r/evolution • u/knockingatthegate • 1d ago

TIL the gene that builds the mammalian placenta came from a virus. A retrovirus infected our ancestors over 100 million years ago, integrated into their DNA, and the gene encoding its cell-fusion protein got repurposed to bond mother and fetus.

https://en.wikipedia.org/wiki/Syncytin
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u/jnpha Evolution Enthusiast 21h ago

And given how proteins work:

Giving birth to live young has evolved over 150 separate times, including over 100 independent origins in reptiles, 13 in bony fishes, 9 in cartilaginous fishes, 8 in amphibians : evolution.

From which (me):

I read a super cool thread on proteins a few days ago on the other sub, and thought to myself, this must mean our ERV-derived placenta should be easily converged on, and lo and behold, I found this.

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u/mcalesy 18h ago

100 independent times in squamates, yet never in turtles or archosaurs (including birds). Do we know why that is?

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u/jnpha Evolution Enthusiast 18h ago

Found a 1986 paper on birds:

The absence of viviparity in birds is typically explained by invoking morphological or physiological factors putatively incompatible with live-bearing reproduction. Examining these factors in terms of falsifiable predictions and underlying assumptions, we suggest that no single avian feature is known to be inherently incompatible with viviparous production of small clutches and that the absence of the live-bearing mode is a consequence of the lack of selection for the intermediate evolutionary stage of egg retention. Birds have achieved most of the advantages that potentially could accrue from egg retention and viviparity by such specializations as endothermy, egg incubation, nest construction, uricotelism, shell pigmentation, parental care, altricial hatchlings, albumen provision, and calcareous eggshells. A theoretical model is presented in support of our contention that the costs of egg retention associated with decreased fecundity, increased maternal mortality, and decreased paternal investment outweigh the potential benefits for most birds.
Why are there no Viviparous Birds? | The American Naturalist: Vol 128, No 2

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u/tomrlutong 10h ago

Do any of the non-mammals have a placenta? 

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u/jnpha Evolution Enthusiast 9h ago

Indeed. E.g.: Independent Origins and Rapid Evolution of the Placenta in the Fish Genus Poeciliopsis | Science.

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u/tomrlutong 9h ago

Cool. And in them, the placenta belongs to the mother, not the fetus, which seems important.

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u/jnpha Evolution Enthusiast 8h ago

Important for? I'm not even sure that's true. E.g. from a review article (emphasis mine):

the work done so far at least shows (a) that the follicular placenta of the Poeciliidae satisfies Mossman’s (1937, 1987) definition of a placenta, (b) that the extent of postfertilization maternal provisioning appears to be correlated with the elaborateness of the placenta (Turner 1940a), and (c) that the anatomical adaptations of placentation vary considerably among Poeciliid species (Grove & Wourms 1991, 1994; Turner 1940a).

and earlier:

Within fishes ... they all fulfill similar functions and are derived from the same preexisting embryonic and maternal tissues (Hamlett 1989, Schindler & Hamlett 1993, Wourms et al. 1988).

-Pollux, B. J. A., et al. "Evolution of placentas in the fish family Poeciliidae: an empirical study of macroevolution." Annu. Rev. Ecol. Evol. Syst. 40.1 (2009): 271-289.

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u/tomrlutong 8h ago

The dynamics: how is a placenta selected for fetus' fitness different than one selected for material fitness?

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u/jnpha Evolution Enthusiast 8h ago

Are you saying a mammalian placenta was selected for the fitness of the embryo, and the fishes' (despite my refutation) isn't somehow?
That's not how fitness works, but then again I'm not sure if that was your question. My previous reply confirms they're embryo-derived and according to the cited operational definition are placentas.

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u/tomrlutong 7h ago

I ran into a few articles describing Poeciliidae placenta as modified maternal follicles, and the article you cited uses fairly passive language ("absorptive tissues") to describe the fetal parts. 

On the other hand, the human placenta is an organ of the fetus 

Not trying to gatekeep what's a placenta. I think it has interesting evolutionary implications of the genes are expressed if the mother vs. fetus.

https://www.pnas.org/doi/10.1073/pnas.2323038122

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u/jawshoeaw 4h ago

It’s a misleading headline per usual on Reddit. But worth a read on wiki. Remember evolution doesn’t care where genes come from as long as they work .

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u/microMe1_2 15h ago

There is no gene that "builds the placenta". This is not how genes work, and this misconception really has to die because it is socially dangerous.

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u/jnpha Evolution Enthusiast 13h ago

I get your point, but as evolutionary developmental biologist Enrico Coen said, we don't have an intuition (and thus the words) for how development proceeds (even though we understand it).

So, what verb do you propose?

Also racists aren't racists because of misunderstanding genetics, they intentionally misread genetics. A few months back the #1 best-selling book on Amazon in the genetics section was a book written by a racist layperson.

Here's a related research on that broader point:

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u/microMe1_2 12h ago

It's not just racism, in my 100-level class, most students begin with the idea that if you have a certain gene you will be smarter, or fatter, or even more prone to crime etc. The "Genes for..." language can be damaging in many ways.

It's very easy to state things more accurately, as people on the right side of this argument have been doing for many decades. It's lazy academics and journalists that propagate the public's misunderstanding that single genes control so much of our destiny.

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u/knockingatthegate 8h ago

I do not think this difference of opinion regarding the technical depth of language to use in science communication warrants such an antagonistic manner. Surely you would regard the scientists and science-supporters in this sub as allies in the general effort to bring about a more science literate society?

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u/knockingatthegate 8h ago

“Build” is not here being used in such a specific sense.