The Pacific oyster, Crassostrea gigas, is an important commercially farmed species in Korea. C. gigas exhibits low genetic diversity in wild populations in Korea. To address this, we bred Japanese broodstock for more than five generations and released them into two populations to increase genetic diversity. We also assessed whether this improvement was achieved by comparing them with a control population. In this study, we analyzed genetic diversity using 16 microsatellite loci of C. gigas. The observed heterozygosity H_O in the farmed population ranged up to 0.494, while in the wild population, it was 0.437. The farmed population had the highest genetic diversity, but the effective population size was low (105). The PD population size for resource creation was 403, which was higher than that of GH. The genetic structure was divided into two groups with K = 2. The first group consisted of the BR population, while the second group included the GH, GW, and PD populations. Therefore, we confirmed significant genetic differences between the farmed, wild, and resource creation populations. This study provides essential genetic information for future fishery resource development and conservation of C. gigas.

Growth variation is one of the most economically important traits in oyster aquaculture, yet the transcriptional programs associated with divergent growth performance remain incompletely understood. In this study, three growth cohorts of the Pacific oyster (Crassostrea gigas) originating from the same production batch were examined to characterize coordinated differences in tissue-associated transcriptional programs across contrasting growth cohorts. A total of 25 RNA-seq libraries were generated from slow-growth (SG) whole-body samples and mantle and gill tissues of medium-growth (MG) and fast-growth (FG) oysters. RNA-seq analysis identified 28,773 expressed genes and 9373 differentially expressed genes (|log₂FC| ≥ 2, FDR ≤ 0.05). Tissue-specific analysis further identified 1407 mantle-specific and 773 gill-specific genes, reflecting distinct functional specialization of the two tissues. Weighted gene co-expression network analysis (WGCNA) resolved six biologically interpretable modules associated with growth state and tissue identity. When module eigengene patterns were examined across the ordered SG–MG–FG continuum, the modules showed sequential shifts from SG-associated basal metabolic and RNA-processing programs to MG-associated tissue-specialization programs and FG-associated mature mantle and gill functional programs. The FG mantle-associated module was enriched for chitin remodeling, shell matrix-related functions, and structural organization, whereas the FG gill-associated module showed enrichment of Toll-like receptor signaling, epithelial organization, and innate immune functions. Expression patterns of representative genes from the six modules were independently validated by quantitative RT-PCR and showed strong concordance with the RNA-seq data. Collectively, these results identify coordinated differences in basal and tissue-associated transcriptional programs across contrasting growth cohorts. This framework provides a transcriptomic basis for interpreting growth-associated tissue maturation and identifies candidate molecular indicators for future validation in oyster selective breeding.

This study investigated seasonal variation in tissue growth, reproductive status, energy reserves, and digestive tubule condition in the Pacific oyster, Magallana gigas, cultured in an off-bottom intertidal rack system in Garorim Bay on the west coast of Korea. Monthly samples of adult oysters were collected from February 2009 to December 2010. Condition index (CI), total carbohydrate content, reproductive stage, female reproductive effort estimated by indirect enzyme-linked immunosorbent assay (ELISA), and digestive tubule atrophy (DTA) were measured concurrently in individual oysters. Gametogenesis was largely synchronized between sexes and was strongly associated with seasonal sea surface temperature ($r=0.84$, $p<0.0001$). Carbohydrate content peaked in early spring and declined during the spawning season. In females, carbohydrate content was negatively correlated with gonad somatic index (GSI) ($r=-0.30$, $p<0.0001$), suggesting that oysters investing more heavily in egg production tended to have lower remaining carbohydrate reserves. GSI showed interannual variation in spawning timing, peaking at 37.9% in June 2009 and 37.0% in July 2010. DTA showed temporal variation, with relatively elevated values in late summer–early autumn 2009 and mid-winter 2010. The late-summer 2009 increase in DTA coincided with a marked decline in salinity; however, monthly mean analyses did not establish a causal relationship between salinity and DTA. Together, these results indicate that the seasonal physiological condition of intertidal rack-cultured oysters reflects a dynamic balance between reproductive investment, carbohydrate reserves, and digestive gland condition, with this balance potentially modified by seasonal environmental variability. These results provide a cross-sectional, multi-parameter baseline for seasonal monitoring of reproductive status, energy reserves, and digestive tubule condition in intertidal rack-cultured oysters in Garorim Bay.