Gene Transfer and the Chimeric Nature of Eukaryotic Genomes

Abstract

Three quarters of the eukaryotic genes that can be traced back to prokaryotes are of bacterial origin, although eukaryotes evolved from the archaeal domain. This observation has inspired a multitude of hypotheses for the origin of the eukaryotic cell, including various fusion and endosymbiotic events. In this chapter, I argue that gene transfer between bacteria and eukaryotes over a long evolutionary time is sufficient to explain the observations of the mixed ancestry of eukaryotic genes. Lateral gene transfer is an evolutionary mechanism that steadily gains acceptance also in eukaryotic genomics. Recent data indeed suggest that most eukaryotic lineages are affected by gene transfer, that the process is ongoing, and that the fraction of genes affected probably is underestimated in most genome-wide analyses. Furthermore, studies of photosynthetic eukaryotes show that lateral gene transfer often operates in parallel with endosymbiotic gene transfer in organisms with secondary plastids. A similar mixture of influx of genes from both the endosymbionts and diverse external sources probably also occurred in the primary endosymbiotic events, which would explain the large amount of bacterial genes in the eukaryotic nucleus.