Embryonic development of goldfish (Carassius auratus): a model for the study of evolutionary change in developmental mechanisms by artificial selection

Unveiling the dynamics of embryogenesis and immune genes expression pattern in the amur common carp (Cyprinus carpio haematopterus)

Amur common carp (Cyprinus carpio haematopterus), is a commercially important fish species that has been genetically improved over the years through selective breeding. Despite its significance in aquaculture, limited knowledge exists regarding its embryogenesis and immune genes associated with its early stages of life. This article represents a detailed study of the embryogenesis and innate immune gene expression analysis of the Amur common carp during its ontogenic developments. The entire embryonic developmental process of ∼44 h could be divided into eight periods, beginning with the formation of the zygote, followed by cleavage, morula, blastula, segmentation, pharyngula, and hatching. The segmentation period, which lasted for ∼ 6 h, exhibited the most significant changes, such as muscle contraction, rudimentary heart formation, increased somites number, and the initiation of blood circulation throughout the yolk. The expression of immune-related genes, namely toll-like receptor (TLR)4, nucleotide-binding oligomerization domain (NOD)1, NOD2 and interleukin (IL)-8 showed stage-specific patterns with varying levels of expression across the developmental stages. The TLR4 gene exhibited the highest expression during the neurella stage, while NOD1 and NOD2 peaked during hatching and IL-8 reached its maximum level during the gastrula stage. This is the first report of the innate immune gene expression during the embryogenesis of Amur common carp.

Exploring the origin of a unique mutant allele in twin-tail goldfish using CRISPR/Cas9 mutants

Artificial selection has been widely applied to genetically fix rare phenotypic features in ornamental domesticated animals. For many of these animals, the mutated loci and alleles underlying rare phenotypes are known. However, few studies have explored whether these rare genetic mutations might have been fixed due to competition among related mutated alleles or if the fixation occurred due to contingent stochastic events. Here, we performed genetic crossing with twin-tail ornamental goldfish and CRISPR/Cas9-mutated goldfish to investigate why only a single mutated allele-chdS with a E127X stop codon (also called chdAE127X)-gives rise to the twin-tail phenotype in the modern domesticated goldfish population. Two closely related chdS mutants were generated with CRISPR/Cas9 and compared with the E127X allele in F2 and F3 generations. Both of the CRISPR/Cas9-generated alleles were equivalent to the E127X allele in terms of penetrance/expressivity of the twin-tail phenotype and viability of carriers. These findings indicate that multiple truncating mutations could have produced viable twin-tail goldfish. Therefore, the absence of polymorphic alleles for the twin-tail phenotype in modern goldfish likely stems from stochastic elimination or a lack of competing alleles in the common ancestor. Our study is the first experimental comparison of a singular domestication-derived allele with CRISPR/Cas9-generated alleles to understand how genetic fixation of a unique genotype and phenotype may have occurred. Thus, our work may provide a conceptual framework for future investigations of rare evolutionary events in domesticated animals.

The Goldfish Genome and Its Utility for Understanding Gene Regulation and Vertebrate Body Morphology

Goldfish, widely viewed as an ornamental fish, is a member of Cyprinidae family and has a very long history in research for both genetics and physiology studies. Among Cyprinidae, the chromosomal locations of orthologs and the amino acid sequences are usually highly conserved. Adult goldfish are 1000 times larger than adult zebrafish (who are in the same family of fishes), which can make it easier to perform several types of experiments compared to their zebrafish cousins. Comparing mutant phenotypes in orthologous genes between goldfish and zebrafish can often be very informative and provide a deeper insight into the gene function than studying the gene in either species alone. Comparative genomics and phenotypic comparisons between goldfish and zebrafish will provide new opportunities for understanding the development and evolution of body forms in the vertebrate lineage.

Effect of temperature on embryonic development and first exogenous feeding of goldfish Carassius auratus (Linnaeus, 1758)

Goldfish or Kinguio is a widely marketed species worldwide due to the ornamental market. There is some lack of acknowledgment of the production of the species under specific climatic conditions. To evaluate the effect of temperature on embryonic development and the first exogenous feeding of goldfish, an experiment was proposed. Fifteen incubators, organized in five treatments (18, 22, 26, 30, and 34 °C) with three replications each, were used to keep the fertilized goldfish eggs until the first exogenous feeding of the larvae. The main development events were observed to understand the possible effects of these temperatures on embryos and larvae of the species. Temperature influences embryo development and the time of first exogenous feeding of goldfish. The temperature of 34 °C was lethal to the species causing 100% of anomalies in the embryos and larvae. The experiment data allow us to conclude that the species presents a maximum thermal limit during embryogenesis, and these data are important to the aquaculture industry and to understand the effect of climate changes on goldfish. The data obtained in this experiment will assist in the management of invasive species and production of the species (aquaculture).

Full-Length RNA Sequencing Provides Insights into Goldfish Evolution under Artificial Selection

Goldfish Carassius auratus is an ideal model for exploring fish morphology evolution. Although genes underlying several ornamental traits have been identified, little is known about the effects of artificial selection on embryo gene expression. In the present study, hybrid transcriptome sequencing was conducted to reveal gene expression profiles of Celestial-Eye (CE) and Ryukin (RK) goldfish embryos. Full-length transcriptome sequencing on the PacBio platform identified 54,218 and 54,106 transcript isoforms in CE and RK goldfish, respectively. Of particular note was that thousands of alternative splicing (AS) and alternative polyadenylation (APA) events were identified in both goldfish breeds, and most of them were inter-breed specific. RT-PCR and Sanger sequencing showed that most of the predicted AS and APA were correct. Moreover, abundant long non-coding RNA and fusion genes were detected, and again most of them were inter-breed specific. Through RNA-seq, we detected thousands of differentially expressed genes (DEGs) in each embryonic stage between the two goldfish breeds. KEGG enrichment analysis on DEGs showed extensive differences between CE and RK goldfish in gene expression. Taken together, our results demonstrated that artificial selection has led to far-reaching influences on goldfish gene expression, which probably laid the genetic basis for hundreds of goldfish variations.

Retinal development and the expression profiles of opsin genes during larval development in Takifugu rubripes

The light-sensitive capacity of fish larvae is determined by the structure of the retina and the opsins expressed in the retinal and nonretinal photoreceptors. In this study, the retinal structure and expression of opsin genes during the early developmental stage of Takifugu rubripes larvae were investigated. Histological examination showed that at 1 days after hatching (dah), seven layers were observed in the retina of T. rubripes larva, including the pigment epithelial layer [retinal pigment epithelium layer (RPE)], photoreceptor layer (PRos/is), outer nuclear layer (ONL), outer plexiform layer (OPL), inner nuclear layer (INL), inner plexiform layer (IPL) and ganglion cell layer (GCL). At 2 dah, optic fibre layer (OFL) can be observed, and all eight layers were visible in the retina. By measuring the thickness of each layer, opposing developmental trends were found in the thickness of ONL, OPL, INL, IPL, GCL and OFL. The nuclear density of ONL, INL and GCL and the ratios of ONL/INL, ONL/GCL and INL/GCL were also measured and the ratio of ONL/GCL ranged from 1.9 at 2 dah to 3.4 at 8 dah and no significant difference was observed between the different developmental stages (P > 0.05). No significant difference was observed for the INL/GCL ratio between the different developmental stages, which ranged from 1.2 at 2 dah to 2.0 at 18 dah (P > 0.05). The results of quantitative real-time polymerase chain reaction (PCR) showed that the expression of RH1, LWS, RH2-1, RH2-2, SWS2, rod opsin, opsin3 and opsin5 could be detected from 1 dah. These results suggest that the well-developed retina and early expression of the opsins of T. rubripes during the period of transition from endogenous to mixed feeding might be critical for vision-based survival skills during the early life stages after hatching.

Pleiotropic functions of chordin gene causing drastic morphological changes in ornamental goldfish

Breeders and fanciers have established many peculiar morphological phenotypes in ornamental goldfish. Among them, the twin-tail and dorsal-finless phenotypes have particularly intrigued early and recent researchers, as equivalent morphologies are extremely rare in nature. These two mutated phenotypes appeared almost simultaneously within a short time frame and were fixed in several strains. However, little is known about how these two different mutations could have co-occurred during such a short time period. Here, we demonstrate that the chordin gene, a key factor in dorsal-ventral patterning, is responsible not only for the twin-tail phenotype but also for the dorsal-finless phenotype. Our F2 backcrossing and functional analyses revealed that the penetrance/expressivity of the dorsal-finless phenotype can be suppressed by the wild-type allele of chdS. Based on these findings, we propose that chdS^wt may have masked the expression of the dorsal-finless phenotype, acting as a capacitor buffering gene to allow accumulation of genetic mutations. Once this gene lost its original function in the twin-tail goldfish lineages, the dorsal-finless phenotype could be highly expressed. Thus, this study experimentally demonstrates that the rapid genetic fixation of morphological mutations during a short domestication time period may be related to the robustness of embryonic developmental mechanisms.

Correlations of expression of nuclear and mitochondrial genes in triploid fish

The expression of nuclear and mitochondrial genes, as well as their coordinated control, regulates cell proliferation, individual development, and disease in animals. However, the potential coregulation between nuclear and mitochondrial genes is unclear in triploid fishes. The two triploids (R2C and RC2) with distinct mitochondrial genomes but similar nuclear genomes exhibit different embryonic development times and growth rates. They are an excellent model for studying how nuclear and mitochondrial genes coordinate. Here, we performed the mRNA-seq of four stages of embryonic development (blastula, gastrula, segmentation, and hatching periods) in the two triploids (R2C and RC2) and their diploid inbred parents (red crucian carp and common carp). After establishing the four patterns of mitochondrial and nuclear gene expression, 270 nuclear genes regulated by mitochondrial genes were predicted. The expression levels of APC16 and Trim33 were higher in RC2 than in R2C, suggesting their potential effects on regulating embryonic development time. In addition, 308 differentially expressed genes filtered from the list of nuclear-encoded mitochondrial genes described by Mercer et al. in 2011 were considered potential genes for which nuclear genes regulate mitochondrial function. The findings might aid in our understanding of the correlation between mitochondrial and nuclear genomes as well as their synergistic effects on embryonic development.

The real Nemo movie: Description of embryonic development in Amphiprion ocellaris from first division to hatching

BACKGROUND

Amphiprion ocellaris is one of the rare reef fish species that can be reared in aquaria. It is increasingly used as a model species for Eco-Evo-Devo. Therefore, it is important to have an embryonic development table based on high quality images that will allow for standardized sampling by the scientific community.

RESULTS

Here we provide high-resolution time-lapse videos to accompany a detailed description of embryonic development in A ocellaris. We describe a series of developmental stages and we define six broad periods of embryogenesis: zygote, cleavage, blastula, gastrula, segmentation, and organogenesis that we further subdivide into 32 stages. These periods highlight the changing spectrum of major developmental processes that occur during embryonic development.

CONCLUSIONS

We provide an easy system for the determination of embryonic stages, enabling the development of A ocellaris as a coral reef fish model species. This work will facilitate evolutionary development studies, in particular studies of the relationship between climate change and developmental trajectories in the context of coral reefs. Thanks to its lifestyle, complex behavior, and ecology, A ocellaris will undoubtedly become a very attractive model in a wide range of biological fields.

Stages of embryonic development in the live-bearing fish, Gambusia holbrooki

BACKGROUND

Divergent morphology and placentation of Poeciliids make them suitable model for investigating how evolutionary selection has altered and conserved the developmental mechanisms. However, there is limited description of their embryonic staging, despite representing a key evolutionary node that shares developmental strategy with placental vertebrates. Here, we describe the embryonic developmental stages of Gambusia holbrooki from zygote to parturition using freshly harvested embryos.

RESULTS

We defined 40 embryonic stages using a numbered (stages 0-39; zygote to parturition, respectively) and named (grouped into seven periods, ie, zygote, cleavage, blastula, gastrula, segmentation, pharyngula, and parturition) staging system. Two sets of quantitative (ie, egg diameter, embryonic total length, otic vesicle closure index, heart rates, the number of caudal fin rays and elements) and qualitative (ie, three-dimensional analysis of images and key morphological criteria) data were acquired and used in combination to describe each stage. All 40 stages are separated by well-defined morphological traits, revealing developmental novelties that are influenced by narrow perivitelline space, placentation, internal gestation, and sex differentiation.

CONCLUSIONS

The principal diagnostic features described are quick, reliable, and easy to apply. This system will benefit researchers investigating molecular ontogeny, particularly sexual differentiation mechanisms in G. holbrooki.

Normal stages of embryonic development of a brood parasite, the rosy bitterling Rhodeus ocellatus (Teleostei: Cypriniformes)

Bitterlings, a group of freshwater teleosts, provide a fascinating example among vertebrates of the evolution of brood parasitism. Their eggs are laid inside the gill chamber of their freshwater mussel hosts where they develop as brood parasites. Studies of the embryonic development of bitterlings are crucial in deciphering the evolution of their distinct early life-history. Here, we have studied 255 embryos and larvae of the rosy bitterling (Rhodeus ocellatus) using in vitro fertilization and X-ray microtomography (microCT). We describe 11 pre-hatching and 13 post-hatching developmental stages spanning the first 14 days of development, from fertilization to the free-swimming stage. In contrast to previous developmental studies of various bitterling species, the staging system we describe is character-based and therefore more compatible with the widely-used stages described for zebrafish. Our bitterling data provide new insights into to the polarity of the chorion, and into notochord vacuolization and yolk sac extension in relation to body straightening. This study represents the first application of microCT scanning to bitterling development and provides one of the most detailed systematic descriptions of development in any teleost. Our staging series will be an important tool for heterochrony analysis and other comparative studies of teleost development, and may provide insight into the co-evolution of brood parasitism.

Relaciones alométricas en estadios tempranos de la especie Brycon moorei Steindachner (Characidae), en condiciones controladas

Para Brycon moorei, los datos sobre morfometría, en estados iniciales del desarrollo, son inexistentes. El objetivo de este estudio fue establecer las principales relaciones alométricas en los estadios tempranos de B. moorei. Se analizaron 12 variables merísticas en un tiempo experimental que transcurrió desde la eclosión hasta los 15 días post-eclosión. En el estadio larval vitelino (LV) se registró, en promedio: longitud total (LT) de 4,74 ± 0,05 mm; longitud estándar (LE) de 4,21 ± 0,07 mm; longitud del hocico (SnL) 0,3 ± 0,01; diámetro del ojo (ED) 0,37 ± 0,007 mm; longitud de la cabeza (HL) 1,02 ± 0,04 mm; altura de la cabeza (HH) 0,88 ± 0,069 mm; altura del cuerpo (BH) 1,26 ± 0,01 mm; longitud hocico-aleta pectoral (SnPF) 1,35 ± 0,034; ausencia de aletas pélvicas, anales y dorsal y el peso fue de 1,8 ± 0,2 mg. En la etapa juvenil (J), las medidas fueron: LT de 25,66 ± 0,64 mm; LE de 21,49 ± 0,63 mm; SnL de 1,69 ± 0,059 mm; ED de 1,71 ± 0,048 mm; HL de 6,33 ± 0,21 mm; HH de 4,89 ± 0,47 mm; BH de 6,13 ± 0,17 mm; SnPF de 6,48 ± 0,19 mm; SnPeF de 10,83 ± 0,28 mm; SnDF de 11,16 ± 0,33 mm; Sn-AF de 3,68 ± 0,34 mm; el peso fue de 221,6 ± 15,84 mg. Para LV, el número de miómeros pre-anales fue 28,65 ± 4,7; los post-anales fueron 23,85 ± 4,27, con un total de 52,47 ± 8,34; para J los miómeros no fueron visibles. Esta información es fundamental para fines ecológicos y productivos en B. moorei.

Developmental potential of somatic and germ cells of hybrids between Carassius auratus females and Hemigrammocypris rasborella males

The cause of hybrid sterility and inviability has not been analyzed in the fin-fish hybrid, although large numbers of hybridizations have been carried out. In this study, we produced allo-diploid hybrids by cross-fertilization between female goldfish (Carassius auratus) and male golden venus chub (Hemigrammocypris rasborella). Inviability of these hybrids was due to breakage of the enveloping layer during epiboly or due to malformation with serious cardiac oedema around the hatching stage. Spontaneous allo-triploid hybrids with two sets of the goldfish genome and one set of the golden venus chub genome developed normally and survived beyond the feeding stage. This improved survival was confirmed by generating heat-shock-induced allo-triploid hybrids that possessed an extra goldfish genome. When inviable allo-diploid hybrid cells were transplanted into goldfish host embryos at the blastula stage, these embryos hatched normally, incorporating the allo-diploid cells. These allo-diploid hybrid cells persisted, and were genetically detected in a 6-month-old fish. In contrast, primordial germ cells taken from allo-diploid hybrids and transplanted into goldfish hosts at the blastula stage had disappeared by 10 days post-fertilization, even under chimeric conditions. In allo-triploid hybrid embryos, germ cells proliferated in the gonad, but had disappeared by 10 weeks post-fertilization. These results showed that while hybrid germ cells are inviable even in chimeric conditions, hybrid somatic cells remain viable.

The onset of piscivory in a freshwater fish species: analysis of behavioural and physiological traits

The onset of piscivory in fish, resulting in a shift from zooplankton or invertebrate to fish prey, was studied on pikeperch (Sander lucioperca) larvae displaying and not displaying piscivorous behaviour at different ages (23, 30, 37, 44 and 52 days post-hatching) using behavioural (attack, capture and swimming activity), morphological (allometry) and digestive enzymatic (trypsin, α-amylase and pepsin) analyses. The shift from zooplanktonic food items (Artemia nauplii) to a piscivorous diet did not occur at the same time for all individuals within the same cohort. Predation tests, conducted under controlled conditions (20°C; ad libitum feeding), showed that some larvae attacked fish prey as early as the age of 3 weeks [11.0 ± 1.3 mm total length (TL)], whereas others did not start until the age of 6 weeks (16.6 ± 1.9 mm TL). Piscivorous individuals were bigger, with larger heads, longer tails, higher acid protease and lower alkaline protease activities, than non-piscivorous conspecifics. In conclusion, high interindividual variability in morphological and digestive system developments linked to the development of predatory abilities could induce cannibalism in fish.

The Genetic Basis of Morphological Diversity in Domesticated Goldfish

Although domesticated goldfish strains exhibit highly diversified phenotypes in morphology, the genetic basis underlying these phenotypes is poorly understood. Here, based on analysis of transposable elements in the allotetraploid goldfish genome, we found that its two subgenomes have evolved asymmetrically since a whole-genome duplication event in the ancestor of goldfish and common carp. We conducted whole-genome sequencing of 27 domesticated goldfish strains and wild goldfish. We identified more than 60 million genetic variations and established a population genetic structure of major goldfish strains. Genome-wide association studies and analysis of strain-specific variants revealed genetic loci associated with several goldfish phenotypes, including dorsal fin loss, long-tail, telescope-eye, albinism, and heart-shaped tail. Our results suggest that accumulated mutations in the asymmetrically evolved subgenomes led to generation of diverse phenotypes in the goldfish domestication history. This study is a key resource for understanding the genetic basis of phenotypic diversity among goldfish strains.

Morphogenesis of Olfactory Organ of Bushymouth Catfish Ancistrus dolichopterus (Teleostei: Loricariidae) before Switching to Exogenous Feeding

Olfaction plays an important role in a fish's life. Its value may differ at different developmental stages, depending on the feeding style of the species. The goal of the present study was to investigate the olfactory organ of a species that feeds mainly on algae- the bushymouth catfish, Ancistrus dolichopterus-at developmental stages from olfactory placode to the definitive olfactory chamber. For this study, we used light and electron (scanning) microscopy. The topography of the olfactory placode of A. dolichopterus is typical for teleostei. Formation of olfactory pit takes place at the same time as rostral elevation formation. Rostral elevation participates in the formation of the nasal bridge and anterior tubular nostril. It was found out that the anlage of olfactory rosette in A. dolichopterus arises earlier than in most teleostei. However, the number of lamellae does not increase until switching to exogenous feeding. We suppose that the early development of olfactory organ is necessary for intraspecific communication, not just for finding food.

Structure, development, and functional morphology of the cement gland of the giant danio, Devario malabaricus

BACKGROUND

Aquatic species in several clades possess cement glands producing adhesive secretions of various strengths. In vertebrates, transient adhesive organs have been extensively studied in Xenopus laevis, other anurans, and in several fish species. However, the development of these structures is not fully understood.

RESULTS

Here, we report on the development and functional morphology of the adhesive gland of a giant danio species, Devario malabaricus. We found that the gland is localized on the larval head, is composed of goblet-like secretory cells framed by basal, bordering, and intercalated apical epithelial cells, and is innervated by the trigeminal ganglion. The gland allows nonswimming larvae to adhere to various substrates. Its secretory cells differentiate by 12 hours postfertilization and begin to disappear in the second week of life. Exogenous retinoic acid disrupts the gland's patterning. More importantly, the single mature gland emerges from fusion of two differentiated secretory cells fields; this fusion is dependent on nonmuscle myosin II function.

CONCLUSIONS

Taken together, our studies provide the first documentation of the embryonic development, structure, and function of the adhesive apparatus of a danioninae. To our knowledge, this is also the first report of a cement gland arising from convergence of two bilateral fields.

Embryonic development of the bullseye puffer Sphoeroides annulatus (Tetraodontidae): A morphofunctional approach to ontogenetic steps

The embryonic development of the bullseye puffer, Sphoeroides annulatus, was characterized on the basis of the theory of saltatory ontogeny. This theory predicts a correlative relationship between the ontogeny-type in an altricial-precocial spectrum and the habitat that a species occupies within an unstable-stable environmental spectrum. Because S. annulatus inhabits a variety of unstable environments along a wide latitudinal range, the hypothesis that this species presents one of the most altricial embryonic developments among tetraodontids was tested. Based on major developmental events that marked the ontogenetic thresholds nine embryonic steps were identified. Developmental features such as small adhesives eggs, lack of vitelline circulation, small free embryos swimming up at hatching guided by positive phototaxis, and small first-feeding larvae actively swam in the water column, suggest that S. annulatus belongs to the reproductive guild of the nonguarders-lithopelagophils. Moreover, a comparative analysis of the developmental sequences, egg size, and first-feeding larvae size between tetraodontids confirms the hypothesis of this study and supports the evolutionary principle of the altricial-precocial spectrum postulated in the theory of saltatory ontogeny.

Frequent nonrandom shifts in the temporal sequence of developmental landmark events during teleost evolutionary diversification

Morphological transformations can be generated by evolutionary changes in the sequence of developmental events. In this study, we examined the evolutionary dynamics of the developmental sequence on a macroevolutionary scale in teleosts. Using the information from previous reports describing the development of 31 species, we extracted the developmental sequences of 19 landmark events involving the formation of phylogenetically conserved body parts; we then inferred ancestral developmental sequences by two different parsimony-based methods-event-pairing and continuous analysis. The phylogenetic comparisons of these sequences revealed event-dependent heterogeneity in the frequency of sequence changes. Most of the sequence changes occurred as exchanges of temporally neighboring events. These heterochronic changes in developmental sequences accumulated along evolutionary time, but the precise distribution of the changes over the teleostean phylogeny remains unclear due to technical limitations.

Mahi-mahi (Coryphaena hippurus) life development: morphological, physiological, behavioral and molecular phenotypes

Background

Mahi‐mahi (Coryphaena hippurus) is a commercially and ecologically important fish species that is widely distributed in tropical and subtropical waters. Biological attributes and reproductive capacities of mahi‐mahi make it a tractable model for experimental studies. In this study, life development of cultured mahi‐mahi from the zygote stage to adult has been described.

Results

A comprehensive developmental table has been created reporting development as primarily detailed observations of morphology. Additionally, physiological, behavioral, and molecular landmarks have been described to significantly contribute in the understanding of mahi life development.

Conclusion

Remarkably, despite the vast difference in adult size, many developmental landmarks of mahi map quite closely onto the development and growth of Zebrafish and other warm‐water, active Teleost fishes.

Mahi‐mahi is a tractable model for experimental studies high‐performance pelagic predatory fish species.

Biological attributes of mahi are reported in a comprehensive developmental table.

Physiological, behavioral and molecular landmarks are described through the life development.

Mahi has a rapid growth rate, but the developmental marks are similar to other teleost fishes.

An alternative evolutionary pathway for the twin-tail goldfish via szl gene mutation

The twin‐tail of ornamental goldfish provides unique evolutionary evidence that the highly conserved midline localization of axial skeleton components can be changed by artificial selection. This morphological change is known to be caused by a nonsense mutation in one of the recently duplicated chordin genes, which are key players in dorsal–ventral (DV) patterning. Since all of the multiple twin‐tail ornamental goldfish strains share the same mutation, it is reasonable to presume that this mutation occurred only once in domesticated goldfish. However, zebrafish with mutated szl gene (another DV patterning‐related gene) also exhibit twin‐tail morphology and higher viability than dino/chordin‐mutant zebrafish. This observation raises the question of whether the szl gene mutation could also reproduce the twin‐tail morphology in goldfish. Here we show that goldfish have at least two subfunctionalized szl genes, designated szlA and szlB, and depletion of these genes in single‐fin goldfish was able to reproduce the bifurcated caudal fin found in twin‐tail ornamental goldfish. Interestingly, several phenotypes were observed in szlA‐depleted fish, while low expressivity of the twin‐tail phenotype was observed in szlB‐depleted goldfish. Thus, even though szl gene mutations may produce twin‐tail goldfish, these szl gene mutations might not be favorable for selection in domestic breeding. These results highlight the uniqueness and rarity of mutations that are able to cause large‐scale morphological changes, such as a bifurcated axial skeleton, with high viability and expressivity in natural and domesticated populations.

Goldfish: an old and new model system to study vertebrate development, evolution and human disease

The goldfish (Carassius auratus) is a domesticated cyprinid teleost closely related to the crucian carp. Goldfish domestication occurred in South China around 1,000 years ago. At least 180 variants and 70 genetically established strains are currently produced. These strains possess diverse phenotypes in body shape, colouration, scales, and fin, eye and hood morphology. These include biologically interesting phenotypes that have not been observed in mutants of zebrafish or medaka. In addition, goldfish strains have been maintained in a non-wild environment for several hundreds of generations, and certain goldfish strains have phenotypes similar to some human diseases. The recent progress in the assembly of the whole-genome sequence of goldfish provides strong tools for a genetic analysis of these phenotypes. The whole-genome duplication (WGD) event occurred in the goldfish genome 8-14 million years ago; this is one of the latest WGD in vertebrates. Goldfish are a useful model for studying genome evolution after the WGD event. This review focuses on the potential for goldfish as a model system in understanding the molecular basis of vertebrate development and evolution and human diseases.

Insertional mutagenesis in ChordinA induced by endogenous ΔTgf2 transposon leads to bifurcation of axial skeletal systems in grass goldfish

The grass goldfish appeared early in the evolutionary history of goldfish, and shows heritable stability in the development of the caudal fin. The twin-tail phenotype is extremely rare, however, some twin-tail individuals were produced in the process of breeding for ornamental value. From mutations in the twin-tail goldfish genome, we identified two kinds of Tgf2 transposons. One type was completely sequenced Tgf2 and the other type was ΔTgf2, which had 858 bp missing. We speculate that the bifurcation of the axial skeletal system in goldfish may be caused by an endogenous ΔTgf2 insertion mutation in Chordin A, as ΔTgf2 has no transposition activity and blocks the expression of Chordin A. The twin-tail showed doubled caudal fin and accumulation of red blood cells in the tail. In addition, in situ hybridization revealed that ventral embryonic tissue markers (eve1, sizzled, and bmp4) were more widely and strongly expressed in the twin-tail than in the wild-type embryos during the gastrula stage, and bmp4 showed bifurcated expression patterns in the posterior region of the twin-tail embryos. These results provide new insights into the artificial breeding of genetically stable twin-tail grass goldfish families.

Embryonic and postembryonic development of the ornamental twin-tail goldfish

Background

Twin‐tail ornamental goldfish have “bifurcated median fins,” a peculiar morphology known to be caused by a mutation in the chdA gene. However, several ambiguities regarding the development of the phenotype remain due to a paucity of detailed observations covering the entire developmental timeframe.

Results

Here, we report a detailed comparative description of embryonic and postembryonic development for two representative twin‐tail ornamental goldfish strains and single‐tail common goldfish. Our observations reveal a polymorphic developmental process for bifurcated median fins; disrupted axial skeletal development at early larval stages; and modified bilateral location of the pelvic fin.

Conclusions

Variations in development of bifurcated median fins and disrupted axial skeletal patterns reflect how artificial selection for adult morphological features influenced molecular developmental mechanisms during the domestication of twin‐tail ornamental goldfish. The polymorphic appearance of bifurcated median fins also implies that, unlike previously proposed hypotheses, the development of these structures is controlled by molecular mechanisms independent of those acting on the pelvic fin. Our present findings will facilitate further study of how modifications of preexisting developmental systems may contribute to novel morphological features. Developmental Dynamics 248:251–283, 2019. © 2019 The Authors. Developmental Dynamics published by Wiley Periodicals, Inc. on behalf of American Association of Anatomists.

This is the first complete study to describe the developmental progression of twin‐tail goldfish. Disrupted axial skeletal morphology in adults develops from a modified osteogenesis process in vertebral elements. The developmental processes for not only the caudal and anal fins, but also pelvic fin, were changed by artificial selection in twin‐tail goldfish. Polymorphic anal and caudal fin development suggested that in addition to the mutation in the chdA gene, other relevant mutations have accumulated in the twin‐tail goldfish. Our developmental observations pave the way to study how the pre‐existing developmental systems were modified by selective pressure for the formation of a novel morphology.

A novel allele of the goldfish chdB gene: Functional evaluation and evolutionary considerations

The twin tail of ornamental goldfish is known to be caused by a nonsense mutation in one chordin paralogue gene. Our previous molecular studies in goldfish revealed that the ancestral chordin gene was duplicated, creating the chdA and chdB genes, and the subsequent introduction of a stop codon allele in the chdA gene ( chdA^E127X) caused the twin‐tail morphology. The chdA^E127X allele was positively selected by breeders, and the allele was genetically fixed in the ornamental twin‐tail goldfish population. However, little is known about the evolutionary history of the chdB paralogue, begging the question: are there the functionally distinct alleles at the chdB locus, and if so, how did they evolve? To address these questions, we conducted molecular sequencing of the chdB gene from five different goldfish strains and discovered two alleles at the chdB gene locus; the two alleles are designated chdB¹ and chdB². The chdB¹ allele is the major allele and was found in all investigated goldfish strains, whereas the chdB² allele is minor, having only been found in one twin‐tail strain. Genetic analyses further suggested that these two alleles are functionally different with regard to survivability ( chdB¹ >  chdB²). These results led us to presume that in contrast to the chdA locus, the chdB locus has tended to be eliminated from the population. We also discuss how the chdB² allele was retained in the goldfish population, despite its disadvantageous function. This study provides empirical evidence of the long‐term retention of a disadvantageous allele under domesticated conditions.

An embryonic staging series up to hatching for Cyprinodon variegatus: An emerging fish model for developmental, evolutionary, and ecological research

Using multiple taxa to research development is necessary for making general conclusions about developmental patterns and mechanisms. We present a staging series for Cyprinodon variegatus as a basis for further study of the developmental biology of fishes in the genus Cyprinodon and for comparative work on teleost fishes beyond the standard models. Cyprinodon are small, euryhaline fishes, widely distributed in fresh, brackish, and hypersaline waters of southern and eastern North America. Cyprinodontids are closely related to fundulids, providing a comparative reference point to the embryological model, Fundulus heteroclitus. Ecologists and evolutionary biologists commonly study Cyprinodon, and we have been using Cyprinodon to study skull variation and its genetic basis among closely related species. We divided embryonic development of C. variegatus into 34 morphologically identifiable stages. We reference our staging series to that already defined for a related model species, Oryzias latipes (medaka) that is studied by a large community of researchers. We provide a description of the early chondrogenesis and ossification of skull and caudal fin bones during the latter stages of embryonic development. We show that Cyprinodon are tractable for studying development. Eggs can be obtained easily from breeding pairs and our study provides a staging system to facilitate future developmental studies.

The Formation of the Goldfish-Like Fish Derived From Hybridization of Female Koi Carp × Male Blunt Snout Bream

Goldfish (Carassius auratus var., GF; 2n = 100) is the most popular ornamental fish in the world. It is assumed that GF evolved from red crucian carp (C. auratus red var., RCC; 2n = 100). However, this hypothesis lacks direct evidence. Furthermore, our knowledge of the role of hybridization in the formation of new species is sparse. In this study, goldfish-like fish with twin tails (GF-L; 2n = 100) was produced by self-mating red crucian carp-like fish (RCC-L; 2n = 100) derived from the distant crossing of koi carp (Cyprinus carpio haematopterus, KOC; 2n = 100; ♀) with blunt snout bream (Megalobrama amblycephala, BSB; 2n = 48; ♂). The phenotypes and genotypes of GF-L and RCC-L were very similar to those of GF and RCC, respectively. Microsatellite DNA and 5S rDNA analyses revealed that GF-L and RCC-L were closely related to GF and RCC, respectively. The presence of a twin tail of GF-L was related to a base mutation in chordinA from G in RCC-L to T in GF-L, indicating that the lineage of RCC-L and GF-L can be used to study gene variation and function. The sequences of 5S rDNA in GF-L and RCC-L were mapped to the genomes of CC and BSB, which revealed that the average similarities of both GF-L and RCC-L to CC were obviously higher than those to BSB, supporting that the genomes of both RCC-L and GF-L were mainly inherited from KOC. GF-L and RCC-L were homodiploids that were mainly derived from the genome of KOC with some DNA fragments from BSB. The reproductive traits of GF-L and RCC-L were quite different from those of their parents, but were the same as those of GF and RCC. RCC-L easily diversified into GF-L, suggesting that RCC and GF evolved within the same period in their evolutionary pathway. This study provided direct evidence of the KOC-RCC-GF evolutionary pathway that was triggered by distant hybridization, which had important significance in evolutionary biology and genetic breeding.

Hormonal induction of ovulation using Ovaprim™ [(D-Arg6, Pro9NEt)-sGnRH+domperidone] and its impact on embryonic development of wild-caught Longspine scraper, Capoeta trutta (Heckel, 1843)

Knowledge of gamete quality is a prerequisite for developing techniques to fertilize eggs and rear offspring for hatchery production. Our objective was to develop assisted reproductive techniques, via hormonal induction of final oocyte maturation (FOM), for Longspine scraper, Capoeta trutta. Fish were administered injections of salmon gonadotropin-releasing hormone analogue containing anti-dopaminergic drug (Ovaprim™) or saline (control). Effects of Ovaprim on induction of ovulation, gamete quality, embryonic development, and larval survival were later examined with serum steroid hormone levels and ovarian histology. The saline group failed to spawn, whereas Ovaprim accelerated FOM and induced spawning. Fish treated with Ovaprim showed an increase in gonadosomatic index, egg diameter, and wet weight relative to controls. Average absolute fecundity, relative fecundity, fertilization, and hatching rates were 8823 eggs/spawn, 53 eggs/g body weight, 95%, and 91%, respectively. Serum 17α,20β-dihydroxy-4-pregnen-3-one (DHP) levels were significantly enhanced by ∼4-fold in Ovaprim-treated fish compared to the saline-injected fish, while 17β-estradiol levels declined upon FOM in hormone treated fish. Embryonic development closely resembled the teleost scheme, despite variations in timing. Larval survival at 6 and 12days post-hatch were 98% and 95%, respectively. Results suggest that Ovaprim is efficient for inducing spawning in C. trutta for stock enhancement or hatchery purposes.

Development, and effect of water temperature on development rate, of pikeperch Sander lucioperca embryos

Knowledge of embryo development is essential to the application of reproductive biotechnology in aquaculture, including for pikeperch Sander lucioperca. We describe pikeperch embryo development and demonstrated effects of temperature on the duration of embryogenesis. Developmental stages in embryos incubated at 15 °C were identified as zygote, 0-1.5 h post-fertilization (hpf); cleavage, 2.5-7.5 hpf; blastula, 9-18.75 hpf; gastrula, 21-39, hpf; segmentation, 45-105 hpf; and hatching, 125-197 hpf. Additional groups of eggs were fertilized and incubated at 10, 15, 20, and 25 °C to document stages of development, development rate, and survival. The optimal fertilization and incubation temperature was shown to be 15 °C, with the highest fertilization, survival, and hatching rates. Embryo development was slower at 10 °C, with 45% of fertilized embryos surviving to hatching. Development was accelerated at 20 °C, and resulted in a 56% survival rate of fertilized embryos. At 25 °C, embryos did not develop to the blastula stage. Pikeperch could be a valuable percid model for research in which flexible incubation temperatures is required.

Open and closed evolutionary paths for drastic morphological changes, involving serial gene duplication, sub-functionalization, and selection

Twin-tail goldfish strains are examples of drastic morphological alterations that emerged through domestication. Although this mutation is known to be caused by deficiency of one of two duplicated chordin genes, it is unknown why equivalent mutations have not been observed in other domesticated fish species. Here, we compared the chordin gene morphant phenotypes of single-tail goldfish and common carp (close relatives, both of which underwent chordin gene duplication and domestication). Morpholino-induced knockdown depleted chordin gene expression in both species; however, while knockdown reproduced twin-tail morphology in single-tail goldfish, it had no effect on common carp morphology. This difference can be explained by the observation that expression patterns of the duplicated chordin genes overlap completely in common carp, but are sub-functionalized in goldfish. Our finding implies that goldfish drastic morphological changes might be enhanced by the subsequent occurrence of three different types of evolutionary event (duplication, sub-functionalization, and selection) in a certain order.

Effect of temperature during embryonic development and first feeding of Trichogaster leeri larvae

Temperature is an environmental factor that influences the development of fish, and when changed abruptly can lead to high mortality. Some species of fish are influenced by this factor, exhibiting a longer time for embryonic development and time to first feeding. This study aims to evaluate the effect of water temperature on embryonic and larval development up to first feeding, to describe the time in hours post fertilization (hpf) of the emergence of different structures and to determine the best hatching rate and survival of animals under different treatments. Five different egg incubation temperatures were used (24, 26, 28, 30 or 32°C, respectively). The eggs were observed at regular intervals of 30 min up to 24 h, every 2 h until 48 h and every 4 h until the display of first feeding in all treatments. Embryonic development was longer for eggs incubated at 24°C and the best results for hatching rate and survival of spawning efficiency were at 28°C. We recommend that incubation of Trichogaster leeri eggs is carried out at 28°C up to the first feeding of larvae.

Goldfish morphology as a model for evolutionary developmental biology

Morphological variation of the goldfish is known to have been established by artificial selection for ornamental purposes during the domestication process. Chinese texts that date to the Song dynasty contain descriptions of goldfish breeding for ornamental purposes, indicating that the practice originated over one thousand years ago. Such a well-documented goldfish breeding process, combined with the phylogenetic and embryological proximities of this species with zebrafish, would appear to make the morphologically diverse goldfish strains suitable models for evolutionary developmental (evodevo) studies. However, few modern evodevo studies of goldfish have been conducted. In this review, we provide an overview of the historical background of goldfish breeding, and the differences between this teleost and zebrafish from an evolutionary perspective. We also summarize recent progress in the field of molecular developmental genetics, with a particular focus on the twin-tail goldfish morphology. Furthermore, we discuss unanswered questions relating to the evolution of the genome, developmental robustness, and morphologies in the goldfish lineage, with the goal of blazing a path toward an evodevo study paradigm using this teleost species as a new model species. For further resources related to this article, please visit the WIREs website.

An alternative developmental table to describe non-model fish species embryogenesis: application to the description of the Eurasian perch (Perca fluviatilis L. 1758) development

BACKGROUND

Fish correspond to the most diversified phylum among vertebrates with a large variety of species. Even if general features are distinguishable during the embryogenesis, several differences in term of timing, organ implementation or step progression always occur between species. Moreover, the developmental timing of wild non-model fish often presents variability within a species. In that context, it is necessary to define a model of developmental table flexible enough to describe fish development by integrating this variability and allow intra- and inter-specific comparisons. The elaboration of a model passes by the definition of new stages that could be easily observable on individuals. The present study aims at proposing such a model and describing accurately the Eurasian perch (Perca fluviatilis) embryogenesis using microscopic techniques among which time lapse video and histological studies. The Eurasian perch belongs to the Percidae family that includes 235 species classified in 11 genera. It is a member of the Perca gender and inhabits the Northern part of Europe and Asia.

RESULTS

At 13 °C, P. fluviatilis development elapses for 15 days from the fertilization to the first oral feeding. The staging division first took into account the cellular status to define periods, then the acquisition of new abilities by the embryo to further define stages. It allowed distinguishing two main stages during the cell cleavage period depending on the synchronization of the cell divisions, two stages during the gastrulation period depending on the cell speed migration and five stages during the organogenesis according to the acquisition of key abilities as proposed in the saltatory theory. During each stage, organs implementation was carefully followed with a particular attention for the visual and digestive systems. In addition, our study shows that embryos hatch at various developmental stages while they all begin to feed at a fixed date, 15 days after the fertilization whatever the spawn and the hatching date. These data give arguments to propose the first oral feeding as the best definition of the embryonic-to-larval transition.

CONCLUSIONS

The present model of developmental table combines flexibility and accuracy allowing detailed description of non-model fish species and intra- and inter-specific comparisons.

Postembryonic staging of wild-type goldfish, with brief reference to skeletal systems

Background: Artificial selection of postembryonic features is known to have established morphological variation in goldfish (Carassius auratus). Although previous studies have suggested that goldfish and zebrafish are almost directly comparable at the embryonic level, little is known at the postembryonic level. Results: Here, we categorized the postembryonic developmental process in the wild‐type goldfish into 11 different stages. We also report certain differences between the postembryonic developmental processes of goldfish and zebrafish, especially in the skeletal systems (scales and median fin skeletons), suggesting that postembryonic development underwent evolutionary divergence in these two teleost species. Conclusions: Our postembryonic staging system of wild‐type goldfish paves the way for careful and appropriate comparison with other teleost species. The staging system will also facilitate comparative ontogenic analyses between wild‐type and mutant goldfish strains, allowing us to closely study the relationship between artificial selection and molecular developmental mechanisms in vertebrates. Developmental Dynamics 244:1485–1518, 2015. © 2015 Wiley Periodicals, Inc.

This study provides the first reliable descriptions of normal post‐embryonic stages of wild type goldfish.

Several post‐embryonic features of goldfish and zebrafish are diverged in these two teleost lineages.

Goldfish larvae and juvenile provide a novel model for the investigation of the evolutionary relationship between domestication and ontogeny.

Gene expression patterns that support novel developmental stress buffering in embryos of the annual killifish Austrofundulus limnaeus

BACKGROUND

The cellular signaling mechanisms and morphogenic movements involved in axis formation and gastrulation are well conserved between vertebrates. In nearly all described fish, gastrulation and the initial patterning of the embryonic axis occur concurrently with epiboly. However, annual killifish may be an exception to this norm. Annual killifish inhabit ephemeral ponds in South America and Africa and permanent populations persist by the production of stress-tolerant eggs. Early development of annual killifish is unique among vertebrates because their embryonic blastomeres disperse randomly across the yolk during epiboly and reaggregate several days later to form the embryo proper. In addition, annual killifish are able to arrest embryonic development in one to three stages, known as diapause I, II, and III. Little is known about how the highly conserved developmental signaling mechanisms associated with early vertebrate development may have shifted in order to promote the annual killifish phenotype. One of the most well-characterized and conserved transcription factors, oct4 (Pou5f1), may have a role in maintaining pluripotency. In contrast, BMP-antagonists such as chordin, noggin, and follistatin, have been previously shown to establish dorsal-ventral asymmetry during axis formation. Transcription factors from the SOXB1 group, such as sox2 and sox3, likely work to induce neural specification. Here, we determine the temporal expression of these developmental factors during embryonic development in the annual killifish Austrofundulus limnaeus using quantitative PCR and compare these patterns to other vertebrates.

RESULTS

Partial transcript sequences to oct4, sox2, sox3, chordin, noggin-1, noggin-2, and follistatin were cloned, sequenced, and identified in A. limnaeus. We found oct4, sox3, chordin, and noggin-1 transcripts to likely be maternally inherited. Expression of sox2, follistatin, and noggin-2 transcripts were highest in stages following a visible embryonic axis.

CONCLUSIONS

Our data suggest that embryonic cells acquire their germ layer identity following embryonic blastomere reaggregation in A. limnaeus. This process of cellular differentiation and axis formation may involve similar conserved signaling mechanisms to other vertebrates. We propose that the undifferentiated state is prolonged during blastomere dispersal, thus functioning as a developmental stress buffer prior to the establishment of embryonic asymmetry and positional identity among the embryonic cells.

Cardiac repair and regenerative potential in the goldfish (Carassius auratus) heart

The remarkable ability of the heart to regenerate has been demonstrated in the zebrafish and giant danio, two fish members of the cyprinid family. Here we use light and electron microscopy to examine the repair response in the heart of another cyprinid, the goldfish (Carassius auratus), following cautery injury to a small portion of its ventricular myocardium. We observed a robust inflammatory response in the first two weeks consisting primarily of infiltrating macrophages, heterophils, and melanomacrophages. These inflammatory cells were identified in the lumen of the spongy heart, within the site of the wound, and attached to endocardial cells adjacent to the site of injury. Marked accumulation of collagen fibers and increased connective tissue were also observed during the first and second weeks in a transition zone between healthy and injured myocardium as well as in adjacent sub-epicardial regions. The accumulation of collagen and connective tissue however did not persist. The presence of capillaries was also noted in the injured area during repair. The replacement of the cauterized region of the ventricle by myocardial tissue was achieved in 6weeks. The presence of ethynyl deoxyuridine-positive cardiac myocytes and partially differentiated cardiac myocytes during repair suggest effective cardiac myocyte driven regeneration mechanisms also operate in the injured goldfish heart, and are similar to those observed in zebrafish and giant danio. Our data suggest the ability for cardiac regeneration may be widely conserved among cyprinids.

The origin of the bifurcated axial skeletal system in the twin-tail goldfish

Twin-tail goldfish possess a bifurcated caudal axial skeleton. The scarcity of this trait in nature suggests that a rare mutation, which drastically altered the mechanisms underlying axial skeleton formation, may have occurred during goldfish domestication. However, little is known about the molecular development of twin-tail goldfish. Here we show that the bifurcated caudal skeleton arises from a mutation in the chordin gene, which affects embryonic dorsal–ventral (DV) patterning. We demonstrate that formation of the bifurcated caudal axial skeleton requires a stop-codon mutation in one of two recently duplicated chordin genes; this mutation may have occurred within approximately 600 years of domestication. We also report that the ventral tissues of the twin-tail strain are enlarged, and form the embryonic bifurcated fin fold. However, unlike previously described chordin-deficient embryos, this is not accompanied by a reduction in anterior–dorsal neural tissues. These results provide insight into large-scale evolution arising from artificial selection.

The ornamental twin-tail goldfish has a bifurcated caudal skeleton that arose during domestication, but the developmental mechanisms that generate this tail are unknown. Here, Abe et al. show that a mutation in the chordin gene affects embryonic dorsal–ventral patterning causing the bifurcated tail skeleton.