Dimensions of Horizontal Gene Transfer in Eukaryotic Microbial Pathogens

Comparative genomic studies of microorganisms have disrupted the paradigm of vertical inheritance with modification. First in bacteria, and more recently in microscopic and even multicellular eukaryotes, horizontal gene transfer (HGT) has been implicated in genomic and ecological evolution. HGT is the exchange of genetic material between organisms that occurs independently of meiotic and mitotic recombination between mating or hybridizing individuals. HGT occurs as viral and plasmid-mediated transfer, and transformation by environmental DNA via known or yet-unknown mechanisms [1]. The existence of environmental gene pools and pan-genomes is supported by decades of functional and phylogenetic studies in bacteria that have highlighted the exchange and proliferation of virulence factors and antibiotic resistance mechanisms [2–4]. Presently, accumulating reports of HGT in eukaryotes raise similar questions of how exposure to such gene pools has impacted the evolution of eukaryotic microbes, and whether or not human activities influence HGT dynamics. Here, we describe the evidence supporting HGT in eukaryotic microbial pathogens from divergent lineages that impact human, animal, and plant health (S1 Table). We consider three interacting dimensions affecting the prevalence of HGT (genetic network structure, selectable functions, and opportunity for contact) in order to better understand how HGTmanifests in this important group of organisms.