From Newsgroup: talk.origins
On 8/19/2025 8:30 PM, John Harshman wrote:
On 8/18/25 6:23 AM, Ernest Major wrote:
The suggestion is that low vitamin C levels are protective against
schistosomes (and potentially in evolutionary history against other
parasites).
https://www.biorxiv.org/content/10.1101/2025.07.22.666193v1.full.pdf
Discussed at
https://www.youtube.com/watch?v=bkO9iFhALL4
There is no citation for the phylogenetic distribution of the loss of ascorbate synthesis. Is it actually true that all insects, all teleosts,
and all passerine birds lack this pathway?
It must be tough to have just enough deficiency to protest against schistosomes but not enough to get scurvy. I'm dubious.
It looks like only some passerine birds and only some insects have lost
the ability to make vitamin C.
They acknowledge that you have to give guinea pigs scurvy before they
are resistant. My take is that there is no selective advantage against parasites that also rely on vitamin C in their diets. As long as any
animal can get enough vitamin C they support parasite reproduction. If
the host is suffering a deficiency the parasites suffer a deficiency.
The hosts would have lost the ability to make vitamin C because they
could get enough in their diet. If they couldn't get enough vitamin C
in their diet they would have more to worry about than parasites. The parasites would be a negative selective pressure when the host's diets
were adequate. The animals obviously need to select against the
parasites when conditions are favorable for the host and parasite
survival. Why would selection be favored for animals that could not get enough to eat?
There are animals like pigs that can make vitamin C, but the synthesis
is compromised in some way and they do not make enough and rely on their
diets to get enough vitamin C. They could check to see if pigs are more
or less resistant than humans eating their normal diets.
Ron Okimoto
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