Identification of the molecular sex-differentiation period in the siberian sturgeon

Investigation of Sexes and Fertility Potential of Female Russian Sturgeon (Acipenser gueldenstaedtii) and Male American Paddlefish (Polyodon spathula) Hybrids

In the present study, 10 allotriploid (3nALT) and 10 allopentaploid (5nALP) six-month-old hybrid fish and two 3nALT and four 5nALP 40-month-old hybrid fish, which resulted by crossing female Russian sturgeon Acipenser gueldenstaedtii (Brandt and Ratzeberg, 1833) and male American paddlefish Polyodon spathula (Walbaum, 1792), were investigated. It was revealed that six-month-old 3nALT and 5nALP hybrids initially had "undifferentiated" gonads, while in the 40-month-old hybrids, only testes were observed in one case of 3nALT and one case of 5nALP hybrids. The testis of 3nALT hybrids was partially developed with spermatogonia, while the testis of one 5nALP hybrid was in the second developmental stage with low spermatogonia density. We could not determine gonad differentiation in any of the cases when the hybrid individuals had the W sex chromosome. We concluded that the gonad differentiation of these interfamilial hybrids follows a similar pattern to interspecific hybrids of different ploidy parent species of the family Acipenseridae, which is consistent with the classical Haldane's rule. However, it cannot be excluded that the testis of this/these hybrid(s) may produce fertile sperm after sexual maturity, depending on additional genetic, hormonal and environmental factors, and further research is required for its evaluation.

hsd17b1 is a key gene for ovarian differentiation of the Siberian sturgeon

This is the first work using gonads from undifferentiated, genetically-sexed Siberian sturgeon describing expression changes in genes related to steroid synthesis and female and male sex differentiation. One factor identified as relevant for ovarian differentiation was the gene coding for the enzyme Hsd17b1, which converts estrone into estradiol-17β. hsd17b1 was highly activated in female gonads at 2.5 months of age, around the onset of sex differentiation, preceding activation of two other genes involved in estrogen production (cyp19a1 and foxl2). hsd17b1 was also strongly repressed in males. Two known foxl2 paralogs are found in Siberian sturgeon-foxl2 and foxl2l-but only foxl2 appeared to be associated with ovarian differentiation. With regard to the male pathway, neither 11-oxygenated androgens nor classic male genes (amh, dmrt1, sox9, and dhh) were found to be involved in male sex differentiation, leaving open the question of which genes participate in early male gonad development in this ancient fish. Taken together, these results indicate an estrogen-dependence of female sex differentiation and 11-oxygenated androgen-independence of male sex differentiation.

Gonadal analysis in the F1 progeny of a gynogenetic Siberian sturgeon Acipenser baerii female

This is the first report of reproduction of a gynogenetic female Siberian sturgeon (Acipenser baerii) and subsequent production of viable offspring with differentiated gonads. Gonads were sampled twice from 30 randomly selected individuals. Gonadal fragments were examined histologically to identify the sex ratio and the sexual maturation stage. The first examined group consisted of 20 % females, 45 % males and 35 % sexually undifferentiated individuals, whereas the second group was composed of 30 % females and 70 % males. The average gonadosomatic index was determined to be 2.72 % and 2.80 % in the first and second groups, respectively. Molecular analysis based on five microsatellite DNA loci was conducted for all the fish for which there were evaluations. The aim of the study was to determine whether the gynogenetic Siberian sturgeon female that produced the offspring that were evaluated was a "superfemale" (WW) or a normal female (ZW). A "superfemale", therefore, is a female of the WW genotype that will produce only females when mated to a male with a ZZ genotype. The presence of female and male offspring clearly indicates that the Siberian sturgeon female that produced the offspring was not a "superfemale".

RNA-Seq transcriptome data of undifferentiated and differentiated gonads of Siberian sturgeon

RNA-Seq transcriptome data from twenty Siberian sturgeon gonads at different developmental stages is described: ten undifferentiated gonads, six gonads of immature males and four gonads from immature females. Siberian sturgeon, Acipenser baerii, is long-lived, late-maturing fish farmed in 50 countries but its production remains on a craftsman scale when compared to industrial species. Sturgeon genetic and physiological studies are less developed than for industrial fish. The data presented hereafter enables fundamental studies on the regulatory mechanisms of sturgeon gonad development, which can further be applied both in aquaculture and in fundamental research.

Full-length transcriptome sequencing and comparative transcriptomic analysis to uncover genes involved in early gametogenesis in the gonads of Amur sturgeon (Acipenser schrenckii)

BACKGROUND

Sturgeons (Acipenseriformes) are polyploid chondrostean fish that constitute an important model species for studying development and evolution in vertebrates. To better understand the mechanisms of reproduction regulation in sturgeon, this study combined PacBio isoform sequencing (Iso-Seq) with Illumina short-read RNA-seq methods to discover full-length genes involved in early gametogenesis of the Amur sturgeon, Acipenser schrenckii.

RESULTS

A total of 50.04 G subread bases were generated from two SMRT cells, and herein 164,618 nonredundant full-length transcripts (unigenes) were produced with an average length of 2782 bp from gonad tissues (three testes and four ovaries) from seven 3-year-old A. schrenckii individuals. The number of ovary-specific expressed unigenes was greater than those of testis (19,716 vs. 3028), and completely different KEGG pathways were significantly enriched between the ovary-biased and testis-biased DEUs. Importantly, 60 early gametogenesis-related genes (involving 755 unigenes) were successfully identified, and exactly 50% (30/60) genes of those showed significantly differential expression in testes and ovaries. Among these, the Amh and Gsdf with testis-biased expression, and the Foxl2 and Cyp19a with ovary-biased expression strongly suggested the important regulatory roles in spermatogenesis and oogenesis of A. schrenckii, respectively. We also found the four novel Sox9 transcript variants, which increase the numbers of regulatory genes and imply function complexity in early gametogenesis. Finally, a total of 236,672 AS events (involving 36,522 unigenes) were detected, and 10,556 putative long noncoding RNAs (lncRNAs) and 4339 predicted transcript factors (TFs) were also respectively identified, which were all significantly associated with the early gametogenesis of A. schrenckii.

CONCLUSIONS

Overall, our results provide new genetic resources of full-length transcription data and information as a genomic-level reference for sturgeon. Crucially, we explored the comprehensive genetic characteristics that differ between the testes and ovaries of A. schrenckii in the early gametogenesis stage, which could provide candidate genes and theoretical basis for further the mechanisms of reproduction regulation of sturgeon.

Morphological characterization of pirarucu Arapaima gigas (Schinz, 1822) gonadal differentiation

Arapaima gigas is a giant air-breathing and bony tongue fish from the Amazon basin and a promising species for aquaculture. A. gigas farming industry is still not established because of the lack of information on its reproductive physiology. Reproduction in captivity cannot be manipulated or stimulated, and the identification of males and females in a broodstock is not easy. We aimed to reveal the morphological sex differentiation of pirarucu as studies involving gonad development are essential to understanding the reproductive physiology of any species. We performed histological analysis on the whole body and extracted the gonads of 150 juveniles. The first sign of ovary differentiation is the sex-specific rearrangement of the germ cells. In 9 cm total length females, the germ cells group into nests and are restricted to the lateral face of the gonad, in close contact with the abdomen wall. With further development, this region invaginates and that later develops into ovigerous lamellae. Meiosis starts soon after ovary differentiation. In males, the germ cells are scattered along the elongated differentiating testis at first, and later become more restricted to the central region where the spermatogonial cysts start to develop. Somatic and germ cells are jointly involved in the cellular reorganization during gonadal differentiation, specifically when the germ cells begin to establish new associations during the development of both the germinal epithelium and stroma. RESEARCH HIGHLIGHTS: In Arapaima gigas, the ovary differentiation occurs in 9 cm TL females and it is marked by the rearrangement of germ and somatic cells; and the germ cells entering meiosis with no formation of ovarian cavity; testis differentiation occurs later and meiosis does not start in males smaller than 80 cm TL.

Transcriptome of tambaqui Colossoma macropomum during gonad differentiation: Different molecular signals leading to sex identity

Tambaqui (Colossoma macropomum) is the major native species in Brazilian aquaculture, and we have shown that females exhibit a higher growth compared to males, opening up the possibility for the production of all-female population. To date, there is no information on the sex determination and differentiation molecular mechanisms of tambaqui. In the present study, transcriptome sequencing of juvenile trunks was performed to understand the molecular network involved in the gonadal sex differentiation. The results showed that before differentiation, components of the Wnt/β-catenin pathway, fox and fst genes imprint female sex development, whereas antagonistic pathways (gsk3b, wt1 and fgfr2), sox9 and genes for androgen synthesis indicate male differentiation. Hence, in undifferentiated tambaqui, the Wnt/β-catenin exerts a role on sex differentiation, either upregulated in female-like individuals, or antagonized in male-like individuals.

Chondrostean sturgeon hepcidin: An evolutionary link between teleost and tetrapod hepcidins

Hepcidin, a cysteine-rich antimicrobial peptide (AMP), plays key roles as a regulatory hormone in iron homeostasis, providing a link between iron metabolism and innate immunity. Unlike many other AMPs displaying a high degree of sequence variability among closely related organisms, hepcidin is highly conserved from teleosts to mammals. However, little is known about the early ancestry of hepcidins in the vertebrate lineage. Here, we first report potential a prototype hepcidin from the Siberian sturgeon Acipenser baerii, a primitive chondrostean species. The A. baerii hepcidin (AbHAMP) gene showed a tripartite exon-intron organization, which encoded a precursor protein comprised of three structural signatures containing eight cysteine residues, a common structure in vertebrate hepcidin genes and proteins. mRNA expression by iron-overloading and bacterial infection and antibacterial activity revealed that AbHAMP might play a role in iron metabolism regulator in the liver, and in direct and/or indirect host immune response in the kidney against invading pathogen. Comparison of gene and protein sequences reveled that AbHAMP possesses intermediate characteristics between tetrapodian and teleostean hepcidins (HAMP1s). Phylogenetically, AbHAMP had a closer genetic affiliation to tetrapodian orthologs than to teleostean orthologs, suggesting that the structures of this chondrostean hepcidin may closely reflect the structures of an evolutionarily ancestral form that might have evolved into extant hepcidins in tetrapods and teleosts, respectively. Based on the identification of hepcidin from the chondrostean group, the emergence of the common ancestral hepcidin should be traced back to in early Osteichthyes: no later than sarcopterygian (lobe-finned fishes) - actinopterygian (ray-finned fishes) split.

Sex-related gonadal gene expression differences in the Russian sturgeon (Acipenser gueldenstaedtii) grown in stable aquaculture conditions

The Russian sturgeon (Acipenser gueldenstaedtii) is a primitive freshwater fish and a source of caviar. In the present study, the gonadal transcriptomes of male and female Russian sturgeons grown in stable aquaculture conditions were analyzed. RNA sequencing of whole-gonad transcriptomes from pools of 4-year old fish (five females, four males), resulted in the identification of 28,170 unique transcripts. Of these, 16,191 could be annotated by similarity to gene sequences from other species. There were 392 transcripts that showed differential abundance by a factor of 20-fold or more between the sexes; 272 of these were annotated; of these, 175 and 97 were in greater abundance in ovaries and testes, respectively. Functional annotation and clustering of the genes with differential abundances of mRNA allowed for identification of several clusters. Thus, a group of transcriptional regulators and factors involved in cell division, especially septins, were in greater abundance in the ovaries; while a different set of transcription factors (including sox6 and sox30) and a group of protein kinases were in greater abundance in the testes. The transcript abundances of nine highly abundant candidate transcripts, as well as of two additional genes previously known to be involved in reproduction, cyp19 (p450 aromatase) and foxl2, were assessed in the individual samples by qRT-PCR. Of these, five (including cyp19 and foxl2) were in greater abundance in ovaries, while the abundance of ighm1 mRNA was greater in testes. Phylogenetic analysis based on the k1c18 keratin gene placed the sturgeon sequence nearest those of other primitive fish species, supporting the ancient origin of the sturgeon. In conclusion, this study details transcriptome differences between male and female sturgeon and identifies key genes that may contribute to sexual determination and differentiation.

De novo transcriptome analysis to search for sex-differentiation genes in the Siberian sturgeon

The sturgeon family includes many species that are lucrative for commercial caviar production, some of which face critical conservation problems. The purpose of this study was to identify genes involved in gonadal sex differentiation in sturgeons, contributing to our understanding of the biological cycle of this valuable species. A high-quality de novo Siberian sturgeon gonadal transcriptome was built for this study using gonadal samples from undifferentiated fish at 3, 5, and 6 months of age; recently sex-differentiated fish at 9 months of age; and immature males and females at 14-17 months of age. Undifferentiated fish were sexed after validation of forkhead box L2 (foxl2) and cytochrome P450, family 19, subfamily A, and polypeptide 1a (cyp19a1a) as sex markers, and the transcriptomes of the 3-month-old undifferentiated fish, 5-6-month-old future females, and 5-6-month-old putative males were compared. The ovarian program was associated with strong activation of genes involved in estrogen synthesis (cyp19a1, foxl2, and estradiol 17-beta-dehydrogenase 1), stem-cell niche building and regulation, and sex-specific nerve cell development. The genes related to the stem-cell niche were: (1) the family of iroquois-class homeodomain proteins 3, 4, and 5 (irx3, irx4, irx5-1, irx5-2, and irx5-3), which are essential for somatic-germ cell interaction; (2) extracellular matrix remodeling genes, such as collagen type XXVIII alpha 1 chain and collagen type II alpha 1 chain, matrix metalloproteinases 24-1 and 24-2, and NADPH oxidase organizer 1, which, along with the somatic cells, provide architectural support for the stem-cell niche; and (3) mitogenic factors, such as lim homeobox 2, amphiregulin, G2/M phase-specific E3 ubiquitin-protein ligase, and connector enhancer of kinase suppressor of ras 2, which are up regulated in conjunction with the anti-apoptotic gene G2/M phase-specific E3 ubiquitin-protein ligase suggesting a potential involvement in regulating the number of germ cells. Genes related to sex-specific nerve cell developments were: the neurofilament medium polypeptides, the gene coding for serotonin receptor 7, 5-hydroxytryptamine receptor 7; neurotensin, isoform CRA-a, the neuron-specific transmembrane protein Delta/Notch-like epidermal growth factor-related receptor; and insulinoma-associated protein 1. The putative testicular program was poorly characterized by elements of the immune response. The classic markers of maleness were not specifically activated, indicating that testicular differentiation occurs at a later stage. In sum, the ovarian program, but not the testicular program, is in place by 5-6 months of age in the Siberian sturgeon. The female program is characterized by estrogen-related genes with well-established roles in gonadal differentiation, but also by several genes with no previously-described function in the ovarian development of fish. These newly-reported genes are involved in stem-cell niche building and regulation as well as sex-specific nerve development.

Experimental evidence of masculinization by continuous illumination in a temperature sex determination teleost (Atherinopsidae) model: is oxidative stress involved?

The present study evaluates the influence of continuous light on phenotypic sex ratios in Chirostoma estor, a temperature sex determination animal model. Relative gene expression levels of 5 day old larvae were performed on two early gonad differentiation genes (sox9 and foxl2), two stress axis activation genes (gcr1 and crf) and four reactive oxygen species (ROS) antagonist effector genes (sod2, ucp2, gsr and cat). Two light treatments were applied from fertilization; control (12L:12D) simulated natural photoperiod and a continuous illumination photoperiod. By the end of the trial (12 weeks after hatching), differentiated and normal gonads were clearly identifiable in both treatments by histological observations. Regarding sex ratio, 73% of phenotypic males were found in continuous illumination compared with 40% in controls. Consistently, the sox9 gene (involved in early testis differentiation) showed an over expression in 64% of the individual larvae analysed compared with foxl2 (ovarian differentiation) suggesting a masculinization tendency in continuous illumination. On the other hand, only 36% of individuals showed the same tendency in the control treatment consistent with phenotypic sex ratios found under normal culture conditions. Relative gene expression results did not show significant difference in sod2, ucp2 and gcr1 levels, but cat, gsr and crf showed significantly higher expression levels in the continuous illumination treatment suggesting that both, the stress axis and ROS response mechanisms were activated at this time. This study suggests, a link between continuous light, oxidative stress and environmental sex determination in vertebrates. However, further research is necessary to describe this possible upstream mechanism that may drive some aspects of sexual plasticity in vertebrates.

Foxl2 and Its Relatives Are Evolutionary Conserved Players in Gonadal Sex Differentiation

Foxl2 is a member of the large family of Forkhead Box (Fox) domain transcription factors. It emerged during the last 15 years as a key player in ovarian differentiation and oogenesis in vertebrates and especially mammals. This review focuses on Foxl2 genes in light of recent findings on their evolution, expression, and implication in sex differentiation in animals in general. Homologs of Foxl2 and its paralog Foxl3 are found in all metazoans, but their gene evolution is complex, with multiple gains and losses following successive whole genome duplication events in vertebrates. This review aims to decipher the evolutionary forces that drove Foxl2/3 gene specialization through sub- and neo-functionalization during evolution. Expression data in metazoans suggests that Foxl2/3 progressively acquired a role in both somatic and germ cell gonad differentiation and that a certain degree of sub-functionalization occurred after its duplication in vertebrates. This generated a scenario where Foxl2 is predominantly expressed in ovarian somatic cells and Foxl3 in male germ cells. To support this hypothesis, we provide original results showing that in the pea aphid (insects) foxl2/3 is predominantly expressed in sexual females and showing that in bovine ovaries FOXL2 is specifically expressed in granulosa cells. Overall, current results suggest that Foxl2 and Foxl3 are evolutionarily conserved players involved in somatic and germinal differentiation of gonadal sex.