Genotyping-by-Sequencing of the regional Pacific abalone (Haliotis discus) genomes reveals population structures and patterns of gene flow

Endangered Pinto/Northern Abalone (Haliotis kamtschatkana) are Panmictic Across Their 3700 km Range Along the Pacific Coast of North America

Connectivity is integral to the dynamics of metapopulations through dispersal and gene flow, and understanding these processes is essential for guiding conservation efforts. Abalone, broadcast-spawning marine snails associated with shallow rocky habitats, have experienced widespread declines, and all seven North American species are threatened. We investigated the connectivity and population genomics of pinto/northern abalone (Haliotis kamtschatkana), the widest-ranging of abalone species. We employed reduced representation sequencing (RADseq) to generate single nucleotide polymorphism (SNP) data, assessing population connectivity and potential adaptive variation at 12 locations across the full range from Alaska to Mexico. Despite depleted populations, our analysis of over 6000 SNPs across nearly 300 individuals revealed that pinto abalone maintains a high genetic diversity with no evidence of a genetic bottleneck. Neutral population structure and isolation by distance were extremely weak, indicating panmixia across the species' range (global F ST = 0.0021). Phylogenetic analysis, principal components analysis, and unsupervised clustering methods all supported a single genetic population. However, slight population differentiation was noted in the Salish Sea and Inside Passage regions, with evidence for higher barriers to dispersal relative to outer coastal areas. This north-central region may also represent the species' ancestral range based on relatively low population-specific F ST values; the northern and southern extremes of the range likely represent range expansions. Outlier analysis did not identify consensus loci implicated in adaptive variation, suggesting limited adaptive differentiation. Our study sheds light on the evolutionary history and contemporary gene flow of this threatened species, providing key insights for conservation strategies, particularly in sourcing broodstock for ongoing restoration efforts.

The Isolation and Characterization of Perlucin in Pacific Abalone, Haliotis discus hannai: A Shell Morphogenic Protein with Potential Responses to Thermal Stress and Starvation

Perlucin is a shell matrix protein that plays a significant role in regulating shell biomineralization. This study aimed to isolate and characterize the perlucin gene and analyze its expression to explore its role in shell formation, regeneration, and responses to thermal stress and starvation in Pacific abalone. The isolated full-length cDNA sequence of Hdh-Perlucin is 1002 bp long, encoding a 163-amino-acid polypeptide with a signal peptide. The mature peptide of Hdh-Perlucin contains a C-type lectin domain with signature motif and six conserved cysteine residues. Gene Ontology analysis suggests that Hdh-Perlucin exhibits carbohydrate-binding activity. Significantly higher expression of Hdh-Perlucin was observed during the juvenile, veliger, and trochophore stages, compared with cell division stage during early development. Upregulated expression was recorded from slow to rapid growth phases and during shell biomineralization, while downregulated expression was noted during starvation. Under thermal stress, expression peaked at 30 °C and 25 °C for 6 and 12 h, respectively, while consistently higher levels were observed at 15 °C throughout the experiment. This study provides the first comprehensive structural and expression analysis of Hdh-Perlucin, highlighting its roles in metamorphosis, shell formation and regeneration, and responses to heat stress and starvation in abalone.

Genomic Evidence for Speciation with Gene Flow in Broadcast Spawning Marine Invertebrates

How early stages of speciation in free-spawning marine invertebrates proceed is poorly understood. The Western Pacific abalones, Haliotis discus, H. madaka, and H. gigantea, occur in sympatry with shared breeding season and are capable of producing viable F1 hybrids in spite of being ecologically differentiated. Population genomic analyses revealed that although the three species are genetically distinct, there is evidence for historical and ongoing gene flow among these species. Evidence from demographic modeling suggests that reproductive isolation among the three species started to build in allopatry and has proceeded with gene flow, possibly driven by ecological selection. We identified 27 differentiation islands between the closely related H. discus and H. madaka characterized by high FST and dA, but not high dXY values, as well as high genetic diversity in one H. madaka population. These genomic signatures suggest differentiation driven by recent ecological divergent selection in presence of gene flow outside of the genomic islands of differentiation. The differentiation islands showed low polymorphism in H. gigantea, and both high FST, dXY, and dA values between H. discus and H. gigantea, as well as between H. madaka and H. gigantea. Collectively, the Western Pacific abalones appear to occupy the early stages speciation continuum, and the differentiation islands associated with ecological divergence among the abalones do not appear to have acted as barrier loci to gene flow in the younger divergences but appear to do so in older divergences.