Table1_Morphological and molecular evolution of hadal amphipod’s eggs provides insights into embryogenesis under high hydrostatic pressure.XLSX

Hadal zones are unique habitats characterized by high hydrostatic pressure (HHP) and scarce food supplies. The ability of eggs of species dwelling in hadal zones to develop into normal embryo under high hydrostatic pressure is an important evolutionary and developmental trait. However, the mechanisms underlying the development of eggs of hadal-dwelling species remain unknown due to the difficulty of sampling ovigerous females. Here, morphological and transcriptome analyses of eggs of the “supergiant” amphipod Alicella gigantea collected from the New Britain Trench were conducted. The morphology of A. gigantea eggs, including size, was assessed and the ultrastructure of the eggshell was investigated by scanning electron microscopy. Transcriptome sequencing and molecular adaptive evolution analysis of A. gigantea eggs showed that, as compared with shallow-water Gammarus species, genes exhibiting accelerated evolution and the positively selected genes were mostly related to pathways associated with “mitosis” and “chitin-based embryonic cuticle biosynthetic process”, suggesting that “normal mitosis maintenance” and “cuticle development and protection” are the two main adaptation strategies for survival of eggs in hadal environments. In addition, the concentration of trimethylamine oxide (TMAO), an important osmotic regulator, was significantly higher in the eggs of hadal amphipods as compared to those of shallow-water species, which might promote the eggs’ adaptation abilities. Morphological identification, evolutionary analysis, and the trimethylamine oxide concentration of A. gigantea eggs will facilitate a comprehensive overview of the piezophilic adaptation of embryos in hadal environments and provide a strategy to analyze embryogenesis under high hydrostatic pressure.

哈达尔海域是具有高静水压（HHP）和稀少食物供应的独特栖息地。哈达尔海域居住物种的卵子在高压下发育成正常胚胎的能力是重要的进化与发育特性。然而，由于采样产卵雌性个体的困难，哈达尔海域物种卵子发育的机制尚不明确。在此，我们对从新不列颠海沟采集的“超级巨型”等足类甲壳动物Alicella gigantea的卵子进行了形态学和转录组分析。评估了A. gigantea卵子的形态，包括大小，并通过扫描电子显微镜研究了卵壳的超微结构。A. gigantea卵子的转录组测序和分子适应性进化分析表明，与浅水区Gammarus物种相比，表现出加速进化和正向选择的基因大多与“有丝分裂”和“基于几丁质的胚胎外皮生物合成途径”相关，这表明“正常有丝分裂维持”和“外皮发育与保护”是哈达尔环境中卵子生存的两种主要适应策略。此外，与浅水物种相比，哈达尔甲壳动物卵子中三甲胺氧化物（TMAO）的浓度显著更高，这可能促进了卵子的适应性。A. gigantea卵子的形态学鉴定、进化分析和三甲胺氧化物浓度将有助于全面了解哈达尔环境中嗜压胚胎的适应性，并提供分析高压下胚胎发生发育的策略。