Abstract
The yolk is a key structure of oviparous animal eggs that provides nutrients for growing embryos. Thus, a vital stage in oogenesis is the uptake of vitellogenin, a large protein that is precursor to the egg yolk. Until a certain stage in reproductive development, passage of vitellogenin to the oocyte is restricted, likely due to high densities of intercellular junctions between granulosa cells. This study investigated junction-related gene expression prior to, and during, vitellogenin uptake in New Zealand short-finned eels (Anguilla australis).
Expression of genes associated with adherens, tight, and gap junctions was compared between wild-caught pre- and early vitellogenic eels. It was expected that expression of target genes would be downregulated in early vitellogenic ovary as granulosa cell connectedness decreased to allow entry of vitellogenin. Contrary to hypothesised outcomes, most genes were either not differentially regulated or upregulated in early vitellogenic eels. This was likely due to sampled eels being in the very early stages of vitellogenesis, where differences in expression between pre- and early vitellogenic oocytes may be difficult to detect. This experiment provided strong incentive to assess junction-related gene expression in fish that had progressed into later stages of vitellogenesis.
Early vitellogenic eels were implanted with a range of doses of estradiol (E2) with or without 11-ketotestosterone (11-KT) for 12 weeks. To compliment this, in vitro ovarian explants were exposed to a similar treatment regime. As expected, oocytes from A. australis treated with 11-KT, in vivo, were in mid-vitellogenesis, which enabled comparison between increasing vitellogenic stages. Analysis of gene expression in ovarian biopsies, from in vivo treated fish, found that 11-KT treatment downregulated expression of several junction-related genes, while treatment with E2-only increased expression of most target genes. No differential expression was observed in ovarian samples treated with hormones in vitro, suggesting that 11-KT does not act directly on the ovary to elicit changes in expression of intercellular junction-related genes. Negative correlations between junction-related gene expression and oocyte size supported oocyte growth as a potential driver of intercellular junction-related gene downregulation.
Overall, this study provided novel insights into possible mechanisms which may facilitate the passage of vitellogenin to the oocyte. This lays the foundation for future work which should aim to visualise the location of junctions in the piscine follicle throughout vitellogenesis, to confirm the tentative conclusions posed in this thesis. The lack of differential regulation in gap junction genes, and downregulation of adherens and tight junction genes in mid-vitellogenesis underscore the importance of junction type in facilitation of vitellogenin oocyte access.