Symbiosis becoming permanent: Survival of the luckiest

Repeated replacement of an intrabacterial symbiont in the tripartite nested mealybug symbiosis

“…Like organelles, these endosymbionts have genomes encoding few genes (12,13), rely on gene products of bacterial origin that are encoded on the host genome (9-11, 14, 15), and in some cases, import protein products encoded by these horizontally transferred genes back into the symbiont (16,17). The names given to these bacteria-endosymbiont, protoorganelle, or bona fide organelle-are a matter of debate (18)(19)(20)(21). What is not in doubt is that long-term interactions between hosts and essential bacteria generate highly integrated and complex symbioses.…”

“…Despite the reduced N e of organelle genomes relative to nuclear genomes, eukaryotes are able to purge deleterious mutations that arise on organelle genomes, perhaps through a combination of host-level selection and the strong negative selective effects of substitutions on genedense organelle genomes (29,30). Extant organelle genomes also encode few genes relative to most bacteria, and it is also likely that a long history of moving genes to the nuclear genome has helped slow or stop organelle degeneration (21,31). Some of the most degenerate insect endosymbionts also seem to have adopted a gene transfer strategy, although the number of transferred genes is far smaller compared with organelles.…”

From microbiology to cell biology: when an intracellular bacterium becomes part of its host cell

“…But other, more modern endosymbioses are available that might tell us something about how organelles came to be, and why they look the way they do. While these newer symbioses will never recapitulate the evolution of the mitochondria and chloroplasts -nothing could -they can reveal the general outcomes that occur as a result of intimate, long-term endosymbiosis, and thus may have happened during their evolution [24 ]. Or they may not, but it seems worthwhile to try this approach, too.…”

Symbiogenesis, gradualism, and mitochondrial energy in eukaryote origin

“…2b), models in which mitochondria are added to a eukaryotic cell that arose gradually. More modern versions of nonsymbiogenic eukaryote origins tend to focus on gradualist mechanisms while largely disregarding the issue of "origin from which kinds of prokaryote" (15,16,17,102,120). By contrast, older gradualist hypotheses of Van Valen and Maiorana (47) and Cavalier-Smith (35) are more courageously explicit on phylogenetic relationships.…”