotic genes(b-cell lymphoma 2),(bcl2-associated agonist of cell death a),(tumor protein p53),(cyclin-dependent kinase 1) studied, mRNA abundance of anti-apoptoticdecreased and pro-apoptoticincreased at 24 HPO. Furthermore,andmRNA transcripts decreased at 24 HPO compared to 0 HPO. DISCUSSION: Thus, TB staining did not detect the loss of oocyte competency if caused by aging. TB staining, however, could be used as a simple and rapid method to evaluate the quality of zebrafish oocytes before fertilization. Taken together, our results indicate the activation of cell death pathways in the advanced stages of oocyte aging in zebrafish.