![]() In fact, a few F2 segregants of chdA E127X/E127X derived from our backcross analysis did not show any mutated phenotypes at the late embryonic stage 9, due to incomplete penetrance presumably stemming from the robustness of dorsal-ventral patterning 9, 15, 16. Previous examinations of dino/chordin zebrafish 10 and backcross analysis of twin-tail goldfish 9 revealed that the phenotypes of these chordin gene mutants are polymorphic. ![]() The absence of twin-tail carp despite clear incentives for breeders to select for such organisms strongly suggests that this morphological mutation does not occur readily in this species, prompting us to investigate why this mutation arose in goldfish, but not in the closely related common carp ( Fig. 1a) 7, 18, 19, there are no records of the appearance of twin-tail morphology in the common carp lineage we should note that other ornamentation-related phenotypes (for example, favorable fur color and texture, body size and tameness) have been independently reproduced in different vertebrate lineages during relatively short periods of domestication 2, 3, 4. Although there is more recent reliable documentation on ornamental common carp breeding history than there is for ornamental goldfish ( Fig. Given the anticipated commercial value of “twin-tail common carp” as an ornamental fish, it can be assumed that breeders would be keen to genetically fix this morphological mutation in breeding populations. ![]() Moreover, common carp is one of the most popular ornamental fish species among breeders worldwide ( Fig. However, we are left with the following question: why has twin-tail morphology not been found in other fish species which underwent both the same chordin gene duplication event and domestication for ornamental purposes? For example, common carp ( Cyprinus carpio, the closest relative of goldfish) has chdA and -B orthologues which were acquired by allotetraploidization (species hybrid polyploidization) ( Fig. 1a) in fact, stably fixed twin-tail strains/species have not been reported for any other vertebrate so far 8, 9. On the basis that the chordin gene plays a crucial role in dorsal-ventral patterning 10, 11, 12, 13, 14, 15, 16, it is presumed that in the absence of the chordin gene duplication, it may be difficult (if not impossible) for the equivalent morphological mutation to be genetically fixed, even in domesticated populations ( Fig. Our previous molecular developmental genetic study revealed that a stop codon mutation in one of two chordin paralogous genes ( chdA and -B, which arose from a recent duplication event prior to domestication) is responsible for twin-tail morphology the stop codon allele located in chdA is designated as chdA E127X ( Fig. The twin-tail phenotype of goldfish, which was established by artificial selection of wildtype (single-tail) Carassius auratus species, is one such representative example 5, 6, 7, 8, 9 ( Fig. ![]() In particular, highly diverged ornamental and pet animals have provided evidence that artificial selection can drastically change morphological features 1, 3, 4 moreover, recent genomic studies of domesticated animals have identified certain genes responsible for the morphological changes that emerged during artificial selection 3, 4. 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.ĭomesticated animals have provided some important insights into how morphological features can be altered during the course of evolution 1, 2, 3, 4. 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. 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. 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). 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. Twin-tail goldfish strains are examples of drastic morphological alterations that emerged through domestication.
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