Characterization of two zebrafish cDNA clones encoding egg envelope proteins ZP2 and ZP3

Transfer of the zp3a gene results in changes in egg adhesiveness and buoyancy in transgenic zebrafish

Reproductive strategies and spawning habits play key roles in the evolution of endemic East Asian cyprinids. However, the molecular mechanisms underlying the regulation of spawning habits are not well understood. We recently identified zona pellucida (Zp) as the top differentially expressed protein between East Asian cyprinids that produce adhesive and semi-buoyant eggs, suggesting that Zp protein may play important roles in the regulation of egg type. In this work, we generated transgenic zebrafish in which oocyte-specific expression of zp genes from rare minnow ( Gobiocypris rarus), an East Asian cyprinid laying adhesive eggs, was driven by a zebrafish zp3.2 gene promoter. We found that the transgenic eggs obtained partial adhesiveness and exhibited alteration in hydration and buoyancy. Abnormal metabolism of vitellogenin (VTG) may contribute to enhanced hydration and/or buoyancy. Our work shows that expression of the exogenous zp3a gene from an adhesive-egg producing fish is sufficient to induce changes in both egg adhesiveness and buoyancy in zebrafish, emphasizing the important role of zp genes in the regulation of spawning habits. Our results thus provide new insights into how endemic East Asian cyprinids may have adapted to the Yangtze river-lake system via changes in spawning habits.

Egg envelope formation of medaka Oryzias latipes requires ZP proteins originating from both the liver and ovary

Teleost oocytes are surrounded by a structure, called chorion or egg envelopes, which is composed of zona pellucida (ZP) proteins. As a result of the gene duplication in teleost, the expression site of the zp genes, coding the major component protein of egg envelopes, changed from the ovary to the maternal liver. In Euteleostei, there are three liver-expressed zp genes, named choriogenin (chg) h, chg hm, and chg l, and the composition of the egg envelope is mostly made up of these Chgs. In addition, ovary-expressed zp genes are also conserved in the medaka genomes, and their proteins have also been found to be minor components of the egg envelopes. However, the specific role of liver-expressed versus ovary-expressed zp genes was unclear. In the present study, we showed that ovary-synthesized ZP proteins first form the base layer of the egg envelope, and then Chgs polymerize inwardly to thicken the egg envelope. To analyze the effects of dysfunction of the chg gene, we generated some chg knockout medaka. All knockout females failed to produce normally fertilized eggs by the natural spawning. The egg envelopes lacking Chgs were significantly thinner, but layers formed by ZP proteins synthesized in the ovary were found in the thin egg envelope of knockout as well as wild-type eggs. These results suggest that the ovary-expressed zp gene is well conserved in all teleosts, including those species in which liver-derived ZP proteins are the major component, because it is essential for the initiation of egg envelope formation.

Medaka, Oryzias latipes, egg envelopes are created by ovarian-expressed ZP proteins and liver-expressed choriogenins

The medaka (Oryzias latipes) egg envelope (chorion) is composed of three major glycoproteins, Zona Interna (ZI)-1, -2, and -3, that originate in the spawning female liver as the precursor proteins Choriogenin (Chg.)H, Chg.Hm, and Chg.L, respectively. These ZI and Chg. proteins contain a structural ZP protein domain that is conserved among the egg envelope proteins of all animals. While ovarian expression of ZP proteins (e.g., ZPCs and ZPB) has been reported in medakas, the functions of these proteins remain unknown. Thus, the present study aimed to determine whether the ovary-expressed medaka ZP protein, mZPC5, is involved in forming the chorion matrix.The mZPC5 gene (mzpc5) was expressed in the ovaries but not the livers of mature female medakas, as shown by reverse transcription-polymerase chain reaction assays with mzpc5-specific primers. In situ hybridization analysis revealed that ovarian mzpc5 expression was restricted to the ooplasm of early (stage I-III) previtellogenic oocytes, and its expression signal weakened with oocyte growth. Following sodium-dodecyl-sulfate polyacrylamide gel electrophoresis (SDS-PAGE) and Western blotting analysis with anti-mZPC5 antibodies, two immunoreactive proteins were detected in the ovary and chorion extracts. These proteins were approximately 50 and 74 kDa in size, like ZI-3 and ZI-2, respectively.Immunohistochemical assays using anti-mZPC5 and anti-Chg.H antibodies localized the mZPC5 protein in the ooplasm of early previtellogenic oocytes. With oocyte growth, mZPC5 tended to accumulate in the chorion, co-localizing with Chg.H.We previously showed that ovary-expressed ZP proteins could not compensate for Chg.L function loss in gene knock-out (chg.l -/-) medakas. As in our previous study, the chg.l-/- females produced oocytes with thin chorions, resulting in infertile soft eggs. However, in the present study, mZPC5 and Chg.H were co-localized in the chg.l-/- chorions. These results suggested that in the medaka previtellogenic oocyte, 1) mZPC5 is secreted from the ooplasm and deposited on the outer surface of its plasma membrane, creating the thin chorion layer; and 2) following the accumulation of liver-derived Chgs., the 3D structure of the chorion matrix is formed cooperatively with mZPC5 and Chgs. during oogenesis. More research is needed to confirm the functions of mZPC5 in chorion structure and physiology.

Screening of Genes Related to Sex Determination and Differentiation in Mandarin Fish (Siniperca chuatsi)

Mandarin fish has an XX/XY sex-determination system. The female mandarin fish is typically larger than the male. Sex identification and the discovery of genes related to sex determination in mandarin fish have important theoretical significance in the elucidation of the regulation and evolutionary mechanism of animal reproductive development. In this study, the chromosome-level genome of a female mandarin fish was assembled, and we found that LG24 of the genome was an X chromosome. A total of 61 genes on the X chromosome showed sex-biased expression. Only six gonadal genes (LG24G00426, LG24G003280, LG24G003300, LG24G003730, LG24G004200, and LG24G004770) were expressed in the testes, and the expression of the other gene LG24G003870 isoform 1 in the ovaries was significantly higher than that in the testes (p < 0.01). Five (except LG24G003280 and LG24G003300) of the seven aforementioned genes were expressed at the embryonic development stage, suggesting their involvement in early sex determination. The expression of LG24G004770 (encoding HS6ST 3-B-like) was also significantly higher in female muscles than in male muscles (p < 0.01), indicating other functions related to female growth. ZP3 encoded by LG24G003870 isoform 1 increased the C-terminal transmembrane domain, compared with that encoded by other fish zp3 isoforms, indicating their different functions in sex determination or differentiation. This study provides a foundation for the identification of sex-determining genes in mandarin fish.

Targeted deletion of liver-expressed Choriogenin L results in the production of soft eggs and infertility in medaka, Oryzias latipes

Egg envelopes (chorions) in medaka, Oryzias latipes, are composed of three major glycoproteins: ZI-1, - 2, and - 3. These gene-encoded chorion glycoproteins are expressed in the liver and/or ovarian oocytes of sexually mature female fish. In medaka, the glycoproteins produced in the female liver are induced by estrogen as Choriogenin (Chg.) H and Chg. H minor (m), which correspond to the zona pellucida (ZP) B (ZPB) protein in mammals, and Chg. L, which corresponds to ZPC in mammals. Chg. H, Chg. Hm, and Chg. L, are then converted to ZI-1, - 2, and - 3, respectively, during oogenesis in medaka ovaries.In the present study, we established a medaka line in which the chg.l gene was inactivated using the transcription activator-like effector nuclease (TALEN) technique. Neither intact chg.l transcripts nor Chg. L proteins were detected in livers of sexually mature female homozygotes for the mutation (homozygous chg.l knockout: chg.l-/-). The chg.l-/- females spawned string-like materials containing "smashed eggs." Closer examination revealed the oocytes in the ovaries of chg.l-/- females had thin chorions, particularly at the inner layer, despite a normal growth rate. In comparing chorions from normal (chg.l+/+) and chg.l-/- oocytes, the latter exhibited abnormal architecture in the chorion pore canals through which the oocyte microvilli pass. These microvilli mediate the nutritional exchange between the oocyte and surrounding spaces and promote sperm-egg interactions during fertilization. Thus, following in vitro fertilization, no embryos developed in the artificially inseminated oocytes isolated from chg.l-/- ovaries. These results demonstrated that medaka ZI-3 (Chg.L) is the major component of the inner layer of the chorion, as it supports and maintains the oocyte's structural shape, enabling it to withstand the pressures exerted against the chorion during spawning, and is essential for successful fertilization. Therefore, gene products of oocyte-specific ZP genes that may be expressed in medaka oocytes cannot compensate for the loss Chg. L function to produce offspring for this species.

Knockin' on Egg's Door: Maternal Control of Egg Activation That Influences Cortical Granule Exocytosis in Animal Species

Fertilization by multiple sperm leads to lethal chromosomal number abnormalities, failed embryo development, and miscarriage. In some vertebrate and invertebrate eggs, the so-called cortical reaction contributes to their activation and prevents polyspermy during fertilization. This process involves biogenesis, redistribution, and subsequent accumulation of cortical granules (CGs) at the female gamete cortex during oogenesis. CGs are oocyte- and egg-specific secretory vesicles whose content is discharged during fertilization to block polyspermy. Here, we summarize the molecular mechanisms controlling critical aspects of CG biology prior to and after the gametes interaction. This allows to block polyspermy and provide protection to the developing embryo. We also examine how CGs form and are spatially redistributed during oogenesis. During egg activation, CG exocytosis (CGE) and content release are triggered by increases in intracellular calcium and relies on the function of maternally-loaded proteins. We also discuss how mutations in these factors impact CG dynamics, providing unprecedented models to investigate the genetic program executing fertilization. We further explore the phylogenetic distribution of maternal proteins and signaling pathways contributing to CGE and egg activation. We conclude that many important biological questions and genotype–phenotype relationships during fertilization remain unresolved, and therefore, novel molecular players of CG biology need to be discovered. Future functional and image-based studies are expected to elucidate the identity of genetic candidates and components of the molecular machinery involved in the egg activation. This, will open new therapeutic avenues for treating infertility in humans.

Proteomics support the threespine stickleback egg coat as a protective oocyte envelope

Ancestrally marine threespine stickleback fish (Gasterosteus aculeatus) have undergone an adaptive radiation into freshwater environments throughout the Northern Hemisphere, creating an excellent model system for studying molecular adaptation and speciation. Ecological and behavioral factors have been suggested to underlie stickleback reproductive isolation and incipient speciation, but reproductive proteins mediating gamete recognition during fertilization have so far remained unexplored. To begin to investigate the contribution of reproductive proteins to stickleback reproductive isolation, we have characterized the stickleback egg coat proteome. We find that stickleback egg coats are comprised of homologs to the zona pellucida (ZP) proteins ZP1 and ZP3, as in other teleost fish. Our molecular evolutionary analyses indicate that across teleosts, ZP3 but not ZP1 has experienced positive Darwinian selection. Mammalian ZP3 is also rapidly evolving, and surprisingly some residues under selection in stickleback and mammalian ZP3 directly align. Despite broad homology, however, we find differences between mammalian and stickleback ZP proteins with respect to glycosylation, disulfide bonding, and sites of synthesis. Taken together, the changes we observe in stickleback ZP protein architecture suggest that the egg coats of stickleback fish, and perhaps fish more generally, have evolved to fulfill a more protective functional role than their mammalian counterparts.

Use of a gene encoding zona pellucida 4 as a female-specific marker for early stage sexual differentiation and size dimorphism in the pacific abalone Haliotis discus hannai

Growth rates of Pacific abalone Haliotis discus hannai are an important trait affecting the economic value in the abalone aquaculture industry. A reverse-transcription polymerase chain reaction (RT-PCR) analyses of tissues from H. discus hannai was conducted for sexually mature gonads to determine male- and female-specific target gene expression, including genes encoding zona pellucida domain 4 (zp4), sperm protein (sp) and lysin (lys), respectively. Sex-specific expression patterns of these gene expression, even in sexually immature abalone, indicate these genes can be used as sensitive and robust sex-specific molecular markers. The RT-PCR procedure was also performed to analyze tissues collected at various developmental stages (50-day intervals) beginning at fertilization to determine when sex differentiation and expression of sex-specific genes was initiated. Detection of zp4 transcript in tissues collected at 200 days post-fertilization (dpf) indicated egg-specific development starts at 150-200 dpf. To evaluate possible sex-specific differences in growth rate, there was conducting of a molecular marker-based sex identification of abalone from a population selected for rapid growth rate. In a group of large H. discus hannai, females were more prevalent than males. To assess the correlation between growth and sex, there was comparison of weights of 3-year-old Pacific abalone in specimens where there had been sex determinations by visual examination and molecular methods. The results indicated females weighed more (55.92 ± 9.38 g, n = 15) than males (43.64 ± 15.55 g, n = 6, P =  0.037), indicating females had a more rapid growth rate than males.

Fishing forward and reverse: Advances in zebrafish phenomics

Understanding how the genome instructs the phenotypic characteristics of an organism is one of the major scientific endeavors of our time. Advances in genetics have progressively deciphered the inheritance, identity and biological relevance of genetically encoded information, contributing to the rise of several, complementary omic disciplines. One of them is phenomics, an emergent area of biology dedicated to the systematic multi-scale analysis of phenotypic traits. This discipline provides valuable gene function information to the rapidly evolving field of genetics. Current molecular tools enable genome-wide analyses that link gene sequence to function in multi-cellular organisms, illuminating the genome-phenome relationship. Among vertebrates, zebrafish has emerged as an outstanding model organism for high-throughput phenotyping and modeling of human disorders. Advances in both systematic mutagenesis and phenotypic analyses of embryonic and post-embryonic stages in zebrafish have revealed the function of a valuable collection of genes and the general structure of several complex traits. In this review, we summarize multiple large-scale genetic efforts addressing parental, embryonic, and adult phenotyping in the zebrafish. The genetic and quantitative tools available in the zebrafish model, coupled with the broad spectrum of phenotypes that can be assayed, make it a powerful model for phenomics, well suited for the dissection of genotype-phenotype associations in development, physiology, health and disease.

Bioinformatic analyses of zona pellucida genes in vertebrates and their expression in Nile tilapia

Zona pellucida (ZP) genes encode ZP glycoproteins which constitute the coat surrounding oocytes and early embryos. Genome-wide identification of ZP genes is still lacking in vertebrates, especially in fish species. Herein, we conducted bioinformatic analyses of the ZP genes of the Nile tilapia and other vertebrates. Totally 16, 9, 17, 27, 21, 20, 26, 19, 14,11, 24, 17, 9, 18, 8, 11, 9, 8, 5, and 4 ZP genes belonging to 5 subfamilies (ZPA, ZPB, ZPC, ZPD, and ZPAX) were found in the sea lamprey, elephant shark, coelacanth, spotted gar, zebrafish, medaka, stickleback, Nile tilapia, Amazon molly, platyfish, seahorse, Northern snakehead, cavefish, tetraodon, clawed frog, turtle, chicken, platypus, kangaroo rat, and human genomes, respectively. The expansion of ZP genes in basal vertebrates was mainly achieved by gene duplication of ZPB, ZPC, and ZPAX subfamilies, while the shrink of ZP gene number in viviparous mammals was achieved by keeping only one copy of the ZP genes in each subfamily or even secondary loss of some subfamilies. The number of ZP gene is related to the environment where the eggs are fertilized and the embryos develop in vertebrates. Transcriptomic analysis showed that 14 ZP genes were expressed in the ovary of Nile tilapia, while two (ZPB2b and ZPC2) were highly expressed in the liver. On the other hand, ZPB1a and ZPB2c were not found to be expressed in any tissue or at any developmental stage of the gonads examined. In the ovary, the expression of ZP genes started from 30 dah (days after hatching), significantly upregulated at 90 dah and maintained this level at 180 dah. The expression of ZPC2 in the liver and ZPC5-2 and ZPAX1 in the ovary was confirmed by in situ hybridization. The ovary- and liver-expressed ZP genes are expressed coordinately with oocyte growth in tilapia.

Transcriptome analysis of three critical periods of ovarian development in Yellow River carp (Cyprinus carpio)

Ovary development is a complex process involving numerous genes; the molecular mechanism underlying the ovary development of carp is still unknown. Here we used Illumina HiSeq™ 2500 to explore the transcriptome of undifferentiated gland (PG), juvenile ovary (OJ) and adult ovary (OA) of Yellow River carp (Cyprinus carpio). A total of 58,749 unigenes were obtained, comprising 45,707 known genes and 13,042 new genes. We identified differentially-expressed genes (DEGs) during development and characterized the functional properties of DEGs by comparison with the gene ontology (GO) and Kyoto Encyclopedia of Genes and Genomes databases. qRT-PCR was used to analyze the expression of 22 DEGs and the results corresponded with those of RNA-Seq. Among DEGs between PG and OJ, some upstream regulators of gonad development were upregulated in PG, such as cyp19a and sox9, while some oocyte-specific genes were upregulated in OJ, such as nobox, bmp15 and zp2. Among DEGs between OJ and OA, many oocyte physiological function-related genes were upregulated in OA, such as fem-1 and foxl2. GO analysis showed a higher number of DEGs from PG-OJ analysis were assigned to reproduction terms. Furthermore, our investigation has also revealed DEGs identified from PG-OJ analysis were enriched in several important functional pathways, such as Fanconi anemia and the notch signal pathway. These data suggested a dynamic shift in gene expression during ovary development, and DEGs between PG and OJ provided crucial candidate gene data for the study of ovarian differentiation. Additionally, a total of 1,776,769 single nucleotide polymorphisms and 157,279 INDEs were revealed from transcriptome data. This result will contribute to knowledge of ovary differentiation of Yellow River carp.

Anguillicola crassus Infection Significantly Affects the Silvering Related Modifications in Steady State mRNA Levels in Gas Gland Tissue of the European Eel

Using Illumina sequencing, transcriptional changes occurring during silvering in swimbladder tissue of the European eel have been analyzed by comparison of yellow and silver eel tissue samples. Functional annotation analysis based on GO terms revealed significant expression changes in a number of genes related to the extracellular matrix, important for the control of gas permeability of the swimbladder, and to reactive oxygen species (ROS) defense, important to cope with ROS generated under hyperbaric oxygen partial pressures. Focusing on swimbladder tissue metabolism, levels of several mRNA species encoding glucose transport proteins were several-fold higher in silver eels, while enzymes of the glycolytic pathway were not affected. The significantly higher steady state level of a transcript encoding for membrane bound carbonic anhydrase, however, suggested that CO₂ production in the pentose phosphate shunt and diffusion of CO₂ was of particular importance in silver eel swimbladder. In addition, the mRNA level of a large number of genes related to immune response and to sexual maturation was significantly modified in the silver eel swimbladder. The modification of several processes related to protein metabolism and transport, cell cycle, and apoptosis suggested that these changes in swimbladder metabolism and permeability were achieved by increasing cell turn-over. The impact of an infection of the swimbladder with the nematode Anguillicola crassus has been assessed by comparing these expression changes with expression changes observed between uninfected yellow eel swimbladder tissue and infected silver eel swimbladder tissue. In contrast to uninfected silver eel swimbladder tissue, in infected tissue the mRNA level of several glycolytic enzymes was significantly elevated, and with respect to extracellular matrix, several mucin genes were many-fold higher in their mRNA level. Modification of many immune related genes and of the functional categories “response to DNA damage stimulus” and “cellular response to stress” illustrated the damaging effect of the nematode infection. This study has identified a range of cellular processes in the swimbladder of silver eels that appear to be altered by nematode infection. These altered cellular processes could contribute to detrimental changes in swimbladder function that, in turn, may lead to impairment of spawning migration.

Four glycoproteins are expressed in the cat zona pellucida

The mammalian oocyte is surrounded by a matrix called the zona pellucida (ZP). This envelope participates in processes such as acrosome reaction induction, sperm binding and may be involved in speciation. In cat (Felis catus), this matrix is composed of at least three glycoproteins called ZP2, ZP3, and ZP4. However, recent studies have pointed to the presence of a fourth protein in several mammals (rat, human, hamster or rabbit), meaning that a reevaluation of cat ZP is needed. For this reason, the objective of this research was to analyze the protein composition of cat ZP by means of proteomic analysis. Using ZP from ovaries and oocytes, several peptides corresponding to four proteins were detected, yielding a coverage of 33.17%, 71.50%, 50.23%, and 49.64% for ZP1, ZP2, ZP3, and ZP4, respectively. Moreover, the expression of four genes was confirmed by molecular analysis. Using total RNA isolated from cat ovaries, the complementary deoxyribonucleic acids encoding cat ZP were partially amplified by reverse-transcribed polymerase chain reaction. Furthermore, ZP1 was totally amplified for the first time in this species. As far as we are aware, this is the first study that confirms the presence of four proteins in cat ZP.

A specific genomic organization and a novel promoter sequence for both ZP2 and ZP3 gene expressions in the Pingxiang red transparent crucian carp, Carassius auratus var. pingxiangnensis

We cloned the full-length cDNA of ZP2 from Carassius auratus var. Pingxiangnensis (CaP_ZP2) and identified a cluster of three ZP genes through its DNA walker. These three genes, CaP_ZP3.1, CaP_ZP2 and CaP_ZP3.2 were located within a 10,855bp region and each comprised of eight exons spanning 1348bp, 1638bp and 1348bp, respectively. Protein bands of egg membrane between 40 and 70kDa were in concordance with the deduced amino acid of these three CaP_ZP genes cDNA and with their molecular mass. This is the first report that two CaP_ZP3 genes were separated by CaP_ZP2 gene. A novel sequence of 1097bp, located between CaP_ZP2 and CaP_ZP3.2, was inserted into the modified pAcGFP1-1 vector in the forward and reverse directions. Results showed that individual sequence served as promoters utilizing common regulatory elements in the forward and reverse directions for both CaP_ZP2 and CaP_ZP3.2 gene expressions. In situ hybridization against CaP_ZP2 confirmed that a strong positive signal was detected in the early development oocytes. Similarly, real-time PCR results also showed that CaP_ZP2 transcription increased mainly in a 4-8month ovary, but was decreased dramatically in a 9-12month ovary.

Expression of two zona pellucida genes is regulated by 17α-ethinylestradiol in adult rare minnow Gobiocypris rarus

Zona pellucida (ZP) proteins are glycoproteins synthesized in liver, ovary or in both tissues in fish. In the present study, we aimed to determine the responsiveness of ZP2 and ZP3 to 17α-ethinylestradiol (EE2) in adult rare minnow Gobiocypris rarus. The full length of ZP3 cDNA was firstly characterized and its tissue distribution revealed that ZP3 mRNA was predominantly expressed in ovary of G. rarus. The gene expression profiles of ZP2, ZP3 and vitellogenin (VTG) were analyzed in gonad and liver of adult G. rarus exposed to EE2 at 1, 5, 25, and 125 ng/L for 3 and 6 days. The results show that ZP2 is more sensitive than ZP3 in gonads of both genders, and VTG in liver is extremely sensitive to EE2 in male fish. However, at lower concentrations (1 and 5 ng/L), the ZP2 in testes shows higher responsiveness to EE2 compared with VTG in rare minnow. The 5' flanking regions of ZP2 and ZP3 were isolated and the comparison of transcription factors in the regions of ZP2 and ZP3 suggested that the disparity for the responsiveness of ZP2 and ZP3 to EE2 could partly be a result of differential cis-elements such as oocyte-specific protein (Osp1) binding sites or/and sex-determining region Y (SRY) binding site.

Variations in the gene expression of zona pellucida proteins, zpb and zpc, in female European eel (Anguilla anguilla) during induced sexual maturation

Vertebrate eggs are surrounded by an extracellular glycoprotein coat termed zona pellucida (ZP). Integrity of ZP is critical for a correct embryo development. Two zona pellucida protein genes (zpb and zpc) from European eel were characterized, specific qPCR assays developed and their expression in immature males and females carried out. An experimental group of silver-stage eel females was maintained at 18 °C and hormonally induced to sexual maturation by weekly injections of carp pituitary extract during 12 weeks. Changes in zpb and zpc expression during sexual maturation were studied in liver and ovary by qPCR. In liver, no changes were recorded during hormonal treatment, while in ovary expression of both genes decreased during sexual development. These results are a first step in the characterization of ZP in European eel and in the understanding of the mechanism underlying egg envelope formation.

Extensive alternative polyadenylation during zebrafish development

The post-transcriptional fate of messenger RNAs (mRNAs) is largely dictated by their 3′ untranslated regions (3′ UTRs), which are defined by cleavage and polyadenylation (CPA) of pre-mRNAs. We used poly(A)-position profiling by sequencing (3P-seq) to map poly(A) sites at eight developmental stages and tissues in the zebrafish. Analysis of over 60 million 3P-seq reads substantially increased and improved existing 3′ UTR annotations, resulting in confidently identified 3′ UTRs for >79% of the annotated protein-coding genes in zebrafish. mRNAs from most zebrafish genes undergo alternative CPA, with those from more than a thousand genes using different dominant 3′ UTRs at different stages. These included one of the poly(A) polymerase genes, for which alternative CPA reinforces its repression in the ovary. 3′ UTRs tend to be shortest in the ovaries and longest in the brain. Isoforms with some of the shortest 3′ UTRs are highly expressed in the ovary, yet absent in the maternally contributed RNAs of the embryo, perhaps because their 3′ UTRs are too short to accommodate a uridine-rich motif required for stability of the maternal mRNA. At 2 h post-fertilization, thousands of unique poly(A) sites appear at locations lacking a typical polyadenylation signal, which suggests a wave of widespread cytoplasmic polyadenylation of mRNA degradation intermediates. Our insights into the identities, formation, and evolution of zebrafish 3′ UTRs provide a resource for studying gene regulation during vertebrate development.

Expression of zona pellucida B proteins in juvenile rare minnow (Gobiocypris rarus) exposed to 17α-ethinylestradiol, 4-nonylphenol and bisphenol A

Zona pellucida (ZP) containing proteins are glycoproteins in teleost chorion and are encoded by several gene subfamilies, mainly including ZPA, ZPB, ZPC and ZPX genes. In teleost species, ZP genes are expressed either in liver under regulation of estrogen or in ovary. In the present study, five ZP gene isoforms were isolated and characterized in Gobiocypris rarus. The putative amino acid sequences of these ZP gene isoforms contain the typical trefoil motif and a ZP domain. These five G. rarus ZP gene isoforms were named as grZPB.1, grZPB.2, grZPB.3, grZPB.4 and grZPB.5. Real-time quantitative reverse transcription PCR (RT-qPCR) analysis indicated that all these ZP mRNA isoforms were exclusively expressed in ovary. G. rarus juveniles at the age of 21 days postfertilization were exposed to 17α-ethinylestradiol (EE2; 0.01, 0.1 and 1 nM), 4-nonylphenol (4-NP; 10, 100 and 1000 nM) or bisphenol A (BPA; 0.1, 1 and 10nM) for 3 days. mRNA expressions of ZPB isoforms following the exposure to xenoestrogen were detected by RT-qPCR. Data were analyzed by the 2(-△△Cq) method. The results indicate that induction by 0.1-1nM EE2 on mRNA expression of the grZPB isoforms is weaker than for vitellogenin. 4-NP exposures at three concentrations had differential effects on the grZPBs. BPA at three concentrations weakly induced mRNA expression of the grZPB isoforms.

Molecular characterization and expression pattern of three zona pellucida 3 genes in the Chinese sturgeon, Acipenser sinensis

Chinese sturgeon (Acipenser sinensis) is a rare and endangered species and also an important resource for the sturgeon aquaculture industry. SMART cDNAs were synthesized from the ovary of A. sinensis, and the full-length cDNAs of three zona pellucida glycoprotein 3 genes (the new gene named AsZP3) were cloned and sequenced. AsZP3.1, AsZP3.2, and AsZP3.3 were 1,388 base pairs (bp), 1,288, and 1,290 bp in length, respectively, and they could be translated into proteins with 440, 394, and 398 amino acids, respectively. High level of amino acids sequence identity was seen between AsZP3.2 and AsZP3.3 (about 82%), but they share low identity with AsZP3.1 (26 and 23%, respectively). The AsZP3.1 has 42-50% amino acids sequence identity values with other fish and lower values with higher vertebrates (38%); AsZP3.2 and AsZP3.3 shared about 30-44% sequence identity with higher vertebrates and other fish. RT-PCR analysis indicated that AsZP3.1 displayed a wide tissue distribution at the mRNA levels including liver, kidney, spleen, heart, and ovary, but AsZP3.2 and AsZP3.3 mRNAs were expressed exclusively in the gonad. All three AsZP3 mRNAs were not detected during embryogenesis and early larval development; furthermore, they were not detected in the gonads of 1- and 2-year-old Chinese sturgeons. All three AsZP3 mRNAs were detected in the testes of 3-year-old males and in the ovaries of 4- and 5-year-old female Chinese sturgeons.

Evolution of the teleostean zona pellucida gene inferred from the egg envelope protein genes of the Japanese eel, Anguilla japonica

A fish egg envelope is composed of several glycoproteins, called zona pellucida (ZP) proteins, which are conserved among vertebrate species. Euteleost fishes synthesize ZP proteins in the liver, while otocephalans synthesize them in the growing oocyte. We investigated ZP proteins of the Japanese eel, Anguilla japonica, belonging to Elopomorpha, which diverged earlier than Euteleostei and Otocephala. Five major components of the egg envelope were purified and their partial amino acid sequences were determined by sequencing. cDNA cloning revealed that the eel egg envelope was composed of four ZPC homologues and one ZPB homologue. Four of the five eel ZP (eZP) proteins possessed a transmembrane domain, which is not found in the ZP proteins of Euteleostei and Otocephala that diverged later, but is found in most other vertebrate ZP proteins. This result suggests that fish ZP proteins originally possessed a transmembrane domain and lost it during evolution. Northern blotting and RT-PCR revealed that all of the eZP transcripts were present in the ovary, but not in the liver. Phylogenetic analyses of fish zp genes showed that ezps formed a group with other fish zp genes that are expressed in the ovary, and which are distinct from the group of genes expressed in the liver. Our results support the hypothesis that fish ZP proteins were originally synthesized in the ovary, and then the site of synthesis was switched to the liver during the evolutionary pathway to Euteleostei.

Nitroreductase-mediated gonadal dysgenesis for infertility control of genetically modified zebrafish

Genetically modified (GM) fish with desirable features such as rapid growth, disease resistance, and cold tolerance, among other traits, have been established in aquaculture. However, commercially available GM fish are restricted because of global concerns over the incomplete assessments of food safety and ecological impact. The ecological impact concerns include gene flow and escape of the GM fish, which may cause extinction of wild natural fish stocks. Infertility control is a core technology for overcoming this obstacle. Although polyploidy technology, GnRH-specific antisense RNA, and RNAi against GnRH gene expression have been used to cause infertility in fish, these approaches are not 100% reliable and are not heritable. In the present study, zebrafish was used as a model to establish an inducible platform of infertility control in GM fish. Nitroreductase, which converts metronidazole substrate into cytotoxin, was fused with EGFP and expressed specifically by oocytes in the Tg(ZP:NTR-EGFP) by a zona pellucida promoter. Through consecutive immersion of metronidazole from 28 to 42 days posthatching, oocyte-specific EGFP expression was eliminated, and atrophy of the gonads was detected by anatomical analysis. These findings reveal that oocyte-specific nitroreductase-mediated catalysis of metronidazole blocks oogenesis and leads to an undeveloped oocyte. Furthermore, oocyte cell death via apoptosis was detected by a TUNEL assay. We found that the gonadal dysgenesis induced by metronidazole resulted in activation of the ovarian killer gene bok, which is a proapoptotic gene member of the Bcl-2 family and led to infertility. These results show that oocyte-specific nitroreductase-mediated catalysis of metronidazole can cause reliable infertility in zebrafish and could potentially be used as a model for other aquaculture fish species.

A microarray analysis of sex- and gonad-biased gene expression in the zebrafish: evidence for masculinization of the transcriptome

Background

In many taxa, males and females are very distinct phenotypically, and these differences often reflect divergent selective pressures acting on the sexes. Phenotypic sexual dimorphism almost certainly reflects differing patterns of gene expression between the sexes, and microarray studies have documented widespread sexually dimorphic gene expression. Although the evolutionary significance of sexual dimorphism in gene expression remains unresolved, these studies have led to the formulation of a hypothesis that male-driven evolution has resulted in the masculinization of animal transcriptomes. Here we use a microarray assessment of sex- and gonad-biased gene expression to test this hypothesis in zebrafish.

Results

By using zebrafish Affymetrix microarrays to compare gene expression patterns in male and female somatic and gonadal tissues, we identified a large number of genes (5899) demonstrating differences in transcript abundance between male and female Danio rerio. Under conservative statistical significance criteria, all sex-biases in gene expression were due to differences between testes and ovaries. Male-enriched genes were more abundant than female-enriched genes, and expression bias for male-enriched genes was greater in magnitude than that for female-enriched genes. We also identified a large number of genes demonstrating elevated transcript abundance in testes and ovaries relative to male body and female body, respectively.

Conclusion

Overall our results support the hypothesis that male-biased evolutionary pressures have resulted in male-biased patterns of gene expression. Interestingly, our results seem to be at odds with a handful of other microarray-based studies of sex-specific gene expression patterns in zebrafish. However, ours was the only study designed to address this specific hypothesis, and major methodological differences among studies could explain the discrepancies. Regardless, all of these studies agree that transcriptomic sex differences in D. rerio are widespread despite the apparent absence of heterogamety. These differences likely make important contributions to phenotypic sexual dimorphism in adult zebrafish; thus, from an evolutionary standpoint, the precise roles of sex-specific selection and sexual conflict in the evolution of sexually dimorphic gene expression are very important. The results of our study and others like it set the stage for further work aimed at directly addressing this exciting issue in comparative genomics.

Purification and characterization of zebrafish hatching enzyme - an evolutionary aspect of the mechanism of egg envelope digestion

There are two hatching enzyme homologues in the zebrafish genome: zebrafish hatching enzyme ZHE1 and ZHE2. Northern blot and RT-PCR analysis revealed that ZHE1 was mainly expressed in pre-hatching embryos, whereas ZHE2 was rarely expressed. This was consistent with the results obtained in an experiment conducted at the protein level, which demonstrated that one kind of hatching enzyme, ZHE1, was able to be purified from the hatching liquid. Therefore, the hatching of zebrafish embryo is performed by a single enzyme, different from the finding that the medaka hatching enzyme is an enzyme system composed of two enzymes, medaka high choriolytic enzyme (MHCE) and medaka low choriolytic enzyme (MLCE), which cooperatively digest the egg envelope. The six ZHE1-cleaving sites were located in the N-terminal regions of egg envelope subunit proteins, ZP2 and ZP3, but not in the internal regions, such as the ZP domains. The digestion manner of ZHE1 appears to be highly analogous to that of MHCE, which partially digests the egg envelope and swells the envelope. The cross-species digestion using enzymes and substrates of zebrafish and medaka revealed that both ZHE1 and MHCE cleaved the same sites of the egg envelope proteins of two species, suggesting that the substrate specificity of ZHE1 is quite similar to that of MHCE. However, MLCE did not show such similarity. Because HCE and LCE are the result of gene duplication in the evolutionary pathway of Teleostei, the present study suggests that ZHE1 and MHCE maintain the character of an ancestral hatching enzyme, and that MLCE acquires a new function, such as promoting the complete digestion of the egg envelope swollen by MHCE.

Choriogenin mRNA as a sensitive molecular biomarker for estrogenic chemicals in developing brackish medaka (Oryzias melastigma)

Teleost choriogenins, precursors of the inner layer subunits of the egg envelope, are regarded as sensitive biomarkers for estrogenic pollutants. In this study, two full-length cDNAs, omChgH and omChgL, which encode the choriogenin H and L forms, respectively, were isolated from a brackish medaka, Oryzias melastigma. 17beta-Estradiol (E2; 10 microg/L)-dependent expression of omChgH and omChgL was observed starting at embryonic stage 34 and restricted exclusively to the liver. In hatchlings, E2 induction of omChgH was stronger than that of omChgL. Static exposure of adult fish to E2 (0, 1, 10, 100, and 500 ng/L), 17alpha-ethinylestradiol (EE2; 0, 1, 10, 100 and 500 ng/L), 4-nonylphenol (NP; 0, 1, 10, 100, and 200 microg/L), and bisphenol A (BPA; 0, 1, 10, 100, and 200 microg/L) in artificial seawater for 7 days resulted in dose-dependent induction of both genes in the liver. In the male livers, the sensitivity of omChgH to these estrogenic compounds was higher than that of omChgL; the lowest-observed-effect concentrations (LOECs) of E2, EE2, NP, and BPA on omChgH were 10 ng/L, 10 ng/L, 100 microg/L and 100 microg/L, respectively, and on omChgL were 100 ng/L, 100 ng/L, 100 microg/L, and 200 microg/L, respectively. All these suggest that omChgH can be used as a highly sensitive biomarker for monitoring estrogenic chemicals in the marine environment.

Transcriptomic analyses reveal novel genes with sexually dimorphic expression in the zebrafish gonad and brain

BACKGROUND

Our knowledge on zebrafish reproduction is very limited. We generated a gonad-derived cDNA microarray from zebrafish and used it to analyze large-scale gene expression profiles in adult gonads and other organs.

METHODOLOGY/PRINCIPAL FINDINGS

We have identified 116638 gonad-derived zebrafish expressed sequence tags (ESTs), 21% of which were isolated in our lab. Following in silico normalization, we constructed a gonad-derived microarray comprising 6370 unique, full-length cDNAs from differentiating and adult gonads. Labeled targets from adult gonad, brain, kidney and 'rest-of-body' from both sexes were hybridized onto the microarray. Our analyses revealed 1366, 881 and 656 differentially expressed transcripts (34.7% novel) that showed highest expression in ovary, testis and both gonads respectively. Hierarchical clustering showed correlation of the two gonadal transcriptomes and their similarities to those of the brains. In addition, we have identified 276 genes showing sexually dimorphic expression both between the brains and between the gonads. By in situ hybridization, we showed that the gonadal transcripts with the strongest array signal intensities were germline-expressed. We found that five members of the GTP-binding septin gene family, from which only one member (septin 4) has previously been implicated in reproduction in mice, were all strongly expressed in the gonads.

CONCLUSIONS/SIGNIFICANCE

We have generated a gonad-derived zebrafish cDNA microarray and demonstrated its usefulness in identifying genes with sexually dimorphic co-expression in both the gonads and the brains. We have also provided the first evidence of large-scale differential gene expression between female and male brains of a teleost. Our microarray would be useful for studying gonad development, differentiation and function not only in zebrafish but also in related teleosts via cross-species hybridizations. Since several genes have been shown to play similar roles in gonadogenesis in zebrafish and other vertebrates, our array may even provide information on genetic disorders affecting gonadal phenotypes and fertility in mammals.

Phylogenetic analysis and identification of pseudogenes reveal a progressive loss of zona pellucida genes during evolution of vertebrates

Vertebrate eggs are surrounded by an extracellular matrix with similar functions and conserved individual components: the zona pellucida (ZP) glycoproteins. In mammals, chickens, frogs, and some fish species, we established an updated list of the ZP genes, studied the relationships within the ZP gene family using phylogenetic analysis, and identified ZP pseudogenes. Our study confirmed the classification of ZP genes in six subfamilies: ZPA/ZP2, ZPB/ZP4, ZPC/ZP3, ZP1, ZPAX, and ZPD. The identification of a Zpb pseudogene in the mouse genome, Zp1 pseudogenes in the dog and bovine genomes, and Zpax pseudogenes in the human, chimpanzee, macaque, and bovine genomes showed that the evolution of ZP genes mainly occurs by death of genes. Our study revealed that the extracellular matrix surrounding vertebrate eggs contains three to at least six ZP glycoproteins. Mammals can be classified in three categories. In the mouse, the ZP is composed of three ZP proteins (ZPA/ZP2, ZPC/ZP3, and ZP1). In dog, cattle and, putatively, pig, cat, and rabbit, the zona is composed of three ZP proteins (ZPA/ZP2, ZPB/ZP4, and ZPC/ZP3). In human, chimpanzee, macaque, and rat, the ZP is composed of four ZP proteins (ZPA/ZP2, ZPB/ZP4, ZPC/ZP3, and ZP1). Our review provides new directions to investigate the molecular basis of sperm-egg recognition, a mechanism which is not yet elucidated.

ZP genes in avian species illustrate the dynamic evolution of the vertebrate egg envelope

The vertebrate egg envelope is composed of a set of related proteins, encoded by the ZP genes. The apparent simplicity of the egg envelope is in contrast to the number of ZP genes identified by conventional cloning and data mining of genome sequences from a number of vertebrates. The vertebrate ZP genes fall into five classes, ZP1, ZP2, ZP3, ZPD and ZPAX. Analysis of chicken genome and EST sequence data has revealed the presence of seven distinct ZP genes, falling into these classes that are expressed in the female reproductive system. Comparison with the repertoire of ZP genes in other vertebrates suggests a major source of diversity in the composition of the egg envelope is a continual process of amplification, diversification and attrition of ZP gene sequences.

Glycoproteins of the vitelline envelope of Amphibian oocyte: Biological and molecular characterization of ZPC component (gp41) inBufo arenarum

The vitelline envelope (VE) participates in sperm-egg interactions during the first steps of fertilization. In Bufo arenarum, this envelope is composed of at least four glycoproteins, with molecular masses of 120, 75, 41, and 38 kDa and molar ratio of 1:1.3:7.4:4.8, respectively. These components were isolated and covalently coupled to silanized glass slides in order to study their sperm-binding capacity. When considering the molar ratio of the glycoproteins in the egg-envelope and assuming that each protein is monovalent for sperm, the assay showed that gp41 and gp38 possess 55 and 25% of total sperm-binding activity. We obtained a full-length cDNA of gp41 (ZPC), comprising a sequence for 486 amino acids, with 43.3% homology with Xenopus laevis ZPC. As in the case of mammalian ZP3 and Xenopus ZPC, Bufo ZPC presented a furin-like (convertase) and a C-terminal transmembrane domain (TMD) reflecting common biosynthetic and secretory pathways. As it was reported for some fishes, we obtained evidence that suggests the presence of more than one zpc gene in Bufo genome, based on different partial cDNA sequences of zpc, Southern blots and two-dimensional SDS-PAGE of deglycosylated egg-envelope components. As far as we are aware, this is the first observation of the presence of different zpc genes in an Amphibian species.

Molecular Characterization and Expression Pattern of Zona Pellucida Proteins in Gilthead Seabream (Sparus aurata)1

The developing oocyte is surrounded by an acellular envelope that is composed of 2-4 isoforms of zona pellucida (ZP) proteins. The ZP proteins comprise the ZP1, ZP2, ZP3, and ZPX isoforms. While ZP1 (ZPB) and ZP3 (ZPC) are present in all species, ZP2 (ZPA) is not found in teleost fish and ZPX is not found in mammals. In the present study, we identify and characterize the ZP1, ZP3 and ZPX isoforms of gilthead seabream. Furthermore, by analyzing the conserved domains, which include the external hydrophobic patch and the internal hydrophobic patch, we show that ZP2 and ZPX are closely related isoforms. ZP proteins are synthesized in either the liver or ovary of most teleosts. Only in rainbow trout has it been shown that zp3 has dual transcription sites. In gilthead seabream, all four mRNA isoforms are transcribed in both the liver and ovary, with zp1a, zp1b, and zp3 being highly expressed in the liver, and zpx being primarily expressed in the ovary. However, determination of the ZP proteins in plasma showed high levels of ZP1b, ZP3, and ZPX, with low or non-detectable levels of ZP1a. In similarity to other teleost ZPs, the hepatic transcription of all four ZP isoforms is under estrogenic control. Previously, we have shown that cortisol can potentiate estrogen-induced ZP synthesis in salmonids, and now we show that this is not the case in the gilthead seabream. The present study shows for the first time the endocrine regulation of a teleost ZPX isoform, and demonstrates the dual-organ transcriptional activities of all the ZP proteins in one species.

Vertebrate species profiling in one step using a single primer

Current species profiling techniques usually require several steps to identify an unknown species in quarantine cases and other forensic applications. Here we have developed a species profiling test that produces unique profiles for all vertebrate species tested using a single primer in a polymerase chain reaction. Samples tested included a range of mammals and other vertebrates such as fish and marsupials; a group of animals yet to be characterized with molecular speciation techniques. Species-specific profiles were shown to be reproducible and able to be generated from less than 10 ng of total DNA, comparable to DNA quantities used in conventional species profiling techniques. A case study demonstrates the utility of the technique by distinguishing between commercial and protected species of the Macropodidae (kangaroo) superfamily.

Tandem-Repeated Zebrafish zp3 Genes Possess Oocyte-Specific Promoters and Are Insensitive to Estrogen Induction1

Zona pellucida (Zp) proteins are glycoproteins in fish chorion and are encoded by multiple gene families, including zp1, zp2, zp3, and potentially other zp genes. Expression of zp genes in teleosts is either in the liver under the induction of estrogen or in the ovaries. In the present study, we isolated and characterized a zebrafish zp3 genomic clone and found three tandem-repeated zp3 genes with high sequence identities. We estimated that there were 10-15 zp2 and 17-21 zp3 genes in a haploid genome. We also found some variant zp2 and zp3 subfamilies, and each subfamily may have multiple members. These zp2 and zp3 genes are distributed in several different chromosomes. Ontogenetic expression of zp2 and zp3 mRNAs was first detected at 3 wk postfertilization, which was about 5 wk earlier than initial vtg1 expression, indicating that ovary development was earlier than vitellogenesis. Both zp2 and zp3 mRNAs were expressed specifically in early-growing oocytes and are insensitive to estrogen induction. Because zp3 genes are organized in tandem repeats, to investigate whether an individual zp3 promoter is capable of driving oocyte-specific expression, green fluorescence protein (gfp)-transgenic zebrafish were developed by using a 3.8-kb zp3 5' upstream region, and we found that the gfp reporter gene was faithfully expressed in developing oocytes in zp3:gfp transgenic females. Thus, the new transgenic line not only provided a convenient living marker for monitoring female gonad development, but also demonstrated that a single zp3 gene promoter is sufficient for oocyte-specific transcription.

Mass spectrometric evidence that proteolytic processing of rainbow trout egg vitelline envelope proteins takes place on the egg

The rainbow trout egg vitelline envelope (VE) is constructed of three proteins, called VEalpha,VEbeta, and VEgamma, that are synthesized and secreted by the liver and transported in the bloodstream to the ovary, the site of VE assembly around eggs. All three proteins possess an N-terminal signal peptide, a zona pellucida domain, a consensus furin-like cleavage site (CFLCS) close to the C terminus, and a short propeptide downstream of the CFLCS. Proteolytic processing at the CFLCS results in loss of the short C-terminal propeptide from precursor proteins and enables incorporation of mature proteins into the VE. Here mass spectrometry (matrix-assisted laser desorption ionization time-of-flight-mass spectrometry and liquid chromatography-mass spectrometry with a micromass-quadrupole TOF hybrid mass and a QSTAR Pulsar i mass spectrometer) was employed with VE proteins isolated from rainbow trout eggs in a peptidomics-based approach to determine the following: 1) the C-terminal amino acid of mature, proteolytically processed VE proteins; 2) the cellular site of proteolytic processing at the CFLCS of VE precursor proteins; and 3) the relationship between proteolytic processing and limited covalent cross-linking of VE proteins. Peptides derived from the C-terminal region were found for all three VE proteins isolated from eggs, indicating that processing at the CFLCS occurs after the arrival of VE precursor proteins at the egg. Consistent with this conclusion, peptides containing an intact CFLCS were also found for all three VE proteins isolated from eggs. Furthermore, peptides derived from the C-terminal propeptides of VE protein heterodimers VEalpha-VEgamma and VEbeta-VEgamma were found, suggesting that a small amount of VE protein can be covalently cross-linked on eggs prior to proteolytic processing at the CFLCS. Collectively, these results provide important evidence about the process of VE formation in rainbow trout and other non-cyprinoid fish and allow comparisons to be made with the process of zona pellucida formation in mammals.

Transcriptome analysis of zebrafish embryogenesis using microarrays

Zebrafish (Danio rerio) is a well-recognized model for the study of vertebrate developmental genetics, yet at the same time little is known about the transcriptional events that underlie zebrafish embryogenesis. Here we have employed microarray analysis to study the temporal activity of developmentally regulated genes during zebrafish embryogenesis. Transcriptome analysis at 12 different embryonic time points covering five different developmental stages (maternal, blastula, gastrula, segmentation, and pharyngula) revealed a highly dynamic transcriptional profile. Hierarchical clustering, stage-specific clustering, and algorithms to detect onset and peak of gene expression revealed clearly demarcated transcript clusters with maximum gene activity at distinct developmental stages as well as co-regulated expression of gene groups involved in dedicated functions such as organogenesis. Our study also revealed a previously unidentified cohort of genes that are transcribed prior to the mid-blastula transition, a time point earlier than when the zygotic genome was traditionally thought to become active. Here we provide, for the first time to our knowledge, a comprehensive list of developmentally regulated zebrafish genes and their expression profiles during embryogenesis, including novel information on the temporal expression of several thousand previously uncharacterized genes. The expression data generated from this study are accessible to all interested scientists from our institute resource database (http://giscompute.gis.a-star.edu.sg/~govind/zebrafish/data_download.html).

How to make an egg: transcriptional regulation in oocytes

The oocyte is a highly differentiated cell. It makes organelles specialized to its unique functions and progresses through a series of developmental stages to acquire a fertilization competent phenotype. This review will integrate the biology of the oocyte with what is known about oocyte-specific gene regulation and transcription factors involved in oocyte development. We propose that oogenesis is reliant on a dynamic gene regulatory network that includes oocyte-specific transcriptional regulators.

Genome-wide identification of female-enriched genes in zebrafish

Characteristic differences in morphology, physiology, and behavior between a male and female are correlated to the differential selection of sex-dependent transcriptomes. By using a cDNA array carrying approximately 9,000 zebrafish unique genes, we identified a group of genes whose expression are enriched in the female fish. A subset of these genes have been confirmed and further grouped as egg-enriched genes, as both somatic- and egg-enriched genes or as somatic-enriched genes by means of RNA gel blot hybridization. Most importantly, a significant proportion of these genes are either functionally unknown or are novel genes. Thus, future studies of this group of genes will help us greatly to understand more about sex-determination and sex-related physiology and behavior. In addition, comparison of zebrafish female-enriched genes with that in Drosophila, we found that only germline genes are shared between vertebrate and invertebrate, suggesting that the process of oogenesis is highly conserved during the evolution.

Interaction of sperm with purified native chicken ZP1 and ZPC proteins

The avian perivitelline membrane (PVM) is the site of initial contact between sperm and egg. It consists of only two major components, which are both homologues of the mammalian zona pellucida (ZP) proteins, and belong to the ZP1 and ZPC families, respectively. We have established a method to isolate large quantities of both native avian ZP proteins and have used these preparations to investigate their sperm-binding capacities. Chicken ZPC forms multimeric structures of defined size and binds to an approximately 180-kDa protein complex present in rooster sperm extracts. Based on experiments using both PVM and isolated proteins, we show that chicken ZP1 is proteolytically degraded by a sperm-associated protease but that chicken ZPC remains intact. An antiserum directed against chicken ZP1 is capable of inhibiting sperm binding to the PVM. Taken together, these data suggest that ZP1, in addition to ZPC, plays a major role in the initial interactions between sperm and egg.

Transgene driving GFP expression from the promoter of the zona pellucida genezpcis expressed in oocytes and provides an early marker for gonad differentiation in zebrafish

Although mechanisms of sex differentiation have been studied intensely in mammals, insects, and worms, little is known about this process in lower vertebrates. To establish a marker for female gonad differentiation in zebrafish, we generated a transgenic line in which 412 bp from the promoter and 5' mRNA leader of the female-specific zebrafish zona pellucida gene zpc are fused to the coding region of green fluorescent protein (GFP). The zpc0.5:GFP transgene is expressed exclusively in oocytes, starting from the onset of female-specific differentiation, and closely resembles the expression pattern of the wild-type zpc. Strong GFP expression persists throughout oogenesis and is visible through the body wall of females. We have also characterized a putative upstream factor of zpc, FIGalpha, and show that distribution of FIGalpha RNA is compatible with its postulated role in the regulation of zpc. The zpc0.5:GFP transgenic line described here will be useful for studying oocyte development and the mechanisms that determine sex-specific gene expression in the zebrafish. It is also the first promoter characterized to date to drive stable and efficient expression specifically in the zebrafish female germline.

Molecular markers of endocrine disruption in aquatic organisms

A wide range of organic contaminant compounds prevalent in the aquatic environment has been shown to exhibit hormone-disrupting activity. The actual potency of such compounds are low compared with endogenous hormones, such as 17beta-estradiol, but may still produce detrimental biological effects. Induced hormone levels are routinely measured using commercial testing kits, though these fail to relate to actual effects. Field and laboratory studies on the biological effects of environmental estrogens have, in the past, largely relied on assays of vitellogenin (vtg) induction in male fish, reduced growth in testes formation, and intersex incidence. Here, we critically review the current and potential application of molecular techniques in assessing the adverse biological reproductive effects of endocrine-disrupting chemicals in aquatic organisms. The role of fish (estrogen, androgen, and progestogen) hormone receptors and invertebrate (ecdysone) hormone receptor, egg production (vtg and chorion) proteins, steroid biosynthesis enzymes (aromatase, sulfotransferase, and hydroxysteroid dehydrogenase), DNA damage, apoptosis, and their potential development as biomarkers are discussed in turn. In each case, the sequences characterized are presented and homologies across species are highlighted. Molecular methods of gauging vtg and zona radiata (ZR) expression and protein concentrations have included immunoassay and reverse transcription polymerase chain reaction (RT-PCR). Suggestions for the isolation for key gene expression products (aromatase, ZR, and vtg, for instance), from a wider range of fish species using degenerate primers, are given. Endocrine disruption in invertebrates has received less attention compared with fish, partly because the knowledge regarding invertebrate endocrinology is limited. Here we review and suggest alternate isolation strategies for key players in the imposex induction process: vitellin (Vn), aromatase, and Ala-Pro-Gly-Trp (APGW) amide neurohormone. Current molecular-level techniques rely on ligand-binding assays, enzyme-linked immunosorbent assay (ELISA), and, more recently, gene expression. In the future, more reliance will be placed on the development of gene expression assays using reporter systems combined with cross-species PCR-based or polyclonal antibody-based assays. We discuss the use of recombinant receptors as a means of primary screening of environmental samples for estrogenicity and antiestrogenicity, which avoids species and seasonal variation in receptor response to ligand binding, a recognized problem of earlier bioassays. Most exciting is the potential that microarray and proteomics approaches have to offer. Such techniques are now used routinely in medical research to identify specific genes and proteins affected by treatment with endocrine disruptors, including estradiol. The technique has yet to be used to screen aquatic organisms, but it has the potential to implicate previously unsuspected estradiol-sensitive genes that may later become molecular markers of endocrine disruption.

Evolution and nomenclature of the zona pellucida gene family

Three subfamilies of genes are acknowledged within the zona pellucida (ZP) gene family. At present, these subfamilies each have two names that are used interchangeably: ZPA or ZP2, ZPB or ZP1, and ZPC or ZP3. The ZPA genes encode the longest protein sequences and the ZPC genes the shortest. Recently, several sequences, which have no clear relationship to the three subfamilies, have been identified. These sequences include two paralogous ZP genes from Xenopus laevis and a single gene from the fish Oryzias latipes. We have conducted extensive phylogenetic analyses of the known ZP genes. As well as establishing the evolutionary relationships among these genes, the analyses make it clear that the dual nomenclature system is no longer feasible, because major paralogous groups are present in the ZPB (ZP1) family of genes of amniotes. We propose a unified system of nomenclature for the ZP gene family that removes the existing ambiguities.

Effects of 17a-ethinylestradiol on the expression of three estrogen-responsive genes and cellular ultrastructure of liver and testes in male zebrafish

In order to monitor the influence of estrogenic compounds on the reproductive physiology of fish, molecular markers for zebrafish vitellogenin, estrogen receptor and ZP2 were developed. For this purpose, sequence information about the zebrafish estrogen receptor and vitellogenin had to be obtained. By means of RT-PCR, a sequence fragment of the zebrafish estrogen receptor alpha was cloned and sequenced. Continuous cDNAs of two zebrafish vitellogenin-like gene products (zfvg1 and zfvg3) were constructed by the help of expressed sequence tags of zebrafish and completely sequenced. The sequences of the estrogen receptor and of the vitellogenins showed significant similarities to corresponding cDNAs of other fish species. Expression of these gene products was measured following exposure to 17alpha-ethinylestradiol and compared with histological endpoints. RT-PCR was used as a semiquantitative technique to record gene expression in adult male zebrafish, which were exposed to 17alpha-ethinylestradiol in time-and dose-response experiments. As for time-dependent expression, all hepatic genes investigated were expressed at considerable amounts from 24 h after onset of exposure to 50 ng/l 17alpha-ethinylestradiol to the end of experiment (17 days). In testes, expression of the estrogen receptor- as well as ZP2-mRNA remained unchanged for the entire experiment, except for the individuals exposed for 17 days, which displayed elevated expression levels of ZP2. In the dose-response experiment, male zebrafish were exposed to 17alpha-ethinylestradiol in concentrations from 0.25-85 ng/l for 4 and 21 days. LOECs for vitellogenin as well as estrogen receptor alpha expression were found to be 2.5 ng/l already after 4 d of exposure. Extension of the exposure time to 21 days resulted in enhanced transcription of vitellogenin-mRNAs at 2.5 ng/l 17alpha-ethinylestradiol, whereas the detection limit could not be lowered. In contrast, in testes no induction of both ZP2 as well as estrogen receptor expression was detected at any concentration tested. To examine estrogen-caused alterations at the ultrastructural level, liver and testes of males exposed to 25 ng/l 17alpha-ethinylestradiol were analysed. Male livers responded with a feminisation reflected by the proliferation of rough endoplasmatic reticulum and Golgi apparatus typical of female hepatocytes during vitellogenesis. However, in testes no signs of feminisation were detectable; rather, destructive phenomena like phagocytosis of sperm cells by Sertoli cells were observed. Thus, in sexually differentiated males no reorganisation of the gonadal tissue towards an ovary could be definitely detected at any level investigated.

Choriogenin mRNA induction in male medaka, Oryzias latipes as a biomarker of endocrine disruption

The fish egg envelope has been known to consist of two distinct layers. The outer layer is thin and is formed around the oocytes during the later stage of previtellogenic development. The inner layer, called zona radiata, is thicker than outer layer and occupies most of the egg envelope. Zona radiata proteins of medaka (Oryzias latipes) consist of two major subunit groups, ZI-1,2 and ZI-3. The precursors of ZI-1,2 and ZI-3 have been named choriogenin H and choriogenin L, respectively. They are synthesized in the liver in response to estrogen, and then released into the blood stream and incorporated into the zona radiata in sexually matured female medaka. However, choriogenin is also induced in male medaka when the fish are exposed to estrogenic chemicals. Choriogenin can be suggested as a possible biomarker of endocrine disruption in fish, however, relatively little has been known about chorogenin gene expression by estrogenic chemicals. In this study, full sequence of choriogenin L DNA was identified, and measurement of choriogenin mRNA induction was established in medaka by use of RT-PCR technique. Also estrogenic effects of industrial chemicals were evaluated by this tool. When mature male medaka was treated with bisphenol A, nonylphenol, and 17alpha-ethinylestradiol for 6 days, respectively, the expression levels of choriogenin L and H mRNA were elevated in treated groups. The induction of choriogenin subunits expression by these chemicals showed a dose-dependent pattern and choriogenin L was found to be more sensitive than choriogenin H.

Purification of serum precursor proteins to vitelline envelope (choriogenins) in masu salmon, Oncorhynchus masou

Three vitelline envelope-related proteins (VERPs), very-high-molecular-weight VERP (vhVERP), high-molecular-weight VERP (hVERP) and low-molecular-weight VERP (lVERP) were purified from female masu salmon serum. The apparent molecular weights of vhVERP, hVERP and lVERP, in their native state, were 520, 88 and 54 kDa, respectively, by gel-filtration chromatography. Very-high-molecular-weight VERP comprises two subunits, corresponding to 175 and 126 kDa. On SDS-PAGE, hVERP and lVERP migrate at 53 and 47 kDa, respectively. Amino acid analysis of vhVERP and hVERP showed that they share a high content of glutamic acid and proline. By contrast, lVERP is rich in glutamic acid and asparatic acid. These features are in good agreement with the amino acid composition of the vitelline envelope. Immuno-biochemical analysis suggested that vhVERP is derived from hVERP by polymerization and/or aggregation. Antibodies against hVERP and lVERP specifically immunostained the vitelline envelope and liver of female masu salmon. In addition, both hVERP and lVERP were induced in the serum of estrogen-treated male fish. Taken together, it is suggested that hVERP and lVERP are homologous molecules with choriogenin H and choriogenin L in medaka, respectively. These results indicate that hVERP and lVERP are precursor proteins to the vitelline envelope (choriogenins) in masu salmon.

Seasonal variations of vitelline envelope proteins, vitellogenin, and sex steroids in male and female eelpout (Zoarces viviparus)

The seasonal variations of vitelline envelope proteins, vitellogenin (VTG), and reproductive steroids were investigated in feral male and female eelpout, Zoarces viviparus. 17beta-Estradiol was present in both sexes with a peak in prespawning fish of 2.6 ng/ml in males and 2.7 ng/ml in females. 11-Ketotestosterone peaked in June at 4.2 and 0.47 ng/ml in males and females, respectively. A surge of testosterone was seen in both sexes in August, just prior to spawning. All steroid levels were low during early pregnancy. The vitelline envelope of the eelpout is composed of two major and one minor protein with molecular weights of 50, 55, and 44 kDa, respectively. An antiserum raised against solubilized vitelline envelope from turbot (Scophthalmus maximus) cross-reacted strongly with the 50-kDa protein from the isolated vitelline envelope and a similar-sized protein in female plasma and plasma from estrogenized males. Interestingly, the 50-kDa protein was also present at low levels in males as demonstrated by ELISA and Western blotting. In males, the 50-kDa protein did not follow the seasonal changes in 17beta-estradiol, but instead showed an almost perfect negative correlation with water temperature. VTG was present in female plasma as shown by Western blotting, but VTG was not detectable in male plasma despite relatively high endogenous estrogen levels. This suggests that the VTG induction by estradiol may be modulated by other factors in the eelpout.

Glycobiology of sperm-egg interactions in deuterostomes

The process of fertilization begins when sperm contact the outermost egg investment and ends with fusion of the two haploid pronuclei in the egg cytoplasm. Many steps in fertilization involve carbohydrate-based molecular recognition between sperm and egg. Although there is conservation of gamete recognition molecules within vertebrates, their homologues have not yet been discovered in echinoderms and ascidians (the invertebrate deuterostomes). In echinoderms, long sulfated polysaccharides act as ligands for sperm receptors. Ascidians employ egg coat glycosides that are recognized by sperm surface glycosidases. Vertebrate egg coats contain zona pellucida (ZP) family glycoproteins, whose carbohydrates bind to sperm receptors. Several candidate sperm receptors for vertebrate ZP proteins have been identified and are discussed here. This brief review focuses on new information concerning fertilization in deuterostomes (the phylogenetic group including echinoderms, ascidians, and vertebrates) and highlights protein-carbohydrate interactions involved in this process.