New insights in oocyte dynamics shed light on the complexities associated with fish reproductive strategies

Information on temporal variations in stock reproductive potential (SRP) is essential in fisheries management. Despite this relevance, fundamental understanding of egg production variability remains largely unclear due to difficulties in tracking the underlying complex fluctuations in early oocyte recruitment that determines fecundity. We applied advanced oocyte packing density theory to get in-depth, quantitative insights across oocyte stages and seasons, selecting the commercially valuable European hake (Merluccius merluccius) as a case study. Our work evidenced sophisticated seasonal oocyte recruitment dynamics and patterns, mostly driven by a low-cost predefinition of fecundity as a function of fish body size, likely influenced also by environmental cues. Fecundity seems to be defined at a much earlier stage of oocyte development than previously thought, implying a quasi-determinate – rather than indeterminate – fecundity type in hake. These results imply a major change in the conceptual approach to reproductive strategies in teleosts. These findings not only question the current binary classification of fecundity as either determinate or indeterminate, but also suggest that current practices regarding potential fecundity estimation in fishes should be complemented with studies on primary oocyte dynamics. Accordingly, the methodology and approach adopted in this study may be profitably applied for unravelling some of the complexities associated with oocyte recruitment and thereby SRP variability.

Specific miRNA-GPCR networks regulate Sox9a/Sox9b activities to promote gonadal renewal in zebrafish

Fertility and endocrine function rely on a tightly regulated synchronicity within the hypothalamic-pituitary gonadal (HPG) axis. FSH/cAMP/MAPK/ Sox9 axis signaling and its regulated specific miRNAs are thought to regulate vertebrate gonadal development and sex differentiation, and yet the regulatory networks are largely unknown. Here we construct small RNA and mRNA libraries from sexually matured ovary and testis of zebrafish to identify specific miRNA-target pairs. Integration of Targetscan prediction and in vivo induced gene expression highlight four specific miRNAs that conditionally target three G protein–coupled receptor (GPCR) x-Sox9 signaling genes, and implicate two regulatory circuits of miR430a-Sox9a in the testis and miR218a-Sox9b in the ovary. Co-injected Sox9a-miR430a mixture increases the proportion of spermatogonia but degenerates primary oocyte, while Sox9b-miR218a mixture induces renewal of ovarian follicles. Co-immunoprecipitation and mass-spectrometry analyses further reveal that miR430a and Sox9a synergistically activate testicular PKC/Rock1 signals while miR218a and Sox9b constrict ovary PKC/PI3K/Rock1 signaling. These results clarify specific miRNAs-GPCR regulatory networks of Sox9a/Sox9b switch, and also provide mechanistic insight into gonadal rejuvenation and plasticity.

Meiotic maturation failure induced by DICER1 deficiency is derived from primary oocyte ooplasm

RNA interference (RNAi) has diverse functions across cellular processes, including a role in the development of the mammalian oocyte. Mouse primary oocytes deficient in the key RNAi enzyme DICER1 exhibit pronounced defects in chromosome congression and spindle formation during meiotic maturation. The cause of this meiotic maturation failure is unknown. In this study, observations of chromosomes and spindle microtubules during prometaphase in DICER1-deficient oocytes indicate that chromosome congression and spindle formation are overtly normal. Spindle breakdown and chromosome displacement occur after the metaphase plate has formed, during the metaphase to anaphase transition. We hypothesised that this defect could be attributed to either RNAi-mediated regulation of nuclear factors, such as the regulation of centromere chromatin assembly, or the regulation of mRNA expression within the cytoplasm. By transplanting germinal vesicles between DICER1-deficient and wild-type primary oocytes, we show that, unexpectedly, the meiotic failure is not caused by a deficiency derived from the germinal vesicle component. Instead, we reveal that the ooplasm of primary oocytes contains DICER1-dependent factors that are crucial for chromosome segregation and meiotic maturation.

Ultrastructure of ovarian follicular epithelium of the Amazonian fish Pseudotylosurus microps (Teleostei: Belonidae): morphological and histochemical characterization of the intercellular deposits

The present paper reports the presence of great quantities of electrondense intercellular material in the follicular epithelium of P. microps. The material apparently is uptaken from circulation and enter the follicle through the intercellular spaces accumulating in the epithelial median-apical intercellular spaces and in perioocytic space. The accumulation starts in the early growth of the primary oocyte and proceed until vitellogenesis. The possible chemical nature and function of the deposits are discussed.

Structure of the female reproductive tract of an adult parous Tasmanian tiger, Thylacinus cynocephalus

The present observations on the now-extinct Thylacinus are based on the reproductive system of an adult thylacine discovered among the specimens of the Hill collection at the Hubrecht Laboratory in the Netherlands. As in other marsupials, the reproductive tract was characterised by the presence of a uterus duplex and a vaginal complex where the ureters passed dorsally over each lateral vaginal canal to enter the bladder. The lateral vaginal canals each entered a urogenital sinus that terminated in a shallow cloaca. The gross dimensions of the reproductive tract of the thylacine were greater than those of any extant dasyurid marsupial. The distance from the rostral pole of the ovaries to the most caudal extremity of the urogenital sinus measured 25 cm. The distinctive aspects of the reproductive tract included a disproportionate enlargement of the corpus uteri that is without parallel in any other marsupial species. The bodies of the right and left uteri measured 10.4 cm 1.2 cm 0.9 cm and 9.1 cm 0.8 cm 0.7 cm respectively. The rostro-caudal length of the right and left cervices measured 2.7 cm and 1.7 cm respectively. The cervical canals entered the vaginal complex by way of a thick median vaginal septum. The elongated caudal component of the vaginal culs-de-sac lacked a median vaginal septum. As in other dasyurid marsupials, the lateral vaginae and associated vaginal complex were of diminutive proportions in relation to the typical marsupial pattern. The histology of the tract was remarkably good for tissue preserved since 1902 and indicated that the tissues were free of pathological changes. A characteristic marsupial pattern of ovarian folliculogenesis was evident where all but a thin peripheral zone of the cytoplasm of the primary oocyte became vacuolated during the pre-antral stage of ovarian follicle development.