Abstract
BackgroundEgg quality can be defined as the egg ability to be fertilized and subsequently develop into a normal embryo. Previous research has shed light on factors that can influence egg quality. Large gaps however remain including a comprehensive view of what makes a bad egg. Initial development of the embryo relies on maternally-inherited molecules, such as transcripts, deposited in the egg during its formation. Bad egg quality is therefore susceptible to be associated with alteration or dysregulation of maternally-inherited transcripts. We performed transcriptome analysis on a large number (N = 136) of zebrafish egg clutches, each clutch being split to monitor developmental success and perform transcriptome analysis in parallel. We aimed at drawing a molecular portrait of the egg in order to characterize the relation between egg transcriptome and developmental success and to subsequently identify new candidate genes involved in fertility.ResultsWe identified 66 transcript that were differentially abundant in eggs of contrasted phenotype (low or high developmental success). Statistical modeling using partial least squares regression and genetics algorithm demonstrated that gene signatures from transcriptomic data can be used to predict developmental success. The identity and function of differentially expressed genes indicate a major dysregulation of genes of the translational machinery in poor quality eggs. Two genes, otulina and slc29a1a, predominantly expressed in the ovary and dysregulated in poor quality eggs were further investigated using CRISPR/Cas9 mediated genome editing. Mutants of each gene revealed remarkable subfertility whereby the majority of their eggs were unfertilizable. The Wnt pathway appeared to be dysregulated in the otulina mutant-derived eggs.ConclusionsHere we show that egg transcriptome contains molecular signatures, which can be used to predict developmental success. Our results also indicate that poor egg quality in zebrafish is associated with a dysregulation of (i) the translational machinery genes and (ii) novel fertility genes, otulina and slc29a1a, playing an important role for fertilization. Together, our observations highlight the diversity of the possible causes of egg quality defects and reveal mechanisms of maternal origin behind the lack of fertilization and early embryonic failures that can occur under normal reproduction conditions.
Highlights
Egg quality can be defined as the egg ability to be fertilized and subsequently develop into a normal embryo
The maternallyprovided transcriptome of the egg is critical in supporting early embryogenesis because transcription from the zygotic genome does not start until the mid-blastula transition (MBT) which occurs approximately 3–4 h postfertilization in zebrafish [4, 5] even though transcription starts earlier for “first wave” zygotic genes [6]
The nucleoplasmin 2 genes were recently found to be crucial for egg quality; suppression of npm2b resulted in embryonic arrest before zygotic genome activation (ZGA) in mouse and zebrafish, and npm2a deficiency in zebrafish led to a complete lack of embryonic development [8, 9]
Summary
Egg quality can be defined as the egg ability to be fertilized and subsequently develop into a normal embryo. The maternallyprovided transcriptome of the egg is critical in supporting early embryogenesis because transcription from the zygotic genome does not start until the mid-blastula transition (MBT) which occurs approximately 3–4 h postfertilization (hpf) in zebrafish [4, 5] even though transcription starts earlier for “first wave” zygotic genes [6] Previous research using both traditional mutational assays [7] as well as more recent transcriptomic analyses have revealed several maternal factors that can influence egg quality. We further conducted functional analyses on two candidate genes that were dysregulated in bad quality eggs using the CRISPR/Cas knockout system and reveal for the first time the essential roles of two new potential fertility-related genes, otulina (OTU deubiquitinase with linear linkage specificity a) and slc29a1a (solute carrier family 29, member 1a) that appear to be important for fertilization. Our observations indicate that poor zebrafish egg quality is associated with a dysregulation of (i) the translational machinery genes and (ii) novel fertility genes, otulina and slc29a1a, playing an important role for fertilization
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