Horizontal Gene Transfer in Eukaryotes: Not if, but How Much?

Horizontal gene transfer (HGT), the movement of genetic material across branches of the tree of life, is well established in prokaryotes and uncontroversial. This is explained in part by relatively compact prokaryote genomes that facilitate assembly and gene prediction, resulting in thousands of complete genomes for analysis. By contrast, their large and often complex genome structure have thwarted HGT studies of eukaryotes. The tide has recently turned with the availability of sufficient high-quality genome data to address quantity and quality of HGT in these taxa. Here, we argue that HGT isa small but significant player in the evolution of microbial eukaryotes and provide examples where HGT has facilitated gain of adaptive functions and in some cases, underpinned major lifestyle transitions.