Those Nematode-Trapping Fungi That are not Everywhere: Hints Towards Soil Microbial Biogeography.

The existence of biogeography for microorganisms is a raising topic in ecology and researchers are employing better distinctions between single species, including the most rare ones, to reveal potential hidden patterns. An important volume of evidence supporting heterogeneous distributions for bacteria, archaea and protists is accumulating, and more recently a few efforts have targeted microscopic fungi. We propose an insight into this latter kingdom by looking at a group of soil nematode-trapping fungi whose species are well-known and easily recognizable. We chose a pure culture approach because of its reliable isolation procedures for this specific group. After morphologically and molecularly identifying all species collected from 2250 samples distributed in 228 locations across Yunnan province of China, we analyzed occurrence frequencies and mapped species, genera, and richness. Results showed an apparent cosmopolitan tendency for this group of fungi, including species richness among sites. However, only four species were widespread across the region, while non-random heterogeneous distributions were observed for the remaining 40 species, both in terms of statistical distribution of species richness reflected by a significant variance-to-mean ratio, as well as in terms of visually discernible spatial clusters of rare species and genera on the map. Moreover, several species were restricted to only one location, raising the question of whether endemicity exists for this microbial group. Finally, environmental heterogeneity showed a marginal contribution in explaining restricted distributions, suggesting that other factors such as geographical isolation and dispersal capabilities should be explored. These findings contribute to our understanding of the cryptic geographic distribution of microorganisms and encourage further research in this direction.