Sequencing Ultraconserved Elements (UCEs) for Marine Population Genomics: A Proof-of-Concept Using a Deep-Sea Mussel Species.

Ultraconserved elements (UCEs) have emerged as a powerful tool for resolving deep evolutionary relationships due to their low DNA quality requirements and broad taxonomic applicability. While their utility for intraspecific and shallow-divergence studies is growing, only a few studies have explored their performance in marine taxa, some of them with metapopulations spanning thousands of kilometers. Here, we employed the UCE approach to investigate the population genomics of Gigantidas platifrons-a deep-sea mussel with a long larval dispersal period that exhibits a panmictic genetic structure across its extensive distribution range in the chemosynthetic ecosystems of the Western Pacific. With its published whole genome and prior restriction site-associated DNA sequencing using IIB restriction enzymes (2b-RAD seq) study, this species is an excellent candidate for evaluating the effectiveness of UCEs. We conducted UCE target capture sequencing on 123 individuals collected from two hydrocarbon seeps and four hydrothermal vents, yielding 1960 UCEs. To assess the impact of different reference choices, we identified 11,870 single-nucleotide polymorphisms (SNPs) by mapping against the published genome and 8936 SNPs by mapping to the representative 1960 UCEs. Both datasets were similar, with over 80% of the SNPs located in intronic and intergenic regions. Analyses based on both datasets consistently implied a clear genetic divergence between the South China Sea (SCS) and Okinawa Trough-Sagami Bay (OT-SB) populations, with predominant gene flow from OT to SB, consistent with previously published 2b-RAD seq findings. Additionally, UCE-based SNPs identified a dynamic decline in population size for individuals in the three regions and revealed selective adaptation signals to their environments. Overall, our study serves as a proof-of-concept demonstrating that UCEs provide a comparable resolution to RAD-Seq in detecting shallow-level genetic divergence and delineating conservation units in a high-dispersal marine species, even when lacking a sequenced genome.