Double maternal-effect: duplicated nucleoplasmin 2 genes, npm2a and npm2b, with essential but distinct functions are shared by fish and tetrapods

Abstract

BackgroundNucleoplasmin 2 (npm2) is an essential maternal-effect gene that mediates early embryonic events through its function as a histone chaperone that remodels chromatin. Recently, two npm2 (npm2a and npm2b) genes have been annotated in zebrafish. Thus, we examined the evolution of npm2a and npm2b in a variety of vertebrates, their potential phylogenetic relationships, and their biological functions using knockout models via the CRISPR/cas9 system.ResultsWe demonstrated that the two npm2 duplicates exist in a wide range of vertebrates, including sharks, ray-finned fish, amphibians, and sauropsids, while npm2a was lost in coelacanth and mammals, as well as some specific teleost lineages. Using phylogeny and synteny analyses, we traced their origins to the early stages of vertebrate evolution. Our findings suggested that npm2a and npm2b resulted from an ancient local gene duplication, and their functions diverged although key protein domains were conserved. We then investigated their functions by examining their tissue distribution in a wide variety of species and found that they shared ovarian-specific expression, a key feature of maternal-effect genes. We also demonstrated that both npm2a and npm2b are maternally-inherited transcripts in vertebrates, and that they play essential, but distinct, roles in early embryogenesis using zebrafish knockout models. Both npm2a and npm2b function early during oogenesis and may play a role in cortical granule function that impact egg activation and fertilization, while npm2b is also involved in early embryogenesis.ConclusionThese novel findings will broaden our knowledge on the evolutionary history of maternal-effect genes and underlying mechanisms that contribute to vertebrate reproductive success. In addition, our results demonstrate the existence of a newly described maternal-effect gene, npm2a, that contributes to egg competence, an area that still requires further comprehension.Electronic supplementary materialThe online version of this article (10.1186/s12862-018-1281-3) contains supplementary material, which is available to authorized users.

What makes a bad egg? Egg transcriptome reveals dysregulation of translational machinery and novel fertility genes important for fertilization

“…While mutations could be detected in F0 females, these mutations were not transmitted to the next generation thus making it impossible to generate non-mosaic homozygote mutant fish for further analysis. Since the mutagenesis efficiency of the CRISPR/Cas9 system was very high [17, 18], the otulina and slc29a1a genes were however sufficiently knocked-out in the transgenic mosaic F0 females to study the phenotype associated with the partial knock out of target genes, as previously shown for other maternal genes in zebrafish [9, 19]. This was evidenced by the substantially lower transcript levels of otulina and slc29a1a in the eggs of these mutant mosaic females as compared to those from control WT pairings (Fig.…”

“…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. The nucleoplasmin 2 ( npm2a and npm2b ) 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]. Further, post-ovulatory ageing induced egg quality defects are associated with low mRNA levels of igf1 (insulin growth factor 1) and beta-tubulin, as well as a small but significant overabundance of keratins 8 and 18, cathepsin Z, and pgs2 (prostaglandin synthase 2) [10, 11].…”

Influence of Parental Fatty Acid Desaturase 2 (fads2) Expression and Diet on Gilthead Seabream (Sparus aurata) Offspring fads2 Expression during Ontogenesis

“…Despite that maternal mRNA transfer would be proven by the presence of transcripts in unfertilized eggs, no differences were found in mRNA expressions between unfertilized and 1.5 h post-fertilized zebrafish eggs, denoting that just fertilized or just spawn eggs are useful to study maternal transfer mRNAs [ 39 ]. In fish, maternal mRNAs produced by the mother based on her genome are incorporated into the developing oocytes at very early stages of oogenesis [ 40 ]. Despite maternal transcripts may originally be distributed throughout the oocyte cytoplasm, after fertilization most of them are pulled into the blastodisc through cytoplasmic streaming or ooplasmic segregation [ 41 , 42 ].…”

“…Despite maternal transcripts may originally be distributed throughout the oocyte cytoplasm, after fertilization most of them are pulled into the blastodisc through cytoplasmic streaming or ooplasmic segregation [ 41 , 42 ]. There, from mid-blastula on-wards, maternal mRNAs become important components of the protein translation machinery contributing to the proper embryogenesis, and their expression is being regulated by their stability and degradation [ 40 ]. In seabream, fads2 mRNA copies were constant during morula (4 hps), blastula (6 hps) and gastrula stages (10 hps) and decreased at the neurula stage (16 hps) suggesting the degradation of the maternal fads2 mRNA at this stage.…”

Histone Chaperones as Cardinal Players in Development

“…In Xenopus and Zebrafish , the transcript variants, Npm2, Npm2a , and Npm2b are expressed predominantly in early oocytes and they take part in cortical granule (CG) formation during folliculogenesis. NPM2 deficiency impairs the egg activation in stage II follicle ( Cheung et al, 2018 ), but the mechanism by which NPM2 affects the oocyte competence is yet to be deciphered. Similar to the Npm2 knockout, loss in fertility along with impaired gonad development has been observed in Asf1b female knock out mice ( Messiaen et al, 2016 ).…”