5. Conclusion
This is the first study to present a high-quality and continuity chromosome-level genome assembly of male O. bidens using nanopore long-read sequencing, achieving a total genome size of 992.9 Mb with a contig N50 of 5.2 Mb. Using Hi-C sequencing technology, 89.31% of the genome sequences were anchored to 38 chromosomes with scaffold N50 of 19.44 Mb corresponding to male karyotypes. A total of 30,922 functional genes were annotated using various databases, and 43.23 % of the male genome was identified as repetitive elements. Over 97% of the BUSCO genes were completely identified in the male O. bidens genome, indicating a complete genome assembly. Therefore, the male O. bidens genome can serve as genetic material for the investigation of the role of male-specific sexual dimorphism in growth. We identified many expanded gene families involved in the synthesis and secretion of hormones in the GnRH signaling pathway. The relevant genes act on growth hormones, regarded as the dominant regulators of sexual dimorphism in growth. These GnRH signaling-related expanded genes were distributed in multiple male O. bidens chromosomes. The gene number was between one and seven, and the top three were chromosomes LG01, LG06, and LG08. Compared to the released femaleO. bidens genome, we found that the male O. bidens LG06 resulted from a reunion of female O. bidens chromosomes GH14 and GH38, carrying male-specific genes involved in DNA replication, recombination, and repair. Further study of LG06 revealed that male O. bidensLG06 originated from the ancient fusion of large fragments from grass carp chromosomes breaking off by genome synteny comparison, suggesting that chromosome number via breakage possibly contributed to genes for their roles in sexual dimorphism traits. This study provides valuable genomic resources for subsequent research on the growth and development, sexual-controlled mechanisms, sex-determination systems, and molecular breeding of O. bidens .