ABSTRACT
Some organelles lack an intraorganellar genome, but some have suggested that these organelles in eukaryotes, such as peroxisomes, endosomes, and vacuoles, may have once been autonomous organisms that evolved into organelles, but that all of their genes in their genomes have been transferred into the nucleus. However, because they have single membranes, it is likely that they originated within the eukaryotic cells. Regardless of their evolution, it is clear that most of the ancestral mitochondrial (i.e., a-proteobacterial) genome has been transferred into the nucleus, and the evolution of the genes of the host genome and the mitochondrial genome has been affected greatly. Also, in the Archaeplastida (plants), almost all of the chloroplast (i.e., cyanobacterial) genome has been transferred into the nucleus, and a few pieces have moved into the mitochondrial genome as well. Here, the coordination between the three genomes (chloroplast, mitochondrial, and nuclear) is critical to the survival of those organisms. In many protists, additional symbiotic events have combined additional genomes into single cells, producing organelles that themselves contained multiple genomes. Because many of the gene products had analogous functions (e.g., genes for glycolysis and the tricarboxylic acid [TCA] cycle), evolution has selected for use of the best enzyme and gene for each compartment of the cell (see Chapter 31). Thus, evolutionary processes and outcomes in eukaryotes are complex and intricate.
