Chromosome-level genome assembly of the disco clam,Ctenoides ales, a first for the bivalve order Limida

Abstract

The bivalve subclass Pteriomorphia, which includes the economically important scallops, oysters, mussels, and ark clams, exhibits extreme ecological, morphological, and behavioral diversity. Among this diversity are five morphologically distinct eye types, making Pteriomorphia an excellent setting to explore the molecular basis for the evolution of novel traits. Of pteriomorphian bivalves, Limida is the only order lacking genomic resources, greatly limiting the potential phylogenomic analyses related to eyes and phototransduction. Here, we present the first limid genome assembly, the disco clam,Ctenoides ales, which is characterized by invaginated eyes, exceptionally long tentacles, and a flashing light display. This genome assembly was constructed with PacBio long reads and Dovetail Omni-CTMproximity-ligation sequencing. The final assembly is ~2.3Gb and over 99% of the total length is contained in 18 pseudomolecule scaffolds. We annotated 41,064 protein coding genes and report a BUSCO completeness of 91.9% for metazoa_obd10. Additionally, we report the completely annotated mitochondrial genome, also a first for Limida. The ~20Kb mitogenome has 12 protein coding genes, 22 tRNAs, 2 rRNA genes, and a 1,589 bp duplicated sequence containing the origin of replication. TheC. alesnuclear genome size is substantially larger than other pteriomorphian genomes, mainly accounted for by transposable element sequences. We inventoried the genome for opsins, the signaling proteins that initiate phototransduction, and found that, unlike its closest eyed-relatives, the scallops,C. aleslacks duplication of the rhabdomeric Gq-coupled opsin that is typically used for invertebrate vision. In fact,C. aleshas uncharacteristically few opsins relative to the other pteriomorphian families, all of which have unique expansions of xenopsins, a recently discovered opsin subfamily. This chromosome-level assembly, along with the mitogenome, will be valuable resources for comparative genomics and phylogenetics in bivalves and particularly for the understudied but charismatic limids.