Extreme Genetic Structure in a Small-Bodied Freshwater Fish, the Purple Spotted Gudgeon, Mogurnda adspersa (Eleotridae)

Freshwater fish are a group that is especially susceptible to biodiversity loss as they often exist naturally in small, fragmented populations that are vulnerable to habitat degradation, pollution and introduction of exotic species. Relatively little is known about spatial dynamics of unperturbed populations of small-bodied freshwater fish species. This study examined population genetic structure of the purple spotted gudgeon (Mogurnda adspersa, Eleotridae), a small-bodied freshwater fish that is widely distributed in eastern Australia. The species is threatened in parts of its range but is common in coastal streams of central Queensland where this study took place. Microsatellite (msat) and mitochondrial DNA (mtDNA) variation was assessed for nine sites from four stream sections in two drainage basins. Very high levels of among population structure were observed (msat FST = 0.18; mtDNA ΦST = 0.85) and evidence for contemporary migration among populations was rare and limited to sites within the same section of stream. Hierarchical structuring of variation was best explained by stream section rather than by drainage basin. Estimates of contemporary effective population size for each site was low (range 28 – 63, Sibship method), but compared favorably with similar estimates for other freshwater fish species, and there was no genetic evidence for inbreeding or recent population bottlenecks. In conclusion, within a stable part of its range, M adspersa exists as a series of small, demographically stable populations that are highly isolated from one another. Complimentary patterns in microsatellites and mtDNA indicate this structuring is the result of long-term processes that have developed over a remarkably small spatial scale. High population structure and limited dispersal mean that recolonisation of locally extinct populations is only likely to occur from closely situated populations within stream sections. Limited potential for recolonisation should be considered as an important factor in conservation and management of this species.

淡水鱼类是一类极易遭受生物多样性丧失的类群，因其天然种群通常规模较小且呈破碎化分布，极易受到生境退化、污染以及外来物种引入的威胁。目前学界对小型淡水鱼类未受干扰种群的空间动态（spatial dynamics）了解相对匮乏。本研究以广泛分布于澳大利亚东部的小型淡水鱼类——紫斑塘鳢（Mogurnda adspersa，Eleotridae）为研究对象，分析其种群遗传结构（population genetic structure）。该物种在其部分分布区内受到生存威胁，但在本研究开展的昆士兰州中部沿海溪流中较为常见。研究对两个流域（drainage basins）内四个河段的9个采样点开展了微卫星（microsatellite，简称msat）与线粒体DNA（mitochondrial DNA，简称mtDNA）变异检测。结果显示，种群间遗传分化水平极高（微卫星FST=0.18；线粒体DNA ΦST=0.85），且种群间的当代迁移（contemporary migration）事件极为罕见，仅发生于同一河段内的采样点之间。种群变异的层级结构（hierarchical structuring）最优拟合模型为河段分组，而非流域分组。各采样点的当代有效种群大小（effective population size）估算值较低（范围为28~63，采用家系法（Sibship method）估算），但与其他淡水鱼类的同类估算结果相比处于合理区间，且未检测到近交（inbreeding）或近期种群瓶颈（population bottlenecks）的遗传信号。综上，在该物种的稳定分布区内，紫斑塘鳢以一系列小型且种群动态稳定的种群形式存在，各群之间高度隔离。微卫星与线粒体DNA的互补分析结果表明，这种遗传结构是在极小空间尺度下长期演化形成的。极高的种群分化水平与有限的扩散能力意味着，局部灭绝种群的重新定殖（recolonisation）仅可能来源于同一河段内邻近的种群。有限的重新定殖潜力应当被纳入该物种保护与管理（conservation and management）的重要考量因素。