Department of Plant Biology, University of Illinois at Urbana-Champaign, Urbana, Illinois, United States of America.
PLoS One. 2011 Jan 14;6(1):e15908. doi: 10.1371/journal.pone.0015908.
Aquatic hyphomycete fungi are fundamental mediators of energy flow and nutrient spiraling in rivers. These microscopic fungi are primarily dispersed in river currents, undergo substantial annual fluctuations in abundance, and reproduce either predominantly or exclusively asexually. These aspects of aquatic hyphomycete biology are expected to influence levels and distributions of genetic diversity over both spatial and temporal scales. In this study, we investigated the spatiotemporal distribution of genotypic diversity in the representative aquatic hyphomycete Tetracladium marchalianum. We sampled populations of this fungus from seven sites, three sites each in two rivers in Illinois, USA, and one site in a Wisconsin river, USA, and repeatedly sampled one population over two years to track population genetic parameters through two seasonal cycles. The resulting fungal isolates (N = 391) were genotyped at eight polymorphic microsatellite loci. In spite of seasonal reductions in the abundance of this species, genotypic diversity was consistently very high and allele frequencies remarkably stable over time. Likewise, genotypic diversity was very high at all sites. Genetic differentiation was only observed between the most distant rivers (∼450 km). Clear evidence that T. marchalianum reproduces sexually in nature was not observed. Additionally, we used phylogenetic analysis of partial β-tubulin gene sequences to confirm that the fungal isolates studied here represent a single species. These results suggest that populations of T. marchalianum may be very large and highly connected at local scales. We speculate that large population sizes and colonization of alternate substrates in both terrestrial and aquatic environments may effectively buffer the aquatic populations from in-stream population fluctuations and facilitate stability in allele frequencies over time. These data also suggest that overland dispersal is more important for structuring populations of T. marchalianum over geographic scales than expected.
水生丝孢真菌是河流中能量流动和营养物质螺旋化的基本介质。这些微观真菌主要分布在河水中，其丰度会发生显著的年度波动，并主要或完全进行无性繁殖。水生丝孢真菌生物学的这些方面预计会影响遗传多样性在空间和时间尺度上的水平和分布。在这项研究中，我们调查了具有代表性的水生丝孢真菌 Tetracladium marchalianum 的基因型多样性的时空分布。我们从美国伊利诺伊州的两条河流中的三个地点和美国威斯康星州的一条河流中的一个地点采集了这种真菌的种群样本，并在两年内对一个种群进行了重复采样，以通过两个季节性周期跟踪种群遗传参数。由此产生的真菌分离物（N=391）在 8 个多态微卫星基因座上进行了基因型分型。尽管该物种的丰度季节性下降，但基因型多样性始终非常高，等位基因频率随时间非常稳定。同样，所有地点的基因型多样性都非常高。仅在距离最远的河流（约 450 公里）之间观察到遗传分化。没有观察到 T. marchalianum 在自然界中进行有性繁殖的明显证据。此外，我们还使用部分β-微管蛋白基因序列的系统发育分析来确认研究中使用的真菌分离物代表单一物种。这些结果表明，T. marchalianum 的种群在局部尺度上可能非常大且高度连接。我们推测，大的种群规模和在陆地和水生环境中替代基质的定殖可能有效地缓冲河流种群的波动，并随着时间的推移促进等位基因频率的稳定性。这些数据还表明，与预期相比，越境扩散对 T. marchalianum 种群在地理尺度上的结构更为重要。
