The genome of the early diverged amphioxus, Asymmetron lucayanum, illuminates the evolution of genome architecture and gene repertoires in cephalochordates

Cephalochordates (amphioxus or lancelet) are considered as living proxies for ancestral chordates due to their key phylogenetic position and slow evolutionary rate. The genomes of living amphioxus thus can help to reveal the genetic basis shaping the evolutionary transition from invertebrates to vertebrates. To gain a comprehensive understanding of the genome architecture in amphioxus, here we generate a chromosome-anchored genome assembly for Asymmetron lucaynum, representing the earliest diverging cephalochordate genus. Our results show that Asymmetron has an enlarged genome compared to those of the other four cephalochordate genomes decoded so far (all in the genus Branchiostoma), causing by pervasive expansions of inter-genic transposable elements (TEs). Nevertheless, both macrosynteny and microsynteny remain highly conserved between Asymmetron and Branchiostoma, enabling reconstruction of the ancestral genomic architecture of the Cephalochordate lineage for tracing genome evolutionary process during Deuterostome and Chordate diversification. By coupling developmental transcriptomic analyses, we further show that purifying selection and constraints on co-transcriptional regulation may have contributed to the maintenance of the conserved microsynteny blocks among cephalochordate species. We also examine the evolutionary history of Hox cluster in cephalochordates and vertebrates, and identify species-specific inversions and TE invasions at this important locus in both Asymmetron and Branchiostoma. Finally, we survey key gene families involved in both innate and adaptive immunity (e.g., TLR, NLR, MHC, and RAG) and uncover their plausible ancestry and evolutionary dynamics in chordates. Taken together, our findings illuminate the genome and gene evolution of cephalochordates and provide valuable resources for understanding the early evolution of chordates and the origin of vertebrates.