Abnormal meiosis in fertile and sterile triploid cyprinid fish

Triploid brown trout, Salmo trutta, develop functional gonads with age and are able to interbreed with diploid counterparts

The study investigated if gonad maturation in triploid brown trout, Salmo trutta, was entirely suppressed or only delayed, and if triploids could interbreed with diploid counterparts. Ten percent of the total number of 3-year-old triploid S. trutta, 15% of 4-year-old fish, and 17% of 5-year-old fish produced semen. Three and 4 years old triploid fish did not produce eggs, but 15% of the 5-year-old fish did so. The quantity and sperm motility of triploid semen did not differ from diploids, but the sperm concentration was significantly lower. When diploid eggs were fertilized with triploid semen (3n × 2n crosses), the percentage of eyed stage embryos, of hatched larvae, and of normal-shaped larvae did not differ from the diploid controls. Circa 90% of 3n × 2n crosses had a ploidy level of 2.4n. In the remaining percentage of 3n × 2n crosses, the ploidy level was ≥2n and <2.4n. In sperm competition experiments where diploid eggs were fertilized with a mixture of diploid and triploid semen, 52% of the originating larvae had a ploidy level of 2n, 43% of 2.4n, and 5% of the fish were not exactly classified. From the start of feeding to an age of 248 days, the mortality rate of 3n × 2n interploid crosses and of 2n × 2n controls was similar. The growth of interploid crosses was significantly higher than that of controls. In triploid mature females, the egg mass per kilogram of body weight was significantly lower than in diploids. The mass of the non-hardened eggs and the percentile weight increase during hardening did not differ from diploid eggs. When triploid eggs were fertilized with diploid semen (2n × 3n crosses), the development rate to normal hatched larvae was less than 10%. All originating larvae had a ploidy level of 3n. From the start of feeding to an age of 248 days, 2n × 3n crosses had a higher mortality rate (15%) than diploid controls (<5%). Growth of this type of interploid crosses was reduced in comparison to controls. Therefore, triploids introduced into natural waters for recreational fisheries or escaping from farms may interbreed with diploid counterparts. This not only alters the genotypes of local populations but also changes the ploidy levels.

Gene expression analyses of GH/IGF axis in triploid crucian carp with growth heterosis

Hybridization and polyploid breeding are the main approaches used to obtain new aquaculture varieties. Allotriploid crucian carp (3n) with rapid growth performance was generated by mating red crucian carp (RCC) with allotetraploids (4n). Fish growth is controlled by the growth hormone (GH)/insulin-like growth factor (IGF) axis. In the present study, we examined the expression characteristics of GH/IGF axis genes in hybrids F1, 4n, 3n, RCC and common carp (CC). The results showed that GHRa, GHRb, IGF1, IGF2, and IGF-1Ra were highly expressed in 3n compared with RCC and CC, whereas IGF3 was undetectable in the liver in RCC, CC and 3n. GHRa and GHRb had low expression in the 4n group. In hybrid F1, GHRa expression was low, whereas GHRb was highly expressed compared to the levels in RCC and CC. Moreover, in hybrid F1, the expression of IGF3 was higher, and the expression of IGF1 and IGF2 was lower than that in the RCC and CC, whereas the expression of IGF-1Ra was similar to that in RCC and CC. For the IGFBP genes, IGFBP1 had higher expression in 3n compared than that in RCC and CC, while other IGFBP genes were not high expressed in 3n. Among the genes detected in this study, 11 genes were nonadditively expressed in 3n, with 5 genes in the transgressive upregulation model. We proposed that the 11 nonadditive expression of GH/IGF axis genes is related to growth heterosis in 3n. This evidence provides new insights into hybridization and polyploid breeding from the perspective of hormone regulation.

Discovery of a novel miRNA involved in the regulation of male infertility in zebrafish

Azoospermia and asthenospermia are common manifestations of male infertility, but it needs further studies to understand the intrinsic regulation mechanism. As a popular model organism, zebrafish is often used to assess reproductive complications. In this study, by analyzing miRNA transcriptome of the mature triploid zebrafish testis afflicted with spermatogenic dysfunctions, leading to the identification of 36 miRNAs that are differentially expressed in comparison with diploid, which are predicted to target 2737 genes. Subsequent functional annotation of these genes pinpointed two miRNAs might association with spermatogenesis. Inhibitory experiments showed that NC_007115.7.7_998413 inhibited conducts a substantial decline in sperm density, and conducted lower embryo fertilization rate than control. And putative target genes qRT-PCR evaluation showed that spata2 was significant down-regulate upon inhibited NC_007115.7.7_998413. In summary, this research positions newly identified miRNA NC_007115.7.998413 as a regulatory factor in male zebrafish reproductive development, enhancing our comprehension of the molecular regulated pathways involved in spermatogenesis.

Evolutionary mechanisms and practical significance of reproductive success and clonal diversity in unisexual vertebrate polyploids

Unisexual reproduction is generally relevant to polyploidy, and unisexual vertebrates are often considered an evolutionary "dead end" due to the accumulation of deleterious mutations and absence of genetic diversity. However, some unisexual polyploids have developed strategies to avoid genomic decay, and thus provide ideal models to unveil unexplored evolutionary mechanisms, from the reproductive success to clonal diversity creation. This article reviews the evolutionary mechanisms for overcoming meiotic barrier and generating genetic diversity in unisexual vertebrates, and summarizes recent research advancements in the polyploid Carassius complex. Gynogenetic gibel carp (Carassius gibelio) is a unique amphitriploid that has undergone a recurrent autotriploidy and has overcome the bottleneck of triploid sterility via gynogenesis. Recently, an efficient strategy in which ploidy changes, including from amphitriploid to amphitetraploid, then from amphitetraploid to novel amphitriploid, drive unisexual-sexual-unisexual reproduction transition and clonal diversity has been revealed. Based on this new discovery, multigenomic reconstruction biotechnology has been used to breed a novel strain with superior growth and stronger disease resistance. Moreover, a unique reproduction mode that combines both abilities of ameiotic oogenesis and sperm-egg fusion, termed as ameio-fusiongensis, has been discovered, and it provides an efficient approach to synthesize sterile allopolyploids. In order to avoid ecological risks upon escape and protect the sustainable property rights of the aquaculture seed industry, a controllable fertility biotechnology approach for precise breeding is being developed by integrating sterile allopolyploid synthesis and gene-editing techniques. This review provides novel insights into the origin and evolution of unisexual vertebrates and into the attempts being made to exploit new breeding biotechnologies in aquaculture.

Production of sterile mono-sex triploid yellow drum (Nibea albiflora): genotypic females and sex-reversed phenotypic males with emphasis on utilization as surrogate broodstock

Production of sterile mono-sex fish is of great significance for sustainable aquaculture as well as germ cell transplantation. In this study, we aimed to produce mono-sex triploid yellow drum, including genotypic females (XXX female) and sex-reversed phenotypic males (XXX male). Firstly, the mono-female triploids were produced through cold-shock treatment on eggs fertilized with sperm from neo-males. Then, the mono-male triploids were produced by the sex reversal of mono-female triploids with oral administration of letrozole (LZ). We comparatively investigated the growth and gonadal development in the mono-sex triploids. The results showed that the triploids displayed similar growth performance to their diploids throughout their first year, but had impaired gonadosomatic index and gametogenesis. No mature gametes were produced in the triploids during their first spawning season. Meanwhile, we analyzed the process of gametogenesis in the both sex of triploids. Ultrastructure of gametogenesis showed that the germ cells arrested at abnormal metaphase 1 in females, while males had irregular meiotic divisions, variable-sized spermatid and degenerated cells. The expression levels of meiosis-related genes (i.e., sycp3 and rec8) confirmed the abnormal meiosis in the triploids. Furthermore, the gonadal development was also determined by the expression patterns of vasa, dmrt1 and cyp19a1a. Abnormal expression of vasa mRNA and protein were detected in triploids. High cyp19a1a expression levels suggested the sex steroid hormones production might be at least partially functional in triploid females. In addition, high dmrt1 expression levels confirmed the masculinization and testicular development of sex-reversed triploid males by LZ. Our findings provide an efficient protocol to produce sterile mono-sex triploid yellow drum and provide new insights into the mechanism of gonadal sterility of triploid fish.

Comparative transcriptomic analysis of reproductive characteristics of reciprocal hybrid lineages derived from hybridization between Megalobrama amblycephala and Culter alburnus

BACKGROUND

Distant hybridization is an important breeding technique for creating new strains with superior traits by integrating two different genomes. Successful hybridization of Megalobrama amblycephala (Blunt snout bream, BSB, 2n = 48) and Culter alburnus (Topmouth culter, TC, 2n = 48) was achieved to establish hybrid lineages (BT and TB), which provide valuable materials for exploring the mechanisms of distant hybridization fertility. In this study, the gonadal tissue transcriptomes of BSB, TC, BT-F1, and TB-F1 were sequenced using Illumina high-throughput sequencing technology to analyze the reproductive characteristics of BT and TB.

RESULTS

Differential gene expression analysis showed that the differentially expressed genes in BT vs BSB and BT vs TC were mainly enriched in signaling pathways not directly associated with meiosis. While, the differentially expressed genes of TB vs BSB and TB vs TC were mainly enriched in pathways related to meiosis, and most of them were down-regulated, indicating that meiosis is suppressed in TB. Under-dominance (UD) genes were enriched in pathways related to meiosis and DNA repair in TB. Over-dominance (OD) genes were enriched in MAPK signaling pathway, expression level dominance-BSB (ELD-B) genes were enriched in pathways related to steroid hormone synthesis and expression level dominance-TC (ELD-T) genes were not significantly enriched in any pathway in both BT and TB.

CONCLUSIONS

These results suggest that meiotic progression may not be affected in BT, whereas it is clearly inhibited in TB. Offspring of M. amblycephala maternal parent may have better genomic compatibility and fertility. Our study provides important information on the molecular mechanisms of breaking reproductive isolation in distantly hybridized fertile lineages.

Phosphoproteomics Reveal New Candidates in Abnormal Spermatogenesis of Pseudomales in Cynoglossus semilaevis

Phosphorylation is a post-translational modification that contributes to versatile protein functions in spermatogenesis, and the variations they generate usually results in abnormal spermatogenesis or sperm dysfunction. The sex-reversal phenomenon exists in Chinese tongue sole under certain conditions such that individuals with a ZW genotype can acquire a male phenotype and are thus called pseudomales. Pseudomale tongue sole can reach sexual maturity but produce only Z-type sperm, and the Z sperm carries paternal epigenetic information. Whether phosphorylation plays a role in the sperm abnormality of pseudomales is unknown. In this study, a phosphoproteomic analysis was performed to compare protein phosphorylation profiles between pseudomale and male testes. Altogether, we identified 14,253 phosphopeptides matching with 4843 proteins, with 1329 differentially phosphorylated peptides corresponding to 1045 differentially phosphorylated proteins (DPPs). Phosphorylation at 781 sites was upregulated and at 548 sites was downregulated. Four motifs were identified among differentially phosphorylated peptides, which were "SP", "SD", "RxxS", and "TP". Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) analyses suggested that the cell cycle and DNA/RNA processing were significantly enriched with the genes encoding DPPs. To analyze DPP function in depth, a protein-protein interaction network was constructed, and Ran-binding protein 2 was found to play a central role in spermatogenesis by regulating several processes such as the cell cycle, eukaryotic translation, ubiquitination, and minichromosome maintenance. In kinase-associated network analyses, two "mitogen-activated protein kinase (Mapk)-centered" clusters were identified that may account for abnormal spermatogenesis in pseudomales. One cluster was centered on Mapk6, which predominantly regulated the cell cycle by interacting with several cyclin-dependent kinases, and the other was centered on the "testis-expressed kinase 1-like (Tesk1l)/Pim1l-Mapk4l- testis-expressed 14 (Tex14)" kinase cascade, which might contribute to spermatogenesis by regulating β-catenin. Taken together, these data suggested the new candidates involved in pseudomale sperm abnormalities and provided clues to discover the phosphorylated regulatory mechanism underlying tongue sole spermatogenesis.

An efficient approach to synthesize sterile allopolyploids through the combined reproduction mode of ameiotic oogenesis and sperm-egg fusion in the polyploid Carassius complex

The association between polyploidy and reproduction transition, which is an intriguing issue in evolutionary genetics, can also be exploited as an approach for genetic improvement in agriculture. Recently, we generated novel amphitriploids (NA3n) by integrating the genomes of the gynogenetic Carassius gibelio and sexual C. auratus, and found gynogenesis was recovered in most NA3n females (NA3n♀I). Here, we discovered a unique reproduction mode, termed ameio-fusiongenesis, which combines the abilities of both ameiotic oogenesis and sperm-egg fusion, in a few NA3n females (NA3n♀II). These females inherited ameiotic oogenesis to produce unreduced eggs from gynogenetic C. gibelio and sperm-egg fusion from sexual C. auratus. Subsequently, we utilized this unique reproduction mode to generate a group of synthetic alloheptaploids by crossing NA3n♀II with Megalobrama amblycephala. They contained all chromosomes of maternal NA3n♀II and a chromosomal set of paternal M. amblycephala. Intergenomic chromosome translocations between NA3n♀II and M. amblycephala were also observed in a few somatic cells. Primary oocytes of the alloheptaploid underwent severe apoptosis owing to incomplete double-strand break repair at prophase I. Although spermatocytes displayed similar chromosome behavior at prophase I, they underwent apoptosis due to chromosome separation failure at metaphase I. Therefore, the alloheptaploid females and males were all sterile. Finally, we established a sustainable clone for the large-scale production of NA3n♀II and developed an efficient approach to synthesize diverse allopolyploids containing genomes of different cyprinid species. These findings not only broaden our understanding of reproduction transition but also offer a practical strategy for polyploidy breeding and heterosis fixing.

Single-cell-resolution transcriptome map revealed novel genes involved in testicular germ cell progression and somatic cells specification in Chinese tongue sole with sex reversal

Female-to-male sex reversals (pseudomales) are common in lower vertebrates and have been found in natural populations, which is a concern under rapid changes in environmental conditions. Pseudomales can exhibit altered spermatogenesis. However, the regulatory mechanisms underlying pseudomale spermatogenesis remain unclear. Here, we characterized spermatogenesis in Chinese tongue sole (Cynoglossus semilaevis), a species with genetic and environmental sex determination, based on a high-resolution single-cell RNA-seq atlas of cells derived from the testes of genotypic males and pseudomales. We identified five germ cell types and six somatic cell types and obtained a single-cell atlas of dynamic changes in gene expression during spermatogenesis in Chinese tongue sole, including alterations in pseudomales. We detected decreased levels of Ca²⁺ signaling pathway-related genes in spermatogonia, insufficient meiotic initiation in spermatocytes, and a malfunction of somatic niche cells in pseudomales. However, a cluster of CaSR genes and MAPK signaling factors were upregulated in undifferentiated spermatogonia of pseudomales. Additionally, we revealed that Z chromosome-specific genes, such as piwil2, dhx37, and ehmt1, were important for spermatogenesis. These results improve our understanding of reproduction after female-to-male sex-reversal and provide new insights into the adaptability of reproductive strategies in lower vertebrates.

Transcription Analysis for Core Networks of lncRNAs–mRNAs: Implication for Potential Role in Sterility of Crassostrea gigas

Simple Summary

This study reveals the expression profiles of lncRNA in the gonads of the Pacific oyster Crassostrea gigas. The potential function of lncRNAs was predicted in the case of antisense and cis-regulatory mechanisms based on their physical positions and their coexpression relationships in the case of trans regulation. Sterility-related DEGs and DELs were chosen for subsequent analysis, demonstrating that trans-regulatory lncRNAs might play a vital role in the gametogenesis of C. gigas. We constructed core networks of lncRNAs–mRNAs for triploid sterile females and hermaphrodites based on pathway results, in which 28 lncRNAs and their 54 trans-regulatory genes were detected. Among 28 sterility-specific lncRNAs, MSTRG.79882.3 and MSTRG.79882.4 for triploid sterile females and MSTRG.33704.1, MSTRG.63844.1, and MSTRG.5675.1 for hermaphrodites play the most significant role.

Abstract

Long noncoding RNA (lncRNA), a type of non-protein-coding transcript, is emerging as a crucial regulator of gene expression. However, few roles of lncRNA in the reproductive process of the Pacific oyster (Crassostrea gigas) have been defined, especially in the regulatory mechanism of sterile triploids gametogenesis. To uncover the potential role of lncRNA, the gonads of diploids, sterile triploids, and partially sterile triploids underwent RNA sequencing. A total of 9618 reliable lncRNAs were identified. The target relationship between lncRNA and mRNA was predicted based on cis, trans, and antisense regulation with bioinformatic software. We chose differentially expressed lncRNAs and mRNAs when sterile triploids were compared to partially sterile triploids and diploids for subsequent functional enrichment analysis. Findings revealed that trans-regulatory lncRNAs might play a significant role in the gametogenesis of C. gigas. Combining pathway results, we constructed core networks of lncRNAs–mRNAs for triploid sterile females and hermaphrodites. Fifty-four genes related to cell division, germline-cell maintenance, and glycogen metabolism were found to be associated with sterility. A total of 28 candidate lncRNAs were predicted to trans-regulate these genes. We speculated that MSTRG.79882.3 and MSTRG.79882.4 for triploid sterile females and MSTRG.33704.1, MSTRG.63844.1, and MSTRG.5675.1 for hermaphrodites were highly important as they were predicted to regulate more sterility-specific genes than others. Our work collectively identified sterility-related lncRNAs and implicated the potential mechanism of lncRNA-mediated regulation in the gametogenesis of sterile triploid oysters.

Structural Abnormalities of Spermatozoa in Triploid Gynogenetic Crucian Carp (Carassius auratus)

The spermatozoa of triploid gynogenetic crucian carp (Carassius auratus) (3nDTCC) possess a spermatogenesis process with a normal genetic background. However, the genetic materials of these spermatozoa do not completely inherit gynogenetic progeny in general. Understanding the intrinsic mechanism may be helpful for developing breeding strategies of gynogenetic fishes. In this study, the spermatozoa ultrastructure was systematically studied in diploid red crucian carp and 3nDTCC to demonstrate their cytological structural differences. In addition, the artificial breeding tests of 3nDTCC(♀) with different ploidy spermatozoa were performed to verify the contributions of genetic materials from 3nDTCC spermatozoa to the gynogenesis progeny. Furthermore, the mRNA expression of centriole-related genes (i.e., cep57, cetn1, rootletin, and nek2) involved in spermatozoa packaging was also determined by quantitative real-time PCR (qPCR) to illustrate the molecular expression characteristics of the spermatozoa packaging process in 3nDTCC. The results reveal the adaptive features of spermatozoa in 3nDTCC, including the loose midpiece structure, abnormal head structure, and abnormal expression of centriole-related genes, which may influence the motility of spermatozoa and make it not involved normally in the genetic composition of the gynogenesis offspring.