Population genetics of the aquatic fungus Tetracladium marchalianum over space and time

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.

Figure 1. Collection sites in Wisconsin and Illinois, USA.

Collections were made from three sites each in the Sangamon River (S1, S2, S3) and the Middle Fork of the Vermilion River (V1, V2, V3), and from one site in Konkapot Creek (K).

Figure 2. Maximum Likelihood tree for 19 isolates of T. marchalianum based on partial β-tubulin gene sequences.

The tree resulting from ML analysis of partial β-tubulin gene sequences (889 unambiguously aligned nucleotides). Support for each branch is shown as the minimum of the Chi²-based and SH-like support values; only values greater than 0.90, indicating significant support for a node, are reported. The scale bar indicates the number of nucleotide substitutions per site. The group to which isolates of T. marchalianum were assigned by Structure is identified when known.