Molecular characterization and expression of an oocyte-specific histone stem-loop binding protein in Carassius gibelio

Molecular characterization and expression patterns of stem-loop binding protein (SLBP) genes in protogynous hermaphroditic grouper, Epinephelus coioides

Stem-loop binding protein (SLBP) binds a stem-loop structure of the mRNA, which is important for the stability of histone mRNAs and translation process. In the present study, two slbp cDNAs (Ecslbp1 and Ecslbp2) were cloned from a protogynous hermaphroditic orange-spotted grouper, Epinephelus coioides. Ecslbp1 cDNA contained a 678 base pair (bp) open reading frame (ORF), encoding a predicted polypeptide of 225 amino acids. Ecslbp2 cDNA contained a 1041 bp, encoding a predicted protein of 346 amino acids. The result of real-time PCR revealed that Ecslbp2 mRNA was exclusively detected in the ovary. Moreover, it was found to be restricted to oocytes according to in situ hybridization (ISH) analysis. Ecslbp2 was found to be hardly detected in gonia and significantly increase in the cytoplasm of primary-growth stage oocytes, but decreased during the process of vitellogenesis. Interestingly, Ecslbp2 expression centralized as a perinuclear speckle in early-primary-growth stage oocytes, which appeared to form into the Balbiani body (Bb) in late-primary-growth stage oocytes. These data indicated that Ecslbp2 might play an important role in the process of oocyte development, and could serve as an oocyte-specific molecular marker for the study of ovary development and sex reversal in groupers.

Oocyte-specific maternal Slbp2 is required for replication-dependent histone storage and early nuclear cleavage in zebrafish oogenesis and embryogenesis

Stem-loop binding protein (SLBP) is required for replication-dependent histone mRNA metabolism in mammals. Zebrafish possesses two slbps, and slbp1 is necessary for retinal neurogenesis. However, the detailed expression and function of slbp2 in zebrafish are still unknown. In this study, we first identified zebrafish slbp2 as an oocyte-specific maternal factor and then generated a maternal-zygotic slbp2 F3 homozygous mutant (MZslbp2Δ4-/-) using CRISPR/Cas9. The depletion of maternal Slbp2 disrupted early nuclear cleavage, which resulted in developmental arrest at the MBT stage. The developmental defects could be rescued in slbp2 transgenic MZslbp2Δ4-/- embryos. However, homozygous mutant MZslbp1Δ1-/- developed normally, indicating slbp1 is dispensable for zebrafish early embryogenesis. Through comparative proteome and transcriptome profiling between WT and MZslbp2Δ4-/- embryos, we identified many differentially expressed proteins and genes. In comparison with those in WT embryos, four replication-dependent histones, including H2a, H2b, H3, and H4, all reduced their expression, while histone variant h2afx significantly increased in MZslbp2Δ4-/- embryos at the 256-cell stage and high stage. Zebrafish Slbp2 can bind histone mRNA stem-loop in vitro, and the defects of MZslbp2Δ4-/- embryos can be partially rescued by overexpression of H2b. The current data indicate that maternal Slbp2 plays a pivotal role in the storage of replication-dependent histone mRNAs and proteins during zebrafish oogenesis.

Molecular characterization and expression pattern of a germ cell marker gene dnd in gibel carp (Carassius gibelio)

As a germ cell marker gene, Dead end (dnd) has been identified and characterized in many vertebrates. Recently, we created a complete germ cell-depleted gonad model by the dnd-specific morpholino-mediated knockdown approach, and revealed sex-biased gene expression alteration through utilizing unisexual gynogenetic superiority in polyploid gibel carp. However, dnd and its expression pattern are still unclear in the gibel carp. In this study, we further analyzed molecular characterization of gibel carp dnd and its dynamic expression pattern during gametogenesis and embryogenesis. Similar to other homologs in vertebrates, gibel carp dnd contains a conserved RRM motif and five other motifs, and is highly evolutionary conserved in genomic organization and neighborhood gene synteny. RT-PCR and Western blot analyses showed its gonad-specific expression intensively in testis and ovary. Section in situ hybridization (SISH) and immunofluorescence localization revealed its dynamic expression pattern specific to oogenic cells and spermatogenetic cells during oogenesis and spermatogenesis. Moreover, its temporal and spatial distribution specific to PGCs were also demonstrated by RT-PCR and whole mount in situ hybridization (WISH) during embryogenesis. Therefore, gibel carp Dnd is a conserved germ cell marker during gametogenesis, and its maternal transcript is also a useful marker for tracing PGC specification and migration.

Complete depletion of primordial germ cells in an All-female fish leads to Sex-biased gene expression alteration and sterile All-male occurrence

Background

Gynogenesis is one of unisexual reproduction modes in vertebrates, and produces all-female individuals with identical genetic background. In sexual reproduction vertebrates, the roles of primordial germ cells on sexual dimorphism and gonadal differentiation have been largely studied, and two distinct functional models have been proposed. However, the role of primordial germ cells remains unknown in unisexual animals, and it is also unclear whether the functional models in sexual reproduction animals are common in unisexual animals.

Results

To solve these puzzles, we attempt to utilize the gynogenetic superiority of polyploid Carassius gibelio to create a complete germ cell-depleted gonad model by a similar morpholino-mediated knockdown approach used in other examined sexual reproduction fishes. Through the germ cell-depleted gonad model, we have performed comprehensive and comparative transcriptome analysis, and revealed a complete alteration of sex-biased gene expression. Moreover, the expression alteration leads to up-regulation of testis-biased genes and down-regulation of ovary-biased genes, and results in the occurrence of sterile all-males with testis-like gonads and secondary sex characteristics in the germ cell-depleted gynogenetic Carassius gibelio.

Conclusions

Our current results have demonstrated that unisexual gynogenetic embryos remain keeping male sex determination information in the genome, and the complete depletion of primordial germ cells in the all-female fish leads to sex-biased gene expression alteration and sterile all-male occurrence.

Electronic supplementary material

The online version of this article (doi:10.1186/s12864-015-2130-z) contains supplementary material, which is available to authorized users.