Abstract
BackgroundHorseshoe crabs are ancient marine arthropods with a long evolutionary history extending back approximately 450 million years, which may benefit from their innate immune systems. However, the genetic mechanisms underlying their abilities of distinguishing and defending against invading microbes are still unclear.ResultsHere, we describe the 2.06 Gbp genome assembly of Tachypleus tridentatus with 24,222 predicted protein-coding genes. Comparative genomics shows that T. tridentatus and the Atlantic horseshoe crab Limulus polyphemus have the most orthologues shared among two species, including genes involved in the immune-related JAK-STAT signalling pathway. Divergence time dating results show that the last common ancestor of Asian horseshoe crabs (including T. tridentatus and C. rotundicauda) and L. polyphemus appeared approximately 130 Mya (121–141), and the split of the two Asian horseshoe crabs was dated to approximately 63 Mya (57–69). Hox gene analysis suggests two clusters in both horseshoe crab assemblies. Surprisingly, selective analysis of immune-related gene families revealed the high expansion of conserved pattern recognition receptors. Genes involved in the IMD and JAK-STAT signal transduction pathways also exhibited a certain degree of expansion in both genomes. Intact coagulation cascade-related genes were present in the T. tridentatus genome with a higher number of coagulation factor genes. Moreover, most reported antibacterial peptides have been identified in T. tridentatus with their potentially effective antimicrobial sites.ConclusionsThe draft genome of T. tridentatus would provide important evidence for further clarifying the taxonomy and evolutionary relationship of Chelicerata. The expansion of conserved immune signalling pathway genes, coagulation factors and intact antimicrobial peptides in T. tridentatus constitutes its robust and effective innate immunity for self-defence in marine environments with an enormous number of invading pathogens and may affect the quality of the adaptive properties with regard to complicated marine environments.
Highlights
Horseshoe crabs are ancient marine arthropods with a long evolutionary history extending back approximately 450 million years, which may benefit from their innate immune systems
More intact Hox clusters may be recovered with more complete assemblies; we can assume that at least one round of whole-genome duplication occurred in both T. tridentatus and L. polyphemus
We found that the two horseshoe crabs T. tridentatus and L. polyphemus had the most orthologues shared among two species, and were enriched for the immune-related Janus kinase (JAK)-Signal Transducer and Activator of Transcription Protein (STAT) signalling pathway
Summary
Horseshoe crabs are ancient marine arthropods with a long evolutionary history extending back approximately 450 million years, which may benefit from their innate immune systems. Horseshoe crabs are marine arthropods, representing an ancient family with an evolutionary history record extending back approximately 450 million years [1]. Based on their static morphology and their position in the arthropod family tree, they have been labelled “living fossils” for a long time [2]. The immune molecular mechanisms of how horseshoe crabs achieve distinguishing “self” and “non-self” antigenic epitopes, known as pathogen-associated molecular patterns (PAMPs), has not yet been established
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