Triploids are rare in nature because of difficulties in meiotic and gametogenic processes, especially in vertebrates. The Carassius complex of cyprinid teleosts contains sexual tetraploid crucian carp/goldfish (C. auratus) and unisexual hexaploid gibel carp/Prussian carp (C. gibelio) lineages, providing a valuable model for studying the evolution and maintenance mechanism of unisexual polyploids in vertebrates. Here we sequence the genomes of the two species and assemble their haplotypes, which contain two subgenomes (A and B), to the chromosome level. Sequencing coverage analysis reveals that C. gibelio is an amphitriploid (AAABBB) with two triploid sets of chromosomes; each set is derived from a different ancestor. Resequencing data from different strains of C. gibelio show that unisexual reproduction has been maintained for over 0.82 million years. Comparative genomics show intensive expansion and alterations of meiotic cell cycle-related genes and an oocyte-specific histone variant. Cytological assays indicate that C. gibelio produces unreduced oocytes by an alternative ameiotic pathway; however, sporadic homologous recombination and a high rate of gene conversion also exist in C. gibelio. These genomic changes might have facilitated purging deleterious mutations and maintaining genome stability in this unisexual amphitriploid fish. Overall, the current results provide novel insights into the evolutionary mechanisms of the reproductive success in unisexual polyploid vertebrates.

由于减数分裂和配子形成过程的困难，三倍体在自然界中很少见，尤其是在脊椎动物中。鲤科硬骨鱼的鲫鱼复合体包含有性四倍体鲫鱼/金鱼（C. auratus）和单性六倍体银鳕鱼/普鲁士鲤（C. gibelio）谱系，为研究脊椎动物单性多倍体的进化和维持机制提供了有价值的模型。在这里，我们对两个物种的基因组进行测序，并将它们的单倍型组装到染色体水平，其中包含两个亚基因组（A 和 B）。测序覆盖度分析显示C. gibelio是具有两组三倍体染色体的双三倍体 (AAABBB)；每个集合都来自不同的祖先。来自不同菌株的C. gibelio的重新测序数据表明单性繁殖已经维持了超过 82 万年。比较基因组学显示减数分裂细胞周期相关基因和卵母细胞特异性组蛋白变体的密集扩展和改变。细胞学分析表明，C. gibelio通过替代的无减数分裂途径产生未减数的卵母细胞；然而， C. gibelio中也存在零星的同源重组和高基因转换率. 这些基因组变化可能有助于清除这种单性二倍体鱼的有害突变并维持基因组稳定性。总的来说，目前的结果为单性多倍体脊椎动物繁殖成功的进化机制提供了新的见解。