Data from: Demographic collapse and low genetic diversity of the Irrawaddy dolphin population inhabiting the Mekong River

In threatened wildlife populations, it is important to determine whether observed low genetic diversity may be due to recent anthropogenic pressure or the consequence of historic events. Historical size of the Irrawaddy dolphin (Orcaella brevirostris) population inhabiting the Mekong River is unknown and there is significant concern for long-term survival of the remaining population as a result of low abundance, slow reproduction rate, high neonatal mortality, and continuing anthropogenic threats. We investigated population structure and reconstructed the demographic history based on 60 Irrawaddy dolphins samples collected between 2001 and 2009. The phylogenetic analysis indicated reciprocal monophyly of Mekong River Orcaella haplotypes with respect to haplotypes from other populations, suggesting long-standing isolation of the Mekong dolphin population from other Orcaella populations. We found that at least 85% of all individuals in the two main study areas: Kratie and Stung Treng, bore the same mitochondrial haplotype. Out of the 21 microsatellite loci tested, only ten were polymorphic and exhibited very low levels of genetic diversity. Both individual and frequency-based approaches suggest very low and non-significant genetic differentiation of the Mekong dolphin population. Evidence for recent bottlenecks was equivocal. Some results suggested a recent exponential decline in the Mekong dolphin population, with the current size being only 5.2% of the ancestral population. In order for the Mekong dolphin population to have any potential for long-term survival, it is imperative that management priorities focus on preventing any further population fragmentation or genetic loss, reducing or eliminating anthropogenic threats, and promoting connectivity between all subpopulations.

对于受威胁的野生动物种群而言，明确其观测到的遗传多样性低下究竟源于近期人为活动压力，还是历史事件遗留的后果，具有重要研究意义。栖息于湄公河的伊洛瓦底江豚（Irrawaddy dolphin, Orcaella brevirostris）的历史种群规模尚不明确，且该种群现存个体数量稀少、繁殖速率缓慢、新生幼崽死亡率高，同时持续面临人为活动威胁，因此其长期生存前景令人高度担忧。本研究基于2001年至2009年间采集的60份伊洛瓦底江豚样本，对其种群结构展开了调查，并重建了种群动态历史。系统发育分析（phylogenetic analysis）结果显示，湄公河种群的江豚单倍型与其他种群的单倍型呈相互单系关系，这表明湄公河江豚种群与其他伊洛瓦底江豚种群长期处于生殖隔离状态。我们发现，在两个主要研究区域——桔井（Kratie）与上丁（Stung Treng）——中，至少85%的个体拥有相同的线粒体单倍型（mitochondrial haplotype）。在检测的21个微卫星位点（microsatellite loci）中，仅10个具有多态性，且遗传多样性水平极低。基于个体和等位基因频率的两种分析方法均显示，湄公河江豚种群的遗传分化（genetic differentiation）程度极低且无统计学显著性。关于近期种群瓶颈（population bottleneck）的相关证据尚不明确。部分分析结果表明，湄公河江豚种群近期呈现指数级下降（exponential decline），当前种群规模仅为祖先种群的5.2%。若要使湄公河江豚种群具备长期生存的可能性，管理工作的优先事项必须聚焦于阻止种群进一步碎片化或遗传多样性流失、降低乃至消除人为活动威胁，以及促进所有亚种群间的基因交流。