Although the endosymbiotic theory of evolution (Margulis 1970) is widely accepted, the series of events that led to the permanent inclusion of mitochondria and chloroplasts in eukaryotic cells are poorly understood. In particular, the diverse biochemical and morphological properties of chloroplasts have led to suggestions that these organelles have been acquired through multiple primary endosymbiotic events (for a review see Delwiche 1999). Furthermore, the presence of three and sometimes four (Gibbs 1962) membranes around certain chloroplasts, some in association with a second nucleus, suggest that secondary endosymbiosis-the incorporation within a eukaryote of a heritable organelle from another eukaryote-has also occurred (Gibbs 1981). Secondary endosymbioses may also have occurred more than once (Delwiche and palmer 1997; Delwiche 1999); in fact, the possibility that some autotrophic eukaryotes lost their photosynthetic organelles only to regain them in a later endosymbiotic event cannot be excluded. The occurrence of such multiple, sequential endosymbiotic events can only be proven if some remnant of the first endosymbiont was retained by the host cell. Nuclear-encoded genes for chloroplast proteins, for example GAPDH, could provide such evidence.
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