ABSTRACT
A defining feature of eukaryotic cells is the presence of organelles, discrete membrane-bound compartments inside of which specific metabolic processes take place. Two such organelles, mitochondria and the plastids (chloroplasts) of algal and plant cells, evolved from once free-living bacteria by endosymbiosis. Decades of research has revealed the extent to which the bacterial progenitors of mitochondria and plastids were transformed during the process of organellogenesis. But the endosymbiotic events that gave rise to these organelles are ancient, and reconstructing the earliest stages of mitochondrial and plastid evolution from the biology of modern-day organisms has proven challenging. This chapter provides an overview of recently evolved protist–microbe symbioses. The examples we discuss provide substrate for discussions about early organelle evolution and, at the same time, underscore the complex ways in which symbiosis has shaped the microbial biosphere over short evolutionary timescales.
