Staying close to home? Genetic differentiation of rough-toothed dolphins near oceanic islands in the central Pacific Ocean

Rough-toothed dolphins have a worldwide tropical and subtropical distribution, yet little is known about the population structure and social organization of this typically open-ocean species. Although it has been assumed that pelagic dolphins range widely due to the lack of apparent barriers and unpredictable prey distribution, recent evidence suggests rough-toothed dolphins exhibit fidelity to some oceanic islands. Using the most comprehensively extensive dataset for this species to date, we assess the isolation and interchange of rough-toothed dolphins at the regional and oceanic scale within the central Pacific Ocean. Using mtDNA and microsatellite genotyping (nDNA), we analyzed samples of insular communities from the main Hawaiian (Kaua‘i n = 93, O‘ahu n = 9, Hawai‘i n = 57), French Polynesian (n = 70) and Samoan (n = 16) archipelagos, and pelagic samples off the Northwestern Hawaiian Islands (n = 18). An overall AMOVA indicated strong genetic differentiation among islands (mtDNA FST = 0.265; p < 0.001; nDNA FST = 0.038; p < 0.001), as well as among archipelagos (mtDNA FST = 0.299; p < 0.001; nDNA FST = 0.055; p < 0.001). Shared haplotypes (n = 4) between the archipelagos may be a product of a relatively recent divergence and/or periodic exchange from poorly understood pelagic populations. Analyses using STRUCTURE and GENELAND identified four separate management units among archipelagos and within the Hawaiian Islands. These results confirm the presence of multiple insular populations within the Pacific and island-specific genetic isolation among populations attached to islands in each archipelago. Insular populations seem most prevalent where oceanographic conditions indicate high local productivity or a discontinuity with surrounding oligotrophic areas. Our findings have important implications for a little studied species that faces increasing anthropogenic threats around oceanic islands.

糙齿海豚（Rough-toothed dolphins）在全球热带与亚热带海域均有分布，但学界对这一典型远洋物种的种群结构与社会组织仍知之甚少。过往研究曾假设，远洋海豚（pelagic dolphins）因缺乏明显地理屏障且猎物分布难以预测，活动范围极为广泛，但最新证据表明，糙齿海豚会对部分大洋岛屿表现出栖息地保真度（fidelity）。本研究借助当前该物种最为全面的数据集，对中太平洋区域及大洋尺度下糙齿海豚的种群隔离与基因交流情况展开评估。研究采用线粒体DNA（mitochondrial DNA, mtDNA）与微卫星基因分型（核DNA，nuclear DNA, nDNA）技术，分析了夏威夷主群岛（考爱岛Kaua‘i：n=93；瓦胡岛O‘ahu：n=9；夏威夷岛Hawai‘i：n=57）、法属波利尼西亚群岛（n=70）、萨摩亚群岛（n=16）的岛屿群落样本，以及西北夏威夷群岛外海的远洋样本（n=18）。总体分子方差分析（Analysis of Molecular Variance, AMOVA）结果显示，岛屿间存在显著的遗传分化（mtDNA FST=0.265；p<0.001；nDNA FST=0.038；p<0.001），群岛间亦呈现类似分化模式（mtDNA FST=0.299；p<0.001；nDNA FST=0.055；p<0.001）。群岛间共享的4个单倍型（haplotype），可能源于相对较近的物种分化事件，以及/或来自尚未被充分认知的远洋种群的周期性基因交流。借助STRUCTURE与GENELAND软件开展的分析，在各群岛内部及夏威夷群岛中，共识别出4个独立的管理单元（management units）。上述结果证实，太平洋海域存在多个岛屿特有种群，且各群岛附属岛屿的种群间存在岛屿特异性的遗传隔离。岛屿种群在海洋学条件显示本地生产力较高，或与周围寡营养区域（oligotrophic areas）存在生境间断的区域最为常见。本研究结果对于这一研究程度较低、且在大洋岛屿周边正面临日益加剧的人为威胁（anthropogenic threats）的物种而言，具有重要的科学意义。