Lateral gene transfers and the evolution of eukaryotes: theories and data.

Vertical transmission of heritable material, a cornerstone of the Darwinian theory of evolution, is inadequate to describe the evolution of eukaryotes, particularly microbial eukaryotes. This is because eukaryotic cells and eukaryotic genomes are chimeric, having evolved through a combination of vertical (parent to offspring) and lateral (trans-species) transmission. Observations on widespread chimerism in eukaryotes have led to new and revised hypothesis for the origin and diversification of eukaryotes that provide specific predictions on the tempo (early vs continuous transfers) and mode (nature of donor and recipient lineages) of lateral gene transfers (LGTs). Analyses of available data indicate that LGTs in eukaryotes largely fall into two categories: (1) LGTs from organelles to the nucleus, only a few of which appear to have occurred at the time of the origin of eukaryotes, and (2) anomalous LGTs involving diverse donor and recipient lineages. Further testing of hypotheses on the origin and diversification of eukaryotes will require complete genome sequences from a number of diverse eukaryotes and prokaryotes combined with sequences of targeted genes from a broad phylogenetic sample.