Organelle Genomes and Endosymbionts

Abstract

Mitochondria and chloroplasts are cell organelles which carry their own genomes. Morphological characteristics, as well as sequence comparisons, clearly support an endosymbiotic origin of these organelles. Mitochondria likely evolved from the uptake of an alphaproteobacterium by a host cell, ultimately leading to the evolution of the eukaryote cell. In contrast, the acquisition of chloroplasts happened several times convergently. Primary endosymbiosis describes the uptake of a cyanobacterium by the last common ancestor of Archaeplastida (comprising plants, green algae, red algae and some other minor taxa). Secondary endosymbiotic events due to the symbiosis of a eukaryote host with chloroplast-bearing algae evolved several times, leading to chloroplasts characterized by three or four cell membranes. Some eukaryotes lack mitochondria, but instead bear derived organelles known as mitosomes or hydrogenosomes. Organelle genomes show usually signs of their prokaryote ancestry (e.g. different genetic code than nuclear DNA) and can differ greatly in size, structure and gene content across taxa. Some sequence fragments of organelle genes became a popular choice for genetic studies, and mitochondrial and chloroplast markers are used as standard for DNA barcoding of animals and plants, respectively. Endosymbiosis is common across eukaryote taxa, and some relationships blur the distinction between organelles and symbionts. Heritable bacterial endosymbionts are broadly classified as either primary symbionts or secondary symbionts. Whereas relationships of primary symbionts are characterized by a mutual obligate relationships, secondary symbionts can be facultative mutualists and reproductive manipulators or have unknown effects on their host. Typically, endosymbiont genomes are highly streamlined, and the smallest reported genomes of all living organisms are described from primary symbionts of insects.