A persistent enigma is the rarity of polyploidy in animals, compared to its prevalence in plants. Although animal polyploids are thought to experience deleterious genomic chaos during initial polyploidization and subsequent rediploidization processes, this hypothesis has not been tested. We provide an improved reference-quality de novo genome for allotetraploid goldfish whose origin dates to ~15 million years ago. Comprehensive analyses identify changes in subgenomic evolution from asymmetrical oscillation in goldfish and common carp to diverse stabilization and balanced gene expression during continuous rediploidization. The homoeologs are coexpressed in most pathways, and their expression dominance shifts temporally during embryogenesis. Homoeolog expression correlates negatively with alternation of DNA methylation. The results show that allotetraploid cyprinids have a unique strategy for balancing subgenomic stabilization and diversification. Rediploidization process in these fishes provides intriguing insights into genome evolution and function in allopolyploid vertebrates.

The knowledge of understanding the molecular traits of the sterile triploid fish is sparse. Herein, we analyzed the microRNA (miRNA) alternations in the testes of the sterile triploid fish produced by crossing the tetraploid fish with the diploid fish, compared with those of tetraploids and diploids used as the controls. A total of 136, 134, and 142 conserved miRNAs and 105, 112, and 119 novel miRNAs were identified in the diploid, triploid, and tetraploid fish, respectively. The genes targeted by the differentially expressed miRNAs were identified and were enriched in the GO term cell surface receptor signaling pathway, cellular process, G-protein coupled receptor signaling pathway, and metabolic process. KEGG pathway enrichment was also assessed to evaluate the target genes with differentially expressed miRNAs and these genes were enriched in four pathways (synthesis and degradation of ketone bodies, pentose and glucuronate interconversions, cyanoamino acid metabolic process, and ascorbate and aldarate metabolism). Nine differentially expressed miRNAs were verified by quantitative real-time PCR analysis (qPCR). The upregulated miRNAs in triploids, including miR-101a, miR-199-5p, miR-214, miR-222, and miR-193a, showed the same results with high-throughput sequencing. Among the selected downregulated miRNAs, miR-7b and miR-153b had significantly lower expression levels in triploids. Dnah3 and Tekt1 genes targeted by miR-199-5p showed lower expression in triploids by qPCR. These verified differentially expressed miRNAs may participate in testicular development and sperm activity by targeting functional genes, which were identified with differential expression in the triploid. This evidence provides insights into the epigenetic regulatory mechanisms of sterility in triploid cyprinids.

As a paracrine factor, Follistatin (Fst) plays an important role in regulating biosynthesis and release of pituitary gonadotropin (GTH), gonadal development, and ovulation cycle. In this study, full‐length complementary DNAs (cDNAs) of the follistatin gene in diploid red crucian carp, Carassius auratus red var., allotriploid crucian carp, and allotetraploid hybrids were obtained. Our data showed that the cDNAs of follistatin of all three fish encoded 322 amino acids. Referring to the structural characteristics of Fst in other species, we found that the Fst protein of these three fish had four domains, respectively. Reverse transcriptase polymerase chain reaction (PCR) indicated that the follistatin gene was widely expressed in all tested tissues, except spleens, in these three fish. Real‐time quantitative PCR analysis of different seasonal profiles showed that allotriploids had significantly higher expression of follistatin messenger RNA in pituitaries during both the prespawning and spawning periods. These results suggest that the elevated expression of the follistatin gene in the pituitaries of allotriploids might lead to sterility of allotriploids by blocking the inhibitory effect of activin on luteinizing hormone β subunit. Furthermore, the results may improve our understanding of reproduction characteristics in triploids, which benefits polyploidy breeding.

activin β A and β B from diploid and allotriploid crucian carp were cloned.The differential expression of activin β A and β B genes in female allotriploid and diploid red crucian carp Carassius auratus red var. were studied and found to be expressed in all the tested tissues; particularly, the expression of activin β A and β B was elevated in the ovaries of allotriploids and differential expression in pituitaries during the nonbreeding season and the breeding season period. The immunohistochemistry indicated that the abnormal triploid ovaries were dominated by small oogonium-like cells with dense signals and that the elevated expression of activin β A and β B in the ovaries of allotriploids may be related to allotriploid sterility. K E Y W O R D Sactivin β A , activin β B , allotriploid, Carassius auratus red. var., differential expression, sterility Activin is a member of the transforming growth factor-β (TGF-β) family of growth and differentiation factors (Vale et al., 1988). Activin was first isolated from gonadal fluids (Mathews, 1994) and commonly consists of three types: activin A, activin B and activin AB. Homodimerisation of the β A subunit generates activin A and the β B subunit generates activin B. The β A and β B subunits can also heterodimerise and form activin AB. In addition to the β A and β B subunits, β C and β E have been identified in mammals (Fang et al., 1996;Hotten et al., 1995) and a β D subunit has been described in the Africa clawed frog Xenopus laevis (Oda et al., 1995), but the β A and β B subunits have been researched more frequently (Woodruff, 1998).

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