Green light irradiation during sex differentiation induces female-to-male sex reversal in the medaka Oryzias latipes

Zebrafish Foxl2l functions in proliferating germ cells for female meiotic entry

Zebrafish sex differentiation is a complicated process and the detailed mechanism has not been fully understood. Here we characterized a transcription factor, Foxl2l, which participates in female oogenesis. We show that it is expressed specifically in proliferating germ cells in juvenile gonads and mature ovaries. We have used CRISPR-Cas9 to generate zebrafish deficient in foxl2l expression. Zebrafish with foxl2l-/- are all males, and this female-to-male sex reversal cannot be reversed by tp53 mutation, indicating this sex reversal is unrelated to cell death. We have generated transgenic fish expressing GFP under the control of foxl2l promoter to track the development of foxl2l + -germ cells; these cells failed to enter meiosis and accumulated as cystic cells in the foxl2l-/- mutant. Our RNA-seq analysis also showed the reduced expression of genes in meiosis and oogenesis among other affected pathways. All together, we show that zebrafish Foxl2l is a nuclear factor controlling the expression of meiotic and oogenic genes, and its deficiency leads to defective meiotic entry and the accumulation of premeiotic germ cells.

5 mC modification of steroid hormone biosynthesis-related genes orchestrates feminization of channel catfish induced by high-temperature

To elucidate the mechanism behind channel catfish feminization induced by high temperature, gonad samples were collected from XY pseudo-females and wild-type females and subjected to high-throughput sequencing for Whole-Genome-Bisulfite-Seq (WGBS) and transcriptome sequencing (RNA-Seq). The analysis revealed 50 differentially methylated genes between wild-type females and XY pseudo-females, identified through the analysis of KEGG pathways and GO enrichment in the promoter of the genome and differentially methylated regions (DMRs). Among these genes, multiple differential methylation sites observed within the srd5a2 gene. Repeatability tests confirmed 7 differential methylation sites in the srd5a2 gene in XY pseudo-females compared to normal males, with 1 specific differential methylation site (16608174) distinguishing XY pseudo-females from normal females. Interestingly, the expression of these genes in the transcriptome showed no difference between wild-type females and XY pseudo-females. Our study concluded that methylation of the srd5a2 gene sequence leads to decreased expression, which inhibits testosterone synthesis while promoting the synthesis of 17β-estradiol from testosterone. This underscores the significance of the srd5a2 gene in the sexual differentiation of channel catfish, as indicated by the ipu00140 KEGG pathway analysis.

Genetic Disruption of cyp21a2 Leads to Systemic Glucocorticoid Deficiency and Tissues Hyperplasia in the Teleost Fish Medaka (Oryzias latipes)

cytochrome P-450, 21-hydroxylase (cyp21a2), encodes an enzyme required for cortisol biosynthesis, and its mutations are the major genetic cause of congenital adrenal hyperplasia (CAH) in humans. Here, we have generated a null allele for the medaka cyp21a2 with a nine base-pair insertion which led to a truncated protein. We have observed a delay in hatching and a low survival rate in homozygous mutants. The interrenal gland (adrenal counterpart in teleosts) exhibits hyperplasia and the number of pomca-expressing cells in the pituitary increases in the homozygous mutant. A mass spectrometry-based analysis of whole larvae confirmed a lack of cortisol biosynthesis, while its corresponding precursors were significantly increased, indicating a systemic glucocorticoid deficiency in our mutant model. Furthermore, these phenotypes at the larval stage are rescued by cortisol. In addition, females showed complete sterility with accumulated follicles in the ovary while male homozygous mutants were fully fertile in the adult mutants. These results demonstrate that the mutant medaka recapitulates several aspects of cyp21a2-deficiency observed in humans, making it a valuable model for studying steroidogenesis in CAH.

Integrative analysis of miRNA-mRNA expression in the brain during high temperature-induced masculinization of female Nile tilapia (Oreochromis niloticus)

Temperature is one of the most important non-genetic sex differentiation factors for fish. The technique of high temperature-induced sex reversal is commonly used in Nile tilapia (Oreochromis niloticus) culture, although the molecular regulatory mechanisms involved in this process remain unclear. The brain is an essential organ for the regulation of neural signals involved in germ cell differentiation and gonad development. To investigate the regulatory roles of miRNAs-mRNAs in the conversion of female to male Nile tilapia gender under high-temperature stress, we compared RNA-Seq data from brain tissues between a control group (28 °C) and a high temperature-treated group (36 °C). The result showed that a total of 123,432,984 miRNA valid reads, 288,202,524 mRNA clean reads, 1128 miRNAs, and 32,918 mRNAs were obtained. Among them, there were 222 significant differentially expressed miRNAs (DE miRNAs) and 810 differentially expressed mRNAs (DE mRNAs) between the two groups. Eight DE miRNAs and eight DE mRNAs were randomly selected, and their expression patterns were validated by qRT-PCR. The miRNA-mRNA co-expression network demonstrated that 40 DE miRNAs targeted 136 protein-coding genes. Functional enrichment analysis demonstrated that these genes were involved in several gonadal differentiation pathways, including the oocyte meiosis signaling pathway, progesterone-mediated oocyte maturation signaling pathway, cell cycle signaling pathway and GnRH signaling pathway. Then, an interaction network was constructed for 8 miRNAs (mir-137-5p, let-7d, mir-1388-5p, mir-124-4-5p, mir-1306, mir-99, mir-130b and mir-21) and 10 mRNAs (smc1al, itpr2, mapk1, ints8, cpeb1b, bub1, fbxo5, mmp14b, cdk1 and hrasb) involved in the oocyte meiosis signaling pathway. These findings provide novel information about the mechanisms underlying miRNA-mediated sex reversal in female Nile tilapia.

High-speed system to generate congenic strains in medaka

The congenic strain, an inbred strain containing a small genomic region from another strain, is a powerful tool to assess the phenotypic effect of polymorphisms and/or mutations in the substituted genomic region. Recent substantial progress in the genetic studies of complex traits increases the necessity of congenic strains and, therefore, a quick breeding system for congenic strains has become increasingly important in model organisms such as mouse and medaka. Traditionally, more than ten generations are necessary to produce a congenic strain. In contrast, a quick method has been reported previously for the mouse, in which the use of genetic markers reduces the required number of backcross generations to about a half that of the traditional method, so that it would take around six generations to obtain a congenic strain. Here, we present an even quicker congenic production system, which takes only about four generations. The system can produce medaka congenic strains having part of the HNI-II (an inbred medaka strain derived from the northern Japanese population, Oryzias sakaizumii) genome in the HdrR-II1 (another inbred strain from the southern Japanese population, O. latipes) background. In this system, the availability of frozen sperm and genotype data of the BC1 male population makes it possible to start marker-assisted congenic production after obtaining the BC2 population. Our evaluation revealed that the system could work well to increase the percentage of recipient genome as expected, so that a congenic strain may be obtained in about one year.

Molecular biological, physiological, cytological, and epigenetic mechanisms of environmental sex differentiation in teleosts: A systematic review

Human activities have been exerting widespread stress and environmental risks in aquatic ecosystems. Environmental stress, including temperature rise, acidification, hypoxia, light pollution, and crowding, had a considerable negative impact on the life histology of aquatic animals, especially on sex differentiation (SDi) and the resulting sex ratios. Understanding how the sex of fish responds to stressful environments is of great importance for understanding the origin and maintenance of sex, the dynamics of the natural population in the changing world, and the precise application of sex control in aquaculture. This review conducted an exhaustive search of the available literature on the influence of environmental stress (ES) on SDi. Evidence has shown that all types of ES can affect SDi and universally result in an increase in males or masculinization, which has been reported in 100 fish species and 121 cases. Then, this comprehensive review aimed to summarize the molecular biology, physiology, cytology, and epigenetic mechanisms through which ES contributes to male development or masculinization. The relationship between ES and fish SDi from multiple aspects was analyzed, and it was found that environmental sex differentiation (ESDi) is the result of the combined effects of genetic and epigenetic factors, self-physiological regulation, and response to environmental signals, which involves a sophisticated network of various hormones and numerous genes at multiple levels and multiple gradations in bipotential gonads. In both normal male differentiation and ES-induced masculinization, the stress pathway and epigenetic regulation play important roles; however, how they co-regulate SDi is unclear. Evidence suggests that the universal emergence or increase in males in aquatic animals is an adaptation to moderate ES. ES-induced sex reversal should be fully investigated in more fish species and extensively in the wild. The potential aquaculture applications and difficulties associated with ESDi have also been addressed. Finally, the knowledge gaps in the ESDi are presented, which will guide the priorities of future research.

Energy as the cornerstone of environmentally driven sex allocation

In recent years, observations of distinct organisms have linked the quality of the environment experienced by a given individual and the sex it will develop. In most described cases, facing relatively harsh conditions resulted in masculinization, while thriving in favorable conditions promoted the development of an ovary. This was shown indistinctively in some species presenting a genetic sex determination (GSD), which were able to sex-reverse, and in species with an environmental sex determination (ESD) system. However, this pattern strongly depends on evolutionary constrains and is detected only when females need more energy for reproduction. Here, I describe the mechanisms involved in this environmentally driven sex allocation (EDSA), which involves two main energy pathways, lipid and carbohydrate metabolism. These pathways act through various enzymes and are not necessarily independent of the previously known transducers of environmental signals in species with ESD: calcium-redox, epigenetic, and stress regulation pathways. Overall, there is evidence of a link between energy level and the sexual fate of individuals of various species, including reptiles, fish, amphibians, insects, and nematodes. As energy pathways are evolutionarily conserved, this knowledge opens new avenues to advance our understanding of the mechanisms that allow animals to adapt their sex according to the local environment.

Circular RNA expression profiles and CircSnd1-miR-135b/c-foxl2 axis analysis in gonadal differentiation of protogynous hermaphroditic ricefield eel Monopterus albus

BACKGROUND

The expression and biological functions of circular RNAs (circRNAs) in reproductive organs have been extensively reported. However, it is still unclear whether circRNAs are involved in sex change. To this end, RNA sequencing (RNA-seq) was performed in gonads at 5 sexual stages (ovary, early intersexual stage gonad, middle intersexual stage gonad, late intersexual stage gonad, and testis) of ricefield eel, and the expression profiles and potential functions of circRNAs were studied.

RESULTS

Seven hundred twenty-one circRNAs were identified, and the expression levels of 10 circRNAs were verified by quantitative real-time PCR (qRT-PCR) and found to be in accordance with the RNA-seq data, suggesting that the RNA-seq data were reliable. Then, the sequence length, category, sequence composition and the relationship between the parent genes of the circRNAs were explored. A total of 147 circRNAs were differentially expressed in the sex change process, and GO and KEGG analyses revealed that some differentially expressed (such as novel_circ_0000659, novel_circ_0004005 and novel_circ_0005865) circRNAs were closely involved in sex change. Furthermore, expression pattern analysis demonstrated that both circSnd1 and foxl2 were downregulated in the process of sex change, which was contrary to mal-miR-135b. Finally, dual-luciferase reporter assay and RNA immunoprecipitation showed that circSnd1 and foxl2 can combine with mal-miR-135b and mal-miR-135c. These data revealed that circSnd1 regulates foxl2 expression in the sex change of ricefield eel by acting as a sponge of mal-miR-135b/c.

CONCLUSION

Our results are the first to demonstrate that circRNAs have potential effects on sex change in ricefield eel; and circSnd1 could regulate foxl2 expression in the sex change of ricefield eel by acting as a sponge of mal-miR-135b/c. These data will be useful for enhancing our understanding of sequential hermaphroditism and sex change in ricefield eel or other teleosts.

Effect of Blue Light on the Growth of the Red Swamp Crayfish Procambraus clarkii Larvae -Seasonal and Sexual Differences

Organisms have the ability to adapt their behavior and physiology in response to seasonal changes in their habitat's environments. Although it is known that a specific light wavelength affects growth and reproduction in various animal taxa, its effect on sexual and seasonal differences in year-round breeding animals remains unclear. Here, we demonstrate that a blue light stimulus promotes or suppresses larval growth in the red swamp crayfish Procambarus clarkia depending on the season. During the spawning season (natural growing period), blue light irradiation accelerates female growth faster than in males, but suppresses growth in both females and males in the overwintering season. Moreover, these seasonal plastic effects of blue light show apparent sexual differences, with female juveniles exhibiting the greatest sensitivity. Our findings provide an opportunity to research how the red swamp crayfish can adapt to various habitable niches from the point of view of light color perception, and can be applied for the development of a more effective aquaculture system, not only for crayfish, but also for other commercially available decapod crustaceans using a specific light environment.

Profile of gene expression changes during estrodiol-17β-induced feminization in the Takifugu rubripes brain

Background

As the critical tissue of the central nervous system, the brain has been found to be involved in gonad development. Previous studies have suggested that gonadal fate may be affected by the brain. Identifying brain-specific molecular changes that occur during estrodiol-17β (E₂) -induced feminization is crucial to our understanding of the molecular control of sex differentiation by the brains of fish.

Results

In this study, the differential transcriptomic responses of the Takifugu rubripes larvae brain were compared after E₂ treatment for 55 days. Our results showed that 514 genes were differentially expressed between E₂-treated-XX (E-XX) and Control-XX (C-XX) T. rubripes, while 362 genes were differentially expressed between E₂-treated-XY (E-XY) and Control-XY (C-XY). For example, the expression of cyp19a1b, gnrh1 and pgr was significantly up-regulated, while st, sl, tshβ, prl and pit-1, which belong to the growth hormone/prolactin family, were significantly down-regulated after E₂ treatment, in both sexes. The arntl1, bhlbe, nr1d2, per1b, per3, cry1, cipc and ciart genes, which are involved in the circadian rhythm, were also found to be altered. Differentially expressed genes (DEGs), which were identified between E-XX and C-XX, were significantly enriched in neuroactive ligand-receptor interaction, arachidonic acid metabolism, cytokine-cytokine receptor interaction and the calcium signaling pathway. The DEGs that were identified between E-XY and C-XY were significantly enriched in tyrosine metabolism, phenylalanine metabolism, arachidonic acid metabolism and linoleic acid metabolism.

Conclusion

A number of genes and pathways were identified in the brain of E₂-treated T. rubripes larvae by RNA-seq. It provided the opportunity for further study on the possible involvement of networks in the brain-pituitary-gonadal axis in sex differentiation in T. rubripes.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12864-021-08158-0.

Comparison of differential expression genes in ovaries and testes of Pearlscale angelfish Centropyge vrolikii based on RNA-Seq analysis

Pearlscale angelfish Centropyge vrolikii is a kind of protogynous hermaphrodite fish with a natural sexual reversion. Under appropriate social conditions, a female fish can transform into a male fish spontaneously. It is an important prerequisite for artificial breeding to understand the process of its gonadal development and sexual reversion. Gonadal development is regulated by many sex-related genes. In this study, we used unreferenced RNA-Seq technology to sequence the ovary at the perinucleolus stage (O_II), ovary at the yolk vesicle stage (O_IV),IV and testis (T), respectively; screened the gonadal differential expression genes (DEGs); and analyzed the expression of these genes in different developmental stages of ovary and different sex gonads. The results showed that a total of 142,589 all-unigene samples were assembled, and gene annotation was performed by COG, GO, KEGG, KOG, Pfam, Swissprot, eggNOG, and NR functional database. Comparative analysis revealed that there were 1919 genes that were up-regulated and 1289 genes were down-regulated in comparison to O_IV vs O_II, while there were 3653 genes that were up-regulated and 2874 genes were down-regulated in comparison of O_IV vs T, there were 3345 genes that were up-regulated and 2995 genes were down-regulated in comparison of the O_II vs the T. At the same time, the results verified by RT-qPCR were consistent with the variation trend of transcriptome data. Among the results, amh, sox9b, dmrt1, dmrt2, cyp11a, cyp17a, and cyp19a were significantly expressed in the testes, while sox3, sox4, sox11, sox17, and hsd3b7 were significantly expressed in the ovaries. And, the expression of the amh, sox9b, dmrt2, and dmrt1 were low in the O_II and O_IV, while significantly increased during the ovotestis in the hermaphroditic period (O_T), and finally reached the highest level in pure testis after sex reversal. The expression of sox3, sox4, hsd3b7, sox11, and sox17 was significantly reduced during the hermaphroditic period (O_T). These results suggested that these genes may play an important role in the process of sex reversal. This study is helpful to further understand the molecular regulation mechanism of gonadal development and sexual reversion in Pearlscale angelfish and also provide important clues for future studies.

Metabolism and Sex Differentiation in Animals from a Starvation Perspective

Animals determine their sex genetically (GSD: genetic sex determination) and/or environmentally (ESD: environmental sex determination). Medaka (Oryzias latipes) employ a XX/XY GSD system, however, they display female-to-male sex reversal in response to various environmental changes such as temperature, hypoxia, and green light. Interestingly, we found that 5 days of starvation during sex differentiation caused female-to-male sex reversal. In this situation, the metabolism of pantothenate and fatty acid synthesis plays an important role in sex reversal. Metabolism is associated with other biological factors such as germ cells, HPG axis, lipids, and epigenetics, and supplys substances and acts as signal transducers. In this review, we discuss the importance of metabolism during sex differentiation and how metabolism contributes to sex differentiation.

Effects of the Thyroid Endocrine System on Gonadal Sex Ratios and Sex-Related Gene Expression in the Pufferfish Takifugu rubripes

To examine the effect and mechanism of thyroid hormone on gonadal sex differentiation, Takifugu rubripes larvae were treated with goitrogen (methimazole, MET, 1000 g/g), and thyroxine (T4, 2nM) from 25 to 80 days after hatching (dah). Gonadal histology and sex ratios of fish were then determined at 80 dah. MET treatment induced masculinization, but T4 treatment did not induce feminization in T. rubripes larvae. Transcriptomic analysis of gonads at 80 dah was then conducted. Among the large number of differentially expressed genes between the groups, the expression of foxl2, cyp19a1a, and dmrt1 was altered. The expression of foxl2, cyp19a1a, dmrt1 and gsdf at 25, 40, 55 days after treatment (dat) was further analyzed by qPCR. MET treatment suppressed the expression of foxl2 and cyp19a1a, and induced the expression of dmrt1 in genetic females (p < 0.05). Additionally, T4 treatment induced an increase in the expression of cyp19a1a in genetic XY gonads only at 25 dat. However, the increase in cyp19a1a expression did not continue to 40 and 55 dat. This may explain why feminization of larvae was not found in the T4-treated group. Thus, the present study provides the first evidence that MET treatment causes masculinization in teleost fish. The effects of MET-induced masculinization in T. rubripes may act primarily via suppression of the expression of foxl2 and cyp19a1a, and stimulation of the expression of dmrt1. Moreover, the effects of higher concentrations of T4 or different concentrations of T3, on sex differentiation require further testing.

High temperature stress response is not sexually dimorphic at the whole-body level and is dependent on androgens to induce sex reversal

The understanding of the molecular and endocrine mechanisms behind environmentally-induced sex reversal in fish is of great importance in the context of predicting the potential effects of climate change, especially increasing temperature. Here, we demonstrate the global effects of high temperature on genome-wide transcription in medaka (Oryzias latipes) during early development. Interestingly, data analysis did not show sexual dimorphic changes, demonstrating that thermal stress is not dependent on genotypic sex. Additionally, our results revealed significant changes in several pathways under high temperature, such as stress response from brain, steroid biosynthesis, epigenetic mechanisms, and thyroid hormone biosynthesis, among others. These microarray data raised the question of what the exact molecular and hormonal mechanisms of action are for female-to-male sex reversal under high temperatures in fish. Complementary gene expression analysis revealed that androgen-related genes increase in females (XX) experiencing high water temperature. To test the involvement of androgens in thermal-induced sex reversal, an androgen antagonist was used to treat XX medaka under a high-temperature setup. Data clearly demonstrated failure of female-to-male sex reversal when androgen action is inhibited, corroborating the importance of androgens in environmentally-induced sex reversal.

Sex determination, gonadal sex differentiation, and plasticity in vertebrate species

A diverse array of sex determination (SD) mechanisms, encompassing environmental to genetic, have been found to exist among vertebrates, covering a spectrum from fixed SD mechanisms (mammals) to functional sex change in fishes (sequential hermaphroditic fishes). A major landmark in vertebrate SD was the discovery of the SRY gene in 1990. Since that time, many attempts to clone an SRY ortholog from nonmammalian vertebrates remained unsuccessful, until 2002, when DMY/dmrt1by was discovered as the SD gene of a small fish, medaka. Surprisingly, however, DMY/dmrt1by was found in only 2 species among more than 20 species of medaka, suggesting a large diversity of SD genes among vertebrates. Considerable progress has been made over the last 3 decades, such that it is now possible to formulate reasonable paradigms of how SD and gonadal sex differentiation may work in some model vertebrate species. This review outlines our current understanding of vertebrate SD and gonadal sex differentiation, with a focus on the molecular and cellular mechanisms involved. An impressive number of genes and factors have been discovered that play important roles in testicular and ovarian differentiation. An antagonism between the male and female pathway genes exists in gonads during both sex differentiation and, surprisingly, even as adults, suggesting that, in addition to sex-changing fishes, gonochoristic vertebrates including mice maintain some degree of gonadal sexual plasticity into adulthood. Importantly, a review of various SD mechanisms among vertebrates suggests that this is the ideal biological event that can make us understand the evolutionary conundrums underlying speciation and species diversity.

Activation of stress response axis as a key process in environment-induced sex plasticity in fish

The determination of sex is an important hallmark in the life cycle of organisms, in which the fate of gonads and then the individual sex are defined. In gonochoristic teleost fish, this process is characterized by a high plasticity, considering that in spite of genotypic sex many environmental factors can cause shifts from one to another molecular pathway, resulting in organisms with mismatching genotypic and phenotypic sexes. Interestingly, in most instances, both female-to-male or male-to-female sex-reversed individuals develop functional gonads with normal gametogenesis and respective progenies with full viability. The study of these mechanisms is being spread to other non-model species or to those inhabiting more extreme environmental conditions. Although water temperature is an important mechanism involved in sex determination, there are other environmental stressors affected by the climate change which are also implicated in stress response-induced masculinization in fish. In this regard, the brain has emerged as the transducer of the environment input that can influence the gonadal fate. Furthermore, the evaluation of other environmental stressors or their synergic effect on sex determination at conditions that simulate the natural environments is growing gradually. Within such scope, the concerns related to climate change impacts rely on the fact that many of biotic and abiotic parameters reported to affect sex ratios are expected to increase concomitantly as a result of increased greenhouse gas emissions and, particularly worrying, many of them are related to male bias in the populations, such as high temperature, hypoxia, and acidity. These environmental changes can also generate epigenetic changes in sex-related genes affecting their expression, with implications on sex differentiation not only of exposed individuals but also in following generations. The co-analysis of multi-stressors with potential inter- and transgenerational effects is essential to allow researchers to perform long-term predictions on climate change impacts in wild populations and for establishing highly accurate monitoring tools and suitable mitigation strategies.

Vitellogenins and choriogenins are biomarkers for monitoring Oryzias curvinotus juveniles exposed to 17 β - estradiol

The effect of estrogens on Oryzias curvinotus juveniles were investigated by sequencing the transcriptome of O. curvinotus juveniles exposed to 17 β - estradiol for 24 h. A total of 69,071,524 and 71,210,528 raw reads were obtained for the control group (NC) and 17 β - estradiol exposure group (E2), respectively. After de novo assembly, total 133,210 unigenes were identified, and 85,837 unigenes (64.44% of 133,210) were annotated. Analysis of the transcriptome showed that exposure to 2 μg/L 17 β - estradiol led to the up-regulation of 19 genes and down-regulation of 18 genes. The eef1b and rps4x was most suitable as controls for quantitative real-time PCR (qPCR) using Reffinder. Different expression genes enrichment analysis found that exposed to 2 μg/L 17 β - estradiol affected various physiological processes, including spliceosome, phototransduction, amino sugar and nuclear sugar metabolism, hypotaurine metabolism, and renin-angiotensin system, etc. Exposing O. curvinotus juveniles to increasing concentrations of 17 β - estradiol (2 ng/L, 20 ng/L, 200 ng/L and 2 μg/L) led to significant up-regulation of vitellogenins (vtgs) and choriogenins (chgs) mRNA expression. The present study is the first high-throughput transcriptome sequencing of O. curvinotus juveniles, which will be useful for future functional analysis of genes related to environmental estrogen exposed, and development of biomarkers.

Threshold and spectral sensitivity of vision in medaka Oryzias latipes determined by a novel template wave matching method

We propose a new analytical method for determining the response threshold in electroretinogram (ERG) in which the wave shows a biphasic slow dc-potential shift. This method uses the recorded wave to the highest intensity stimuli in each wavelength tested as a template wave f(t), and it was compared with other recorded waves obtained under lower intensities g(t). Our test recordings in medaka Oryzias latipes were analogous between the template and the compared waveforms, although there were differences in amplitude and time lag (τ, peak time difference) which occurred as a result of the difference in stimulus intensity. Cross-correlation analysis was applied. Based on the obtained cross-correlation function C_fg(τ) in each comparison, τ was determined as the time lag at which the cross-correlation coefficient R_fg(τ) showed the maximum value. Determined thresholds that were based on both the experimenter's visual inspection and this new method agreed well when the adoption condition was set to satisfy R(τ) ≥ 0.7 and τ ≤ 150 ms in scotopic or τ ≤ 120 ms in photopic conditions. We concluded that this "template wave matching method" is a quick and reliable objective assessment that can be used to determine the threshold. This study analyzed ERG recordings in response to 6 kinds of wavelength light stimuli (380 nm to 620 nm) at different photon flux densities. We report the threshold levels and relative spectral sensitivities in scotopic and photopic vision of medaka.