Chromosomal level reference genome of Tachypleus tridentatus provides insights into evolution and adaptation of horseshoe crabs

Horseshoe crabs including Tachypleus tridentatus are a group of marine arthropods living fossil species which have existed on the earth for 500 million years. However, the genetic mechanisms underlying their unique adaptive ability are still unclear. Here, we assembled the first chromosome-level T. tridentatus genome, and proofed that this genome is of very high quality with contig N50 1.69 Mb. By comparison with other arthropods, some gene families of T. tridentatus experienced significant expansion, whichare related to several signaling pathways, endonuclease activity, and metabolic processes. Based on the comparative analysis of genomics and 27 transcriptomes from 9 tissues, we found that the expanded Dscam genes usually locate at the key hub positions of immune network. Furthermore, the Dscam genes showed higher levels of expression in the yellow connective tissue, the birthplace of blood cells with strong differentiation capability, than the other 8 tissues. Besides, Dscam genes ar...