Mitochondria and Chloroplasts Evolved by Endosymbiosis

KEY CONCEPTS
-Both mitochondria and chloroplasts are descended from bacterial ancestors.
-Most of the genes of the mitochondrial and chloroplast genomes have been transferred to the nucleus during the organelle’s evolution.
How is it that an organelle evolved so that it contains genetic information for some of its functions, whereas the information for other functions is encoded in the nucleus? FIGURE 1 shows the endosymbiotic hypothesis for mitochondrial evolution, in which primitive cells captured bacteria that provided the function of cellular respiration and over time evolved into mitochondria. At first, the proto-organelle must have contained all of the genes needed to specify its functions. A similar mechanism has been proposed for the origin of chloroplasts.

FIGURE 1 Mitochondria originated by an endosymbiotic event when a bacterium was captured by a eukaryotic cell.
Sequence homologies suggest that mitochondria and chloroplasts evolved separately from lineages that are common with different eubacteria, with mitochondria sharing an origin with α-purple bacteria and chloroplasts sharing an origin with cyanobacteria. The closest known relative of mitochondria among the bacteria is Rickettsia (the causative agent of typhus, Rocky Mountain spotted fever, and several other infectious diseases carried by arthropod vectors), which is an obligate intracellular parasite that is probably descended from free-living bacteria. This reinforces the idea that mitochondria originated in an endosymbiotic event involving an ancestor that is also common to Rickettsia.
The endosymbiotic origin of the chloroplast is emphasized by the relationships between its genes and their counterparts in bacteria.
The organization of the rRNA genes in particular is closely related to that of a cyanobacterium, which pins down more precisely the last common ancestor between chloroplasts and bacteria. Not surprisingly, cyanobacteria are photosynthetic.
At least two changes must have occurred as the bacterium became integrated into the recipient cell and evolved into the mitochondrion (or chloroplast). The organelles have far fewer genes than an independent bacterium and have lost many of the gene functions that are necessary for independent life (such as metabolic pathways). The majority of genes encoding organelle functions are in fact now located in the nucleus, so these genes must have been transferred there from the organelle.
Transfer of DNA between an organelle and the nucleus has occurred over evolutionary history and still continues. The rate of transfer can be measured directly by introducing a gene that can
function only in the nucleus (because it contains a nuclear intron, or because the protein must function in the cytosol) into an organelle. In terms of providing the material for evolution, the transfer rates from organelle to nucleus are roughly equivalent to the rate of single gene mutation. DNA introduced into mitochondria is transferred to the nucleus at a rate of 2 × 10^-5 per generation.
Experiments to measure transfer in the reverse direction, from nucleus to mitochondrion, suggest that the rate is much lower, less than 10 . When a nuclear-specific antibiotic resistance gene is introduced into chloroplasts, its transfer to the nucleus and successful expression can be detected by screening seedlings for resistance to the antibiotic. This shows that transfer occurs at a rate of 1 in 16,000 seedlings, or 6 × 10^-5 per generation. Transfer of a gene from an organelle to the nucleus requires physical movement of the DNA, of course, but successful expression also requires changes in the coding sequence.
Organelle proteins that are encoded by nuclear genes have special sequences that allow them to be imported into the organelle after they have been synthesized in the cytoplasm. These sequences are not required by proteins that are synthesized within the organelle.
Perhaps the process of effective gene transfer occurred at a period when compartments were less rigidly defined, so that it was easier both for the DNA to be relocated and for the proteins to be incorporated into the organelle regardless of the site of synthesis.
Phylogenetic analyses show that gene transfers have occurred independently in many different lineages. It appears that transfers of mitochondrial genes to the nucleus occurred only early in animal cell evolution, but it is possible that the process is still continuing in plant cells. The number of transfers can be large; there are more than 800 nuclear genes in Arabidopsis, whose sequences are related to genes in the chloroplasts of other plants. These genes are candidates for evolution from genes that originated in the chloroplast.