The following points highlight the top two theories based on origin of mitochondria and chloroplasts. The theories are: 1. Endosymbiont Theory 2. Direct Filiation Theory.
1. Endosymbiont Theory:
This theory is based on the similarities of chloroplasts and mitochondria with prokaryotic cells. These organelles possess their own genetic material (DNA) as well as the machinery for protein synthesis. According to this theory, eukaryotic cells developed from primitive “proto-eukaryotic cells” which were anaerobic and utilized the glycolytic pathway for energy generation.
These proto-eukaryotic cells were large in size and were able to take nutrients into the cell from the outer environment through phagocytosis.
Mitochondria are proposed to have evolved from primitive prokaryotes that could utilize oxygen for respiration (aerobic prokaryotes) following their ingestion into the proto-eukaryotic cells through the process of phagocytosis (Fig. 20.8). The O2 utilizing prokaryotic cells became symbionts of the proto-eukaryotic cells which had ingested them.
It is envisaged that during evolution, these prokaryotic symbionts lost their unnecessary functions and evolved to become the present day mitochondria.
Today, some of the mitochondrial proteins are encoded by the cell nucleus and are synthesized in the cytoplasm on cytoplasmic ribosomes, while several others are encoded by the mtDNA and are synthesized within the mitochondria itself in association with the mitochondrial ribosomes.
Migration of DNA from mitochondria and chloroplasts into the nuclear DNA has been documented; this kind of DNA is called promiscuous DNA. The existence of promiscuous DNA explains why nuclear genes code for some of the mitochondrial proteins and also suggests the manner in which nucleus may have acquired this capability; it also supports the endosymbiont theory of origin of the organelles.
Chloroplasts are proposed to have evolved from the prokaryotes capable of photosynthesis (Fig. 20.8). These photosynthetic prokaryotes are thought to have been ingested by the proto-eukaryotic cells and in due course established symbiotic relationship with them.
During the evolution, they lost their unnecessary functions and developed into the present-day chloroplasts. It has been suggested that the chloroplasts of different plant species containing different chlorophyll pigments evolved from different symbiotic events.
For example, blue green algae containing chlorophyll a and phycobilins are believed to have become symbionts in the proto-eukaryotic cells which developed into red algae.
Similarly, hypothetical photosynthetic prokaryotes containing chlorophyll a and c (yellow prokaryotes) became symbionts in the proto-eukaryotic cells which ultimately evolved into brown algae, diatoms and dinoflagellates.
The yellow prokaryotes are now extinct. Another group of hypothetical photosynthetic prokaryotes containing chlorophylls a and b (green prokaryotes) were ingested by the proto-eukaryotic cells which later evolved into green algae and higher plants. The green prokaryotes have also become extinct.
The bases for the endosymbiont theory lie in the similarities of the organelles (chloroplasts and mitochondria) with prokaryotic cells and in their dissimilarities from the eukaryotic cells. The major points of these similarities and dissimilarities are summarized in Table 20.6.
2. Direct Filiation Theory:
According to this theory, the chloroplasts and mitochondria did not evolve from prokaryotic cells ingested from outside but they are believed to have developed within the primitive “proto-eukaryotic cells”. It is proposed that cytoplasmic vesicles were formed by the invagination of the plasma membranes of the proto-eukaryotic cells.
These cytoplasmic vesicles also enclosed some genetic material (DNA) and gradually evolved into the present day organelles. For the evolution of mitochondria and chloroplasts, three models have been proposed.
(1) Model of Raff and Mahler:
In the proto-eukaryotic cell, plasma membrane contained the respiratory mechanisms of electron transfer and oxidative phosphorylation. The plasma membrane invaginated to produce free cytoplasmic vesicles which enclosed the respiratory pathways. Later it acquired plasmid or extra-chromosomal fragments of DNA.
This DNA contained genes for tRNA, rRNA and mRNA coding for the hydrophobic proteins of the respiratory membrane.
(2) Model of Cavalier-Smith:
This is a general model that suggests the origin of mitochondria, chloroplasts, nucleus, Golgi complex and lysosomes etc. According to this model, budding off and fusion of plasma membrane (a process of endocytosis) enclosed some extra chromosomal DNA segment (or plasmid).
Some of these structures developed to become mitochondria, while some others developed to become chloroplasts; a similar process is believed to be involved in the formation of nuclear membrane that enclosed the chromosomes to produce the nucleus.
(3) Model of Reijnders:
According to this model, DNA of the proto-eukaryotic cell duplicated and the two copies of the DNA were enclosed by separate membranes to make separate compartments. Much of the DNA of one compartment was eliminated leaving only that part which was necessary for mitochondrial function; this compartment ultimately developed into mitochondria.
There was no loss of DNA from the other compartment and it finally developed into the nucleus. Evolution of chloroplasts is proposed to have occurred through a similar process.