Tracing the primordial Chlamydiae: extinct parasites of plants?

• The ancestor of extant Chlamydiae was already adapted to an intracellular lifestyle.
• Adaptation to intracellular niche preceded the emergence of Archaeplastida.
• We discuss the potential contributions of the chlamydial ancestor to the evolution of plants.

Chlamydiae are obligate intracellular bacteria found as symbionts and pathogens in a wide range of eukaryotes, including protists, invertebrates, and vertebrates. It was recently proposed that an ancient chlamydial symbiont facilitated the establishment of primary plastids in a tripartite symbiosis with cyanobacteria and early eukaryotes. In this review, we summarize recent advances in understanding of the lifestyle and the evolutionary history of extant Chlamydiae. We reconstruct and describe key features of the ancient chlamydial symbiont. We propose that it was already adapted to an intracellular lifestyle before the emergence of Archaeplastida, and that several observations are compatible with an essential contribution of Chlamydiae to the evolution of algae and plants.