MadSci Network: Cell Biology

Re: what can happen when moitochondria evolves? ?

Date: Mon Jun 25 11:45:06 2001
Posted By: Mike Klymkowsky, Professor
Area of science: Cell Biology
ID: 993075312.Cb

Evolving mitochondria.

The question of mitochondrial evolution is closely linked to mitochondrial origins.

Based on genomic sequence analysis, it has been proposed that eukarya (organisms like ourselves, with cells that contain a true nucleus) arose through the fusion (some 2,000,000,000 years ago) of a bacterium and an archea (the "eubacterial/archeal merger").

The bacterium gave rise to mitochondria, while the archea produced the surrounding cell (and its nucleus).

From an evolutionary point of view, this dramatically changed in the bacterium's environment and the selection pressures acting on it.

The environment within the eukaryotic cell is more or less constant.

In such a constant environment, many of the mitochondrion's genes, previously valuable for adapting to changing environments, became redundant.

It requires energy to replicate DNA (genes) and energy spend on unnecessary DNA is wasted.

Deletion of unnecessary genes speeded mitochondrial replication.

Those mitochondria replicated faster would swamp out their slower brothers.

Since only a relatively small number of mitochondria are passed from one generation to the next (typically the mother's), fast replicating mitochondria usefully became the only mitochondria present.

A similar process of host/symbiont evolution has beenoccurring in aphids. Obligate bacteria (known as bacteriocytes) are found in a specialized organelle known as a symbiosome.

These bacteria supply the aphid with essential amino acids, not found in its food - the sap of plants.

This association began 150 to 250 million years ago, and now both symbiont and host are completely dependent upon one another.

Here is a technical reference to the evolution of such symbionts, which indicates that it is similar in general terms to the evolution of mitochondria.

Once the unnecessary genes were deleted, a new strategy emerged to "streamline" the mitochondrial genome -- the transfer of mitochondrial genes to the nucleus.

This involves a complex balancing act. The more genes transferred out of the mitochondria, the faster the mitochondria can replicate.

There are cells with essentially no mitochondrial DNA -- they are known as rho0 cells. They have small "petite" mitochondria that do not support respiration, so these cells obtain all of their energy from glycolysis. In most organisms, the rho0 phenotype would be lethal.

When a mitochondrial gene essential for respiration (or some other essential mitochondrial function) is transferred to the nucleus, it must be modified so that its gene product can be reimported back to the mitochondria.

This trick that can be done, and many originally mitochondria/bacterial genes now reside in the nucleus. In fact, in some organisms the mitochondria have retained the DNA for only five genes! (out of the thousands they started with).

Moving genes and resetting Muller's ratchet.

Moving genes to the nucleus has another advantage associated with sexual reproduction. Mutations accumulate in mitochondrial DNA genes. Through a process known as Muller's ratchet, more and more mutations will accumulate -- leading to a decrease in the overall fitness of the mitochondria (with respect to its metabolic functions).

A new organism obtains its mitochondria from its mother. There no way to remove mitochondrial mutations except to eliminate the organism that carries them (here is a link to human diseases related to defective mitochondria).

If mitochondrial genes are transferred to the nucleus, however, then the process of meiotic recombination can sort alleles away from one another. Those gametes/organisms that contain deleterious mutations are eliminated, while those that contain neutral or beneficial alleles survive.

Thus sex acts to reset the mutational load of the species. This appears to be one of the major reason that sexual reproduction evolved and has become the predominant mode of reproduction among higher organisms.

An obvious question then is why haven't all mitochondrial genes moved to the nucleus? follow this link and the "related articles" link on pubmed.


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