Abstract
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.
Highlights
Within-species genetic diversity is distributed spatially among populations and is variable over time
We find no evidence of cryptic species in T. marchalianum; this finding is supported by published phylogenies based on 18S and internal transcribed spacer (ITS) rDNA [54,81]
These dynamics do not appear to influence the genetic make-up of T. marchalianum populations
Summary
Within-species genetic diversity is distributed spatially among populations and is variable over time. In the case of aquatic hyphomycete fungi, which are microscopic, submerged in water, and grow into and through opaque substrates, population genetic studies can generate unprecedented insight into otherwise unobservable biological phenomena. Aquatic hyphomycetes are saprotrophic fungi that are fundamental mediators of energy flow and nutrient spiraling in stream food webs [1,2,3,4]. More than 90% of the carbon in wooded stream food webs originates from terrestrial vegetation [5,6] but is initially inaccessible to aquatic invertebrates, which generally do not produce the enzymes necessary to digest recalcitrant plant compounds [7]. In the process of obtaining their own nutrition, aquatic hyphomycetes digest plant debris (e.g. [8,9]), thereby promoting decomposition and making carbon and nutrients available to aquatic invertebrates [10,11]
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