Early onset of embryonic mortality in sub-fertile families of rainbow trout (Oncorhynchus mykiss)

Comparisons among rainbow trout, Oncorhynchus mykiss, populations of maternal transcript profile associated with egg viability

Background

Transcription is arrested in the late stage oocyte and therefore the maternal transcriptome stored in the oocyte provides nearly all the mRNA required for oocyte maturation, fertilization, and early cleavage of the embryo. The transcriptome of the unfertilized egg, therefore, has potential to provide markers for predictors of egg quality and diagnosing problems with embryo production encountered by fish hatcheries. Although levels of specific transcripts have been shown to associate with measures of egg quality, these differentially expressed genes (DEGs) have not been consistent among studies. The present study compares differences in select transcripts among unfertilized rainbow trout eggs of different quality based on eyeing rate, among 2 year classes of the same line (A1, A2) and a population from a different hatchery (B). The study compared 65 transcripts previously reported to be differentially expressed with egg quality in rainbow trout.

Results

There were 32 transcripts identified as DEGs among the three groups by regression analysis. Group A1 had the most DEGs, 26; A2 had 15, 14 of which were shared with A1; and B had 12, 7 of which overlapped with A1 or A2. Six transcripts were found in all three groups, dcaf11, impa2, mrpl39_like, senp7, tfip11 and uchl1.

Conclusions

Our results confirmed maternal transcripts found to be differentially expressed between low- and high-quality eggs in one population of rainbow trout can often be found to overlap with DEGs in other populations. The transcripts differentially expressed with egg quality remain consistent among year classes of the same line. Greater similarity in dysregulated transcripts within year classes of the same line than among lines suggests patterns of transcriptome dysregulation may provide insight into causes of decreased viability within a hatchery population. Although many DEGs were identified, for each of the genes there is considerable variability in transcript abundance among eggs of similar quality and low correlations between transcript abundance and eyeing rate, making it highly improbable to predict the quality of a single batch of eggs based on transcript abundance of just a few genes.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12864-021-07773-1.

Climate change impacts on fish reproduction are mediated at multiple levels of the brain-pituitary-gonad axis

Anthropogenic emissions of carbon dioxide in the atmosphere have generated rapid variations in atmospheric composition which drives major climate changes. Climate change related effects include changes in physico-chemical proprieties of sea and freshwater, such as variations in water temperature, salinity, pH/pCO₂ and oxygen content, which can impact fish critical physiological functions including reproduction. In this context, the main aim of the present review is to discuss how climate change related effects (variation in water temperature and salinity, increases in duration and frequency of hypoxia events, water acidification) would impact reproduction by affecting the neuroendocrine axis (brain-pituitary-gonad axis). Variations in temperature and photoperiod regimes are known to strongly affect sex differentiation and the timing and phenology of spawning period in several fish species. Temperature mainly acts at the level of gonad by interfering with steroidogenesis, (notably on gonadal aromatase activity) and gametogenesis. Temperature is also directly involved in the quality of released gametes and embryos development. Changes in salinity or water acidification are especially associated with reduction of sperm quality and reproductive output. Hypoxia events are able to interact with gonad steroidogenesis by acting on the steroids precursor cholesterol availability or directly on aromatase action, with an impact on the quality of gametes and reproductive success. Climate change related effects on water parameters likely influence also the reproductive behavior of fish. Although the precise mechanisms underlying the regulation of these effects are not always understood, in this review we discuss different hypothesis and propose future research perspectives.

Transcriptome analysis of egg viability in rainbow trout, Oncorhynchus mykiss

Background

Maternal transcripts are accumulated in the oocyte during oogenesis to provide for protein synthesis from oocyte maturation through early embryonic development, when nuclear transcription is silenced. The maternal mRNAs have short poly(A) tails after undergoing post-transcriptional processing necessary for stabilizing them for storage. The transcripts undergo cytoplasmic polyadenylation when they are to be translated. Transcriptome analyses comparing total mRNA and elongated poly(A) mRNA content among eggs of different quality can provide insight into molecular mechanisms affecting egg developmental competence in rainbow trout. The present study used RNA-seq to compare transcriptomes of unfertilized eggs of rainbow trout females yielding different eyeing rates, following rRNA removal and poly(A) retention for construction of the libraries.

Results

The percentage of embryos to reach the 32-cell stage at 24 h post fertilization was significantly correlated to family eyeing rate, indicating that inviable embryos were developmentally compromised before zygotic genome activation. RNA sequencing identified 2 differentially expressed transcripts (DETs) from total mRNA sequencing comparing females with low-quality (< 5% eyeing), medium-quality (30–50% eyeing), and high-quality (> 80% eyeing) eggs. In contrast, RNA sequencing from poly(A) captured transcripts identified 945 DETs between low- and high-quality eggs, 1012 between low- and medium-quality eggs, and only 2 between medium- and high-quality eggs. The transcripts of mitochondrial genes were enriched with polyadenylated transcript sequencing and they were significantly reduced in low-quality eggs. Similarly, mitochondrial DNA was reduced in low-quality eggs compared with medium- and high-quality eggs. The functional gene analysis classified the 945 DETs between low- and high-quality eggs into 31 functional modules, many of which were related to ribosomal and mitochondrial functions. Other modules involved transcription, translation, cell division, apoptosis, and immune responses.

Conclusions

Our results indicate that differences in egg quality may be derived from differences in maternal nuclear transcript activation and cytoplasmic polyadenylation before ovulation, as opposed to accumulation and storage of maternal nuclear transcripts during oogenesis. Transcriptome comparisons suggest low-quality eggs suffered from impaired oxidative phosphorylation and translation. The DETs identified in this study provide insight into developmental competence in rainbow trout eggs.

Electronic supplementary material

The online version of this article (10.1186/s12864-019-5690-5) contains supplementary material, which is available to authorized users.

Stressor timing, not cortisol, is an important embryo viability determinant in female rainbow trout Oncorhynchus mykiss

The objective of this study was to determine whether the timing of elevated cortisol levels in adult female rainbow trout Oncorhynchus mykiss, achieved by silastic implants within the body cavity, had an effect on embryo viability. Cortisol-implanted fish experienced a significant increase in circulating levels of plasma cortisol compared with sham-implanted fish, and plasma cortisol remained elevated relative to sham-implanted fish for 4 months in each of the three treatment groups (0-4, 4-8 and 8-12 months). There were no significant differences in embryo viability, egg diameters or plasma 17β-oestradiol levels between the cortisol and sham-implanted treatments in any of the groups. There was a significant difference in the number of subfertile females (<80% embryo viability) when the three treatments were compared. The majority of the females (75%) implanted immediately postspawn (0-4 months) produced subfertile eggs, which was significantly higher than those treated 4-8 (33%) or 8-12 (17%) months postspawn. These results imply that silastic implantation can affect oocyte development, independent of plasma cortisol levels, leading to a reduction in embryo viability.

Egg and sperm quality in fish

Fish egg quality can be defined as the ability of the egg to be fertilized and subsequently develop into a normal embryo. Similarly, sperm quality can be defined as its ability to successfully fertilize an egg and subsequently allow the development of a normal embryo. In the wild or under aquaculture conditions, the quality of fish gametes can be highly variable and is under the influence of a significant number of external factors or broodstock management practices. For these reasons, the topic of gamete quality has received increasing attention. Despite the significant efforts made towards a better understanding of the factors involved in the control of gamete quality, the picture is far from being complete and the control of gamete quality remains an issue in the aquaculture industry. Some of the factors responsible for the observed variability of gamete quality remain largely unknown or poorly understood. In addition very little is known about the cellular and molecular mechanisms involved in the control of egg and sperm quality. In the present review, the molecular and cellular characteristics of fish gametes are presented with a special interest for the mechanisms that could participate in the regulation of gamete quality. Then, after defining egg and sperm quality, and how can it can be accurately estimated or predicted, we provide an overview of the main factors that can impact gamete quality in teleosts.

Reduced embryonic survival in rainbow trout resulting from paternal exposure to the environmental estrogen 17alpha-ethynylestradiol during late sexual maturation

Exposure of fishes to environmental estrogens is known to affect sexual development and spawning, but little information exists regarding effects on gametes. This study evaluated embryonic survival of offspring from male rainbow trout (Oncorhynchus mykiss) exposed to 17alpha-ethynylestradiol (EE(2)) using an in vitro fertilization protocol. Males were exposed at either 1800 or 6700 degree days ( degrees d) (i.e. 161 or 587 days post-fertilization (dpf)) to test for effects on testes linked to reproductive ontogeny. At 1800 degrees d, fish were beginning testicular differentiation and were exposed to 109 ng EE(2)/l for 21 days. At 6700 degrees d, fish have testes containing spermatocytes and spermatids and were exposed for 56 days to either 0.8, 8.3, or 65 ng EE(2)/l. Semen was collected at full sexual maturity in each group and used to fertilize eggs pooled from several non-exposed females. Significant decreases in embryonic survival were observed only with the 6700 degrees d exposure. In 0.8 and 8.3 ng EE(2)/l treatments, embryo survival was significantly reduced at 19 dpf when compared with the control. In contrast, an immediate decrease in embryonic survival at 0.5 dpf was observed in the 65 ng EE(2)/l treatment. Blood samples collected at spawning from 6700 degrees d exposed males revealed a significant decrease in 11-ketotestosterone and a significant increase in luteinizing hormone levels for the 65 ng EE(2)/l treatment when compared with the other treatment groups. Results indicate that sexually maturing male rainbow trout are susceptible to EE(2) exposure with these fish exhibiting two possible mechanisms of reduced embryonic survival through sperm varying dependant on EE(2) exposure concentrations experienced.