Re: Science has a news article up about "living fossils"

On 3/18/24 8:09 AM, Ernest Major wrote:

On 18/03/2024 14:23, John Harshman wrote:

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I'm afraid your understanding is wrong. Cyt b is part of the electron transport chain. While it's true that most of the genes retained by animal mitochondria are crucial parts of ATP production, so are many of the genes lost from the mitochondrial genome after transfer to the nuclear genome. Many of the proteins involved have to be imported into the mitochondrion, which doesn't seem at all optimal. This seems more like constructive neutral evolution than adaptive evolution. Now of course loss of a crucial gene can only be neutral if it's already been transferred to the nucleus, but that sort of transfer is quite common. The usual fate of such transfers ("numts") is to decay over time, but during the short period when they're functional, the mitochondrial gene could potentially be lost.

 For a mitochondrial gene to be transferred to the nuclear genome, not only does a copy have to be integrated into the nuclear genome, and transcribed, but the resulting protein has to be imported into the mitochrondrion. If I understand correctly that usually (universally?) requires the acquisition of a mitochondrial targeting sequence.
 

Quite so, making it a significantly rarer event. Then we enter a stage in which there are two versions of the mitochondrial protein, and one of them is superfluous. Given the much higher mutation rate of the mitochondrial genome, the mitochondrial gene is much more likely to suffer a deactivating mutation, after which deletions will chip away at its sequence, eventually disposing of it entirely.
A quick search finds a review of metazoan mitochondrial genomes, and there's more variation than I had supposed. The bulk of it is in tRNA genes, but some of the important protein-coding genes are also missing in some species. Check out table 2. It even, very surprisingly, appears that a couple of genes have been added to the mitochondrial genome in a few of the assayed species.
https://www.nature.com/articles/hdy200862