Mock communities highlight the diversity of host-associated eukaryotes.

Host-associated microbes are ubiquitous. Every multicellular eukaryote, and even many unicellular eukaryotes (protists), hosts a diverse community of microbes. High-throughput sequencing (HTS) tools have illuminated the vast diversity of host-associated microbes and shown that they have widespread influence on host biology, ecology and evolution (McFall-Ngai et al. ). Bacteria receive most of the attention, but protists are also important components of microbial communities associated with humans (Parfrey et al. ) and other hosts. As HTS tools are increasingly used to study eukaryotes, the presence of numerous and diverse host-associated eukaryotes is emerging as a common theme across ecosystems. Indeed, HTS studies demonstrate that host-associated lineages account for between 2 and 12% of overall eukaryotic sequences detected in soil, marine and freshwater data sets, with much higher relative abundances observed in some samples (Ramirez et al. ; Simon et al. ; de Vargas et al. ). Previous studies in soil detected large numbers of predominantly parasitic lineages such as Apicomplexa, but did not delve into their origin [e.g. (Ramirez et al. )]. In this issue of Molecular Ecology, Geisen et al. () use mock communities to show that many of the eukaryotic organisms detected by environmental sequencing in soils are potentially associated with animal hosts rather than free-living. By isolating the host-associated fraction of soil microbial communities, Geisen and colleagues help explain the surprisingly high diversity of parasitic eukaryotic lineages often detected in soil/terrestrial studies using high-throughput sequencing (HTS) and reinforce the ubiquity of these host-associated microbes. It is clear that we can no longer assume that organisms detected in bulk environmental sequencing are free-living, but instead need to design studies that specifically enumerate the diversity and function of host-associated eukaryotes. Doing so will allow the field to determine the role host-associated eukaryotes play in soils and other environments and to evaluate hypotheses on assembly of host-associated communities, disease ecology and more.