Thalidomide and its metabolite 5-hydroxythalidomide induce teratogenicity via the cereblon neosubstrate PLZF.

Thalidomide causes teratogenic effects by inducing protein degradation via cereblon (CRBN)-containing ubiquitin ligase and modification of its substrate specificity. Human P450 cytochromes convert thalidomide into two monohydroxylated metabolites that are considered to contribute to thalidomide effects, through mechanisms that remain unclear. Here, we report that promyelocytic leukaemia zinc finger (PLZF)/ZBTB16 is a CRBN target protein whose degradation is involved in thalidomide- and 5-hydroxythalidomide-induced teratogenicity. Using a human transcription factor protein array produced in a wheat cell-free protein synthesis system, PLZF was identified as a thalidomide-dependent CRBN substrate. PLZF is degraded by the ubiquitin ligase CRL4CRBN in complex with thalidomide, its derivatives or 5-hydroxythalidomide in a manner dependent on the conserved first and third zinc finger domains of PLZF. Surprisingly, thalidomide and 5-hydroxythalidomide confer distinctly different substrate specificities to mouse and chicken CRBN, and both compounds cause teratogenic phenotypes in chicken embryos. Consistently, knockdown of Plzf induces short bone formation in chicken limbs. Most importantly, degradation of PLZF protein, but not of the known thalidomide-dependent CRBN substrate SALL4, was induced by thalidomide or 5-hydroxythalidomide treatment in chicken embryos. Furthermore, PLZF overexpression partially rescued the thalidomide-induced phenotypes. Our findings implicate PLZF as an important thalidomide-induced CRBN neosubstrate involved in thalidomide teratogenicity.

The dwarf neon rainbowfish Melanotaenia praecox, a small spiny-rayed fish with potential as a new Acanthomorpha model fish: I. Fin ray ontogeny and postembryonic staging.

BACKGROUND: Fish fins with highly variable color patterns and morphologies have many functions. In Actinopterygii, the free parts of fins are supported by "soft rays" and "spiny rays." Spiny rays have various functions and are extremely modified in some species, but they are lacking in popular model fish such as zebrafish and medaka. Additionally, some model fish with spiny rays are difficult to maintain in ordinary laboratory systems. RESULTS: Characteristics of the small, spiny-rayed rainbowfish Melanotaenia praecox render it useful as an experimental model species. Neither fish age nor body size correlate well with fin development during postembryonic development in this species. A four-stage developmental classification is proposed that is based on fin ray development. CONCLUSIONS: Melanotaenia praecox is an ideal species to rear in laboratories for developmental studies. Our classification allows for postembryonic staging of this species independent of individual age and body size. Development of each fin ray may be synchronized with dorsal fin development. We discuss the differences in mechanisms regulating soft, spiny, and procurrent ray development.

The dwarf neon rainbowfish Melanotaenia praecox, a small spiny-rayed fish with potential as a new Acanthomorpha model fish: II. Establishment of a microinjection procedure for genetic engineering.

BACKGROUND: Rainbowfish is a clade of colorful freshwater fish. Melanotaenia praecox is a small rainbowfish species with biological characteristics that make it potentially useful as an experimental model species. We anticipate that M. praecox could become a new model used in various fields, such as ecology, evolution, and developmental biology. However, few previous studies have described experimental set-ups needed to understand the molecular and genetic mechanisms within this species. RESULTS: We describe detailed procedures for genetic engineering in the rainbowfish M. praecox. By using these procedures, we successfully demonstrated CRISPR/Cas-mediated knockout and Tol2 transposon-mediated transgenesis in this species. Regarding the CRISPR/Cas system, we disrupted the tyrosinase gene and then showed that injected embryos lacked pigmentation over much of their body. We also demonstrated that a Tol2 construct, including a GFP gene driven by a ubiquitous promoter, was efficiently integrated into the genome of M. praecox embryos. CONCLUSIONS: The establishment of procedures for genetic engineering in M. praecox enables investigation of the genetic mechanisms behind a broad range of biological phenomena in this species. Thus, we suggest that M. praecox can be used as a new model species in various experimental biology fields.

Genomic screening of fish-specific genes in gnathostomes and their functions in fin development.

In this study, we comprehensively searched for fish-specific genes in gnathostomes that contribute to development of the fin, a fish-specific trait. Many previous reports suggested that animal group-specific genes are often important for group-specific traits. Clarifying the roles of fish-specific genes in fin development of gnathostomes, for example, can help elucidate the mechanisms underlying the formation of this trait. We first identified 91 fish-specific genes in gnathostomes by comparing the gene repertoire in 16 fish and 35 tetrapod species. RNA-seq analysis narrowed down the 91 candidates to 33 genes that were expressed in the developing pectoral fin. We analyzed the functions of approximately half of the candidate genes by loss-of-function analysis in zebrafish. We found that some of the fish-specific and fin development-related genes, including fgf24 and and1/and2, play roles in fin development. In particular, the newly identified fish-specific gene qkia is expressed in the developing fin muscle and contributes to muscle morphogenesis in the pectoral fin as well as body trunk. These results indicate that the strategy of identifying animal group-specific genes is functional and useful. The methods applied here could be used in future studies to identify trait-associated genes in other animal groups.

Insights regarding skin regeneration in non-amniote vertebrates: Skin regeneration without scar formation and potential step-up to a higher level of regeneration.

Skin wounds are among the most common injuries in animals and humans. Vertebrate skin is composed of an epidermis and dermis. After a deep skin injury in mammals, the wound heals, but the dermis cannot regenerate. Instead, collagenous scar tissue forms to fill the gap in the dermis, but the scar does not function like the dermis and often causes disfiguration. In contrast, in non-amniote vertebrates, including fish and amphibians, the dermis and skin derivatives are regenerated after a deep skin injury, without a recognizable scar remaining. Furthermore, skin regeneration can be compared with a higher level of organ regeneration represented by limb regeneration in these non-amniotes, as fish, anuran amphibians (frogs and toads), and urodele amphibians (newts and salamanders) have a high capacity for organ regeneration. Comparative studies of skin regeneration together with limb or other organ regeneration could reveal how skin regeneration is stepped up to a higher level of regeneration. The long history of regenerative biology research has revealed that fish, anurans, and urodeles have their own strengths as models for regeneration studies, and excellent model organisms of these non-amniote vertebrates that are suitable for molecular genetic studies are now available. Here, we summarize the advantages of fish, anurans, and urodeles for skin regeneration studies with special reference to three model organisms: zebrafish (Danio rerio), African clawed frog (Xenopus laevis), and Iberian ribbed newt (Pleurodele waltl). All three of these animals quickly cover skin wounds with the epidermis (wound epidermis formation) and regenerate the dermis and skin derivatives as adults. The availability of whole genome sequences, transgenesis, and genome editing with these models enables cell lineage tracing and the use of human disease models in skin regeneration phenomena, for example. Zebrafish present particular advantages in genetics research (e.g., human disease model and Cre-loxP system). Amphibians (X. laevis and P. waltl) have a skin structure (keratinized epidermis) common with humans, and skin regeneration in these animals can be stepped up to limb regeneration, a higher level of regeneration.

Zebrafish can regenerate endoskeleton in larval pectoral fin but the regenerative ability declines.

We show for the first time endoskeletal regeneration in the developing pectoral fin of zebrafish. The developing pectoral fin contains an aggregation plate of differentiated chondrocytes (endochondral disc; primordium for endoskeletal components, proximal radials). The endochondral disc can be regenerated after amputation in the middle of the disc. The regenerated disc sufficiently forms endoskeletal patterns. Early in the process of regenerating the endochondral disc, epithelium with apical ectodermal ridge (AER) marker expression rapidly covers the amputation plane, and mesenchymal cells start to actively proliferate. Taken together with re-expression of a blastema marker gene, msxb, and other developmental genes, it is likely that regeneration of the endochondral disc recaptures fin development as epimorphic limb regeneration does. The ability of endoskeletal regeneration declines during larval growth, and adult zebrafish eventually lose the ability to regenerate endoskeletal components such that amputated endoskeletons become enlarged. Endoskeletal regeneration in the zebrafish pectoral fin will serve as a new model system for successful appendage regeneration in mammals.

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.

Fgf signalling controls diverse aspects of fin regeneration.

Studies have shown that fibroblast growth factor (Fgf) signalling is necessary for appendage regeneration, but its exact function and the ligands involved during regeneration have not yet been elucidated. Here, we performed comprehensive expression analyses and identified fgf20a and fgf3/10a as major Fgf ligands in the wound epidermis and blastema, respectively. To reveal the target cells and processes of Fgf signalling, we performed a transplantation experiment of mesenchymal cells that express the dominant-negative Fgf receptor 1 (dnfgfr1) under control of the heat-shock promoter. This mosaic knockdown analysis suggested that Fgf signalling is directly required for fin ray mesenchyme to form the blastema at the early pre-blastema stage and to activate the regenerative cell proliferation at a later post-blastema stage. These results raised the possibility that the early epidermal Fgf20a and the later blastemal Fgf3/10a could be responsible for these respective processes. We demonstrated by gain-of-function analyses that Fgf20a induces the expression of distal blastema marker junbl, and that Fgf3 promotes blastema cell proliferation. Our study highlights that Fgfs in the wound epidermis and blastema have distinct functions to regulate fin regeneration cooperatively.

Evolutionary developmental transition from median to paired morphology of vertebrate fins: Perspectives from twin-tail goldfish.

Vertebrate morphology has been evolutionarily modified by natural and/or artificial selection. The morphological variation of goldfish is a representative example. In particular, the twin-tail strain of ornamental goldfish shows highly diverged anal and caudal fin morphology: bifurcated anal and caudal fins. Recent molecular developmental genetics research revealed that a stop codon mutation in one of the two recently duplicated chordin genes is important for the highly diverged fin morphology of twin-tail goldfish. However, some issues still need to be discussed in the context of evolutionary developmental biology (evo-devo). For example, the bifurcated anal and caudal fins of twin-tail goldfish provided early researchers with insights into the origin of paired fins (pectoral and pelvic fins), but no subsequent researchers have discussed this topic. In addition, although the fossil jawless vertebrate species Euphanerops is also known to have had a bifurcated anal fin, how the bifurcated anal fin of twin-tail goldfish is related to that of fossil jawless vertebrate species has never been investigated. In this review, we present an overview of the early anatomical and embryological studies of twin-tail goldfish. Moreover, based on the similarity of embryonic features between the secondarily bifurcated competent stripe in twin-tail goldfish and the trunk bilateral competent stripes in conventional gnathostomes, we hypothesized that they share the same molecular developmental mechanisms. We also postulate that the bifurcated anal fin of Euphanerops might be caused by the same type of modification of dorsal-ventral patterning that occurs in the twin-tail goldfish, unlike the previously suggested evolutionary process that required the co-option of paired fin developmental mechanisms. Understanding the molecular developmental genetics of twin-tail goldfish allows us to further investigate the evolutionary developmental mechanisms of the origin of paired fins.

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 chdB 1 and chdB 2 . The chdB 1 allele is the major allele and was found in all investigated goldfish strains, whereas the chdB 2 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 ( chdB 1 > chdB 2 ). 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 chdB 2 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 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.

Upstream regulation for initiation of restricted Shh expression in the chick limb bud.

BACKGROUND: The organizing center, which serves as a morphogen source, has crucial functions in morphogenesis in animal development. The center is necessarily located in a certain restricted area in the morphogenetic field, and there are several ways in which an organizing center can be restricted. The organizing center for limb morphogenesis, the ZPA (zone of polarizing activity), specifically expresses the Shh gene and is restricted to the posterior region of the developing limb bud. RESULTS: The pre-pattern along the limb anteroposterior axis, provided by anterior Gli3 expression and posterior Hand2 expression, seems insufficient for the initiation of Shh expression restricted to a narrow, small spot in the posterior limb field. Comparison of the spatiotemporal patterns of gene expression between Shh and some candidate genes (Fgf8, Hoxd10, Hoxd11, Tbx2, and Alx4) upstream of Shh expression suggested that a combination of these genes' expression provides the restricted initiation of Shh expression. CONCLUSIONS: Taken together with results of functional assays, we propose a model in which positive and negative transcriptional regulatory networks accumulate their functions in the intersection area of their expression regions to provide a restricted spot for the ZPA, the source of morphogen, Shh. Developmental Dynamics 246:417-430, 2017. © 2017 Wiley Periodicals, Inc.

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.

Mechanism of pectoral fin outgrowth in zebrafish development.

Fins and limbs, which are considered to be homologous paired vertebrate appendages, have obvious morphological differences that arise during development. One major difference in their development is that the AER (apical ectodermal ridge), which organizes fin/limb development, transitions into a different, elongated organizing structure in the fin bud, the AF (apical fold). Although the role of AER in limb development has been clarified in many studies, little is known about the role of AF in fin development. Here, we investigated AF-driven morphogenesis in the pectoral fin of zebrafish. After the AER-AF transition at ∼36 hours post-fertilization, the AF was identifiable distal to the circumferential blood vessel of the fin bud. Moreover, the AF was divisible into two regions: the proximal AF (pAF) and the distal AF (dAF). Removing the AF caused the AER and a new AF to re-form. Interestingly, repeatedly removing the AF led to excessive elongation of the fin mesenchyme, suggesting that prolonged exposure to AER signals results in elongation of mesenchyme region for endoskeleton. Removal of the dAF affected outgrowth of the pAF region, suggesting that dAF signals act on the pAF. We also found that the elongation of the AF was caused by morphological changes in ectodermal cells. Our results suggest that the timing of the AER-AF transition mediates the differences between fins and limbs, and that the acquisition of a mechanism to maintain the AER was a crucial evolutionary step in the development of tetrapod limbs.

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

BACKGROUND: Highly divergent morphology among the different goldfish strains (Carassius auratus) may make it a suitable model for investigating how artificial selection has altered developmental mechanisms. Here we describe the embryological development of the common goldfish (the single fin Wakin), which retains the ancestral morphology of this species. RESULTS: We divided goldfish embryonic development into seven periods consisting of 34 stages, using previously reported developmental indices of zebrafish and goldfish. Although several differences were identified in terms of their yolk size, epiboly process, pigmentation patterns, and development rate, our results indicate that the embryonic features of these two teleost species are highly similar in their overall morphology from the zygote to hatching stage. CONCLUSIONS: These results provide an opportunity for further study of the evolutionary relationship between domestication and development, through applying well-established zebrafish molecular biological resources to goldfish embryos.

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.

Dynamic coupling of pattern formation and morphogenesis in the developing vertebrate retina.

During embryonic development, pattern formation must be tightly synchronized with tissue morphogenesis to coordinate the establishment of the spatial identities of cells with their movements. In the vertebrate retina, patterning along the dorsal-ventral and nasal-temporal (anterior-posterior) axes is required for correct spatial representation in the retinotectal map. However, it is unknown how specification of axial cell positions in the retina occurs during the complex process of early eye morphogenesis. Studying zebrafish embryos, we show that morphogenetic tissue rearrangements during eye evagination result in progenitor cells in the nasal half of the retina primordium being brought into proximity to the sources of three fibroblast growth factors, Fgf8/3/24, outside the eye. Triple-mutant analysis shows that this combined Fgf signal fully controls nasal retina identity by regulating the nasal transcription factor Foxg1. Surprisingly, nasal-temporal axis specification occurs very early along the dorsal-ventral axis of the evaginating eye. By in vivo imaging GFP-tagged retinal progenitor cells, we find that subsequent eye morphogenesis requires gradual tissue compaction in the nasal half and directed cell movements into the temporal half of the retina. Balancing these processes drives the progressive alignment of the nasal-temporal retina axis with the anterior-posterior body axis and is controlled by a feed-forward effect of Fgf signaling on Foxg1-mediated cell cohesion. Thus, the mechanistic coupling and dynamic synchronization of tissue patterning with morphogenetic cell behavior through Fgf signaling leads to the graded allocation of cell positional identity in the eye, underlying retinotectal map formation.

Morphological evolution and diversity of pectoral fin skeletons in teleosts.

The Teleostei class has the most species of the fishes. Members of this group have pectoral fins, enabling refined movements in the water. Although teleosts live in a diverse set of environments, the skeletal pattern of pectoral fins in teleosts is considered to show little morphological variability. Here, in order to elucidate variations in pectoral fin skeletons and to identify their evolutionary processes, we compared the pectoral fin skeletons from 27 species of teleosts. We identified several variations and a diversity of pectoral fin skeletal patterns within some teleost groups. Taken together with previous reports on teleost skeletons, our findings reveal that in the course of teleost evolution, there are a mixture of conserved and non-conserved components in the pectoral fin skeletons of teleosts, and that teleosts may have experienced the variation and conservation of the number and shape of the proximal radials, the loss of the mesocoracoid, and the change in the distal radial-fin ray relationship.

Developmental independence of median fins from the larval fin fold revises their evolutionary origin.

The median fins of modern fish that show discrete forms (dorsal, anal, and caudal fins) are derived from a continuous fold-like structure, both in ontogeny and phylogeny. The median fin fold (MFF) hypothesis assumes that the median fins evolved by reducing some positions in the continuous fin fold of basal chordates, based on the classical morphological observation of developmental reduction in the larval fin folds of living fish. However, the developmental processes of median fins are still unclear at the cellular and molecular levels. Here, we describe the transition from the larval fin fold into the median fins in zebrafish at the cellular and molecular developmental level. We demonstrate that reduction does not play a role in the emergence of the dorsal fin primordium. Instead, the reduction occurs along with body growth after primordium formation, rather than through actively scrapping the non-fin forming region by inducing cell death. We also report that the emergence of specific mesenchymal cells and their proliferation promote dorsal fin primordium formation. Based on these results, we propose a revised hypothesis for median fin evolution in which the acquisition of de novo developmental mechanisms is a crucial evolutionary component of the discrete forms of median fins.

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 chdSwt 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.