The Extent, Role, and Timing of Endosymbiotic Gene Transfer in Plastids

Endosymbiotic gene transfer (EGT), the transfer of endosymbiont genes to the host nucleus, is considered a fundamental process of plastid endosymbiosis. Together with retargeting of the protein products of EGTs to the plastid where they function, EGTs are viewed as a hallmark of plastids as genetically integrated organelles. Despite its central role in one of the biggest evolutionary transitions on our planet, and a long history of inquiry into plastid evolution, our knowledge about the extent of EGTs, their roles in the host cell and timing of acquisition, is still patchy. This chapter summarizes our current knowledge about EGT, framing the discussion in the more general context of horizontal gene transfer (HGT), and highlighting the issues that research in this field is facing. While the need to investigate gene transfer in the context of plastid endosymbiosis is universally acknowledged, there is no consensus on the methodology used to research EGT and HGT, making comparisons between studies difficult. However, some patterns are beginning to emerge and the central role of EGT in plastid establishment is now being shifted toward a shared role between EGT, HGT, and contributions by the host.