Aquaporins (AQPs) are highly-selective transmembrane water transporters that are involved in the adaptation to environmental challenges. However, the structure, function, and evolution of AQPs in bivalves remain largely unknown. In this study, AQP gene family members were identified in nine bivalve species, and their abundance rangs from 7 to 15. Nine AQPs (MmAQPs) were identified in the genome of hard clam (Mercenaria mercenaria), which is a euryhaline bivalve that has evolved sophisticated osmoregulatory mechanisms and salinity adaptation. Structurally, all MmAQPs contain 6 or 12 transmembrane α-helices, a major intrinsic protein (MIP) domain, and 2 asparagine-proline-alanine (NPA) motifs. MmAQPs were classified into three subfamilies based on phylogenetic analysis: AQP1-like, AQP3-like, and AQP8-like. No AQP11-like subfamily member was identified in the genome of hard clam. Tandem duplication resulted in a lineage-specific expansion in AQP8-like subfamily in hard clams. MmAQP8 genes showed different expression sensitivity to different environmental stressors. The gene expression patterns of three MmAQP8 were similar under heat, hypoxia, and air exposure stress, but differed greatly under salinity stress, indicating that tandem duplication events may accelerate the functional divergence of AQP8 genes in hard clams. AQP3-like members may have undergone gene loss during evolution, resulting in weakened glycerol and urea penetration in hard clams. Three orthologs of MmAQPs were detected in the genomes of Cyclina sinensis and Archivesica marissinica through synteny analyses. Tissue expression profiles showed that MmAQP genes were highly expressed in the foot and hepatopancreas. Under environmental stress, the expression levels of most of the MmAQP genes changed significantly to maintain metabolic homeostasis. Several MmAQP genes were downregulated to reduce water permeability under salinity and air exposure stress. Several MmAQP genes were significantly upregulated to promote the transmembrane transport of ammonia and reactive oxygen species and activate anti-apoptotic responses to resist stress. This study provides a comprehensive understanding of the AQP gene family in hard clams, and lays a foundation for further studies to explore the functions of AQPs in bivalves.

中文翻译：

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Mercenaria mercenaria水通道蛋白基因家族成员的全基因组鉴定、结构和进化特征及表达分析

水通道蛋白 (AQP) 是高度选择性的跨膜水转运蛋白，参与适应环境挑战。然而，双壳类动物 AQP 的结构、功能和进化在很大程度上仍然未知。本研究在 9 个双壳类动物中鉴定出 AQP 基因家族成员，其丰度范围为 7 到 15。在硬蛤基因组中鉴定出 9 个 AQP（MmAQP）（雇佣兵雇佣兵)，这是一种进化出复杂的渗透压调节机制和盐度适应性的广盐双壳类动物。在结构上，所有 MmAQP 都包含 6 或 12 个跨膜 α-螺旋、一个主要内在蛋白 (MIP) 结构域和 2 个天冬酰胺-脯氨酸-丙氨酸 (NPA) 基序。基于系统发育分析，MmAQPs 分为三个亚家族：AQP1-like、AQP3-like 和 AQP8-like。在硬蛤的基因组中没有发现 AQP11 样亚科成员。串联复制导致硬蛤中 AQP8 样亚科的谱系特异性扩展。MmAQP8基因对不同的环境应激源表现出不同的表达敏感性。三种MmAQP8的基因表达模式在热、缺氧和空气暴露胁迫下相似，但在盐胁迫下差异很大，表明串联复制事件可能会加速硬蛤中 AQP8 基因的功能分化。AQP3 样成员可能在进化过程中经历了基因丢失，导致甘油和尿素在硬蛤中的渗透减弱。在基因组中检测到 MmAQP 的三个直系同源物青蛤和马里西尼档案馆通过同线性分析。组织表达谱显示 MmAQP 基因在足部和肝胰腺中高表达。在环境压力下，大多数 MmAQP 基因的表达水平发生显着变化以维持代谢稳态。几个 MmAQP 基因被下调以降低盐度和空气暴露胁迫下的透水性。几个 MmAQP 基因被显着上调，以促进氨和活性氧的跨膜转运，并激活抗凋亡反应以抵抗压力。本研究提供了对硬蛤AQP基因家族的全面认识，为进一步研究双壳类动物AQP的功能奠定了基础。