> “I’m still confused. My jaw is still on the floor,” said Prosanta Chakrabarty, an ichthyologist at Louisiana State University and the curator of fishes at its Museum of Natural Science. “It’s like if they had a cow and a giraffe make a baby.” Then he quickly corrected himself, because the lineages of those two ruminants split only a few dozen million years ago. The evolutionary paths of paddlefish and sturgeons diverged 184 million years ago. For those fish to breed is more like “if a human came out of a platypus egg,” he said.
While this is very surprising, it is not that surprising.
The distance in time from the point when some evolution lineages have split is a very poor predictor of the genetic distance between them.
While no living being from today is really a "living fossil" that is identical to some ancient ancestor, there still exist both conservative evolution lines, where those who live today do not have any obvious differences from ancestors living even 500 million years ago, and progressive evolution lines, where after switching to a different way of life and after even only one million years the descendants may have changed beyond recognition.
> American paddlefish (Polyodon spathula), which are diploid, have 60 pairs of chromosomes, while Russian sturgeons (Acipenser gueldenstaedtii), which are tetraploid, have four sets totaling about 250 chromosomes (the chromosomes are so numerous and some are so small that it’s hard to count them reliably). Nearly 200 million years of independent evolution should have seeded the two species’ DNA with countless genetic mismatches and incompatibilities — from missing and added genes, to rearrangements and relocations of genes, to mutational tweaks to gene expression. It seemed certain that hybrid cells would struggle to figure out how to line up their chromosomes during cell division and what genes to turn on or off.
As the researchers said:
> “We never wanted to play around with hybridization,” said Mozsár. “It was just a negative control, which found, somehow, a way to live.”
Such differences in the number and organization of chromosomes are the reason why the hybrids of distantly related species are usually sterile.
Only when gametes are produced by the hybrids for their reproduction, during the meiosis the homologous chromosomes must pair and in such hybrids this fails.
On the other hand, when the first hybrid cell is formed by the fusion of gametes, and also during its later divisions, which will grow the hybrid body, the number and organization of the chromosomes matters very little.
What matters is just the amount of differences in individual genes and in their regulation, which should be small enough so that their functions remain compatible, which must be true in this case.
It is normal in animals for the number of chromosomes to fluctuate due to fusions or fissions and for the order of the genes to be modified, but such changes do not modify the complete set of genes or their functions and these changes may happen without any changes in the appearance of the animal.
> The distance in time from the point when some evolution lineages have split is a very poor predictor of the genetic distance between them.
It seems to be a very good predictor with some extreme outliers yeah? It's reliable enough that this event ran against the intuition of specialists who are trained and experienced in this domain.
It's easy to look after the fact and say "oh ya 180m years but they didn't change much" it's another thing to have predicted this as a likely outcome. They didn't, I wouldn't have, I doubt you would have either?
It is a reasonably good predictor only for animals which are relatively closely related and only when in none of the two lineages there has been any significant modification of the way of life, like starting to eat a different kind of food or adapting to an environment where different locomotion or sensory skills are needed.
In this case the accumulated genetic distance consists mainly in random mutations with no or little adaptive value, for which the approximation of a constant "molecular clock" is valid.
The attempts to extrapolate this "molecular clock" hypothesis to other circumstances have produced a huge number of ridiculous research papers whose results have low usefulness.
The approximation of a constant "molecular clock" may be also be applicable in other cases then what I have mentioned in the beginning when the genetic distance does not refer to the complete genome, but only to some groups of genes that have not been involved in the specific adaptations which have been caused in one or both lineages by changes in the way of life, so they have still evolved with a constant mutation rate.
The approximation of a constant "molecular clock" fails in many cases because even if mutations really happen at a relatively constant rate, most of them lead to immediate extinction of the mutated cells and the mutations that we see in the genomes that are examined now are only a small fraction of all mutations, those that have been preserved in survivors.
How large is that fraction depends on where in the genome the mutation has happened, i.e. whether it was in a place with large or small influence on the chances of survival and also on whether the mutated animal lived at that time in an environment with great or little adaptive pressure.
There have been attempts at hybridization between sturgeon and paddlefish in the past. Paddlefish are the most closely related extant species to the sturgeons, so while it's surprising, it's not exactly mind-blowing. It's not like they hybridized a sturgeon with a carp.
>“The case with myself and others who have taken sturgeon from our waters and tried to make the hybrid is we get nothing. It does not develop,” Semmens said. “Maybe the egg will start to develop, but it does not complete development. It does not hatch. These guys over in Hungary don’t have the same species of sturgeon to work with that we do. We have the shovelnose sturgeon. They have Russian sturgeon. It turns out there are some differences there, and that was important.” [0]
"The expectation was that if you took paddlefish milk and fertilized a sturgeon egg, nothing would happen. When they crossed the Russian sturgeon eggs with milk or spermatozoa in the milk, the expectation was it would not hybridize from the paddlefish"
This is a quote from an expert who didn't expect it to happen. And you're presenting that to me to suggest that it was expected. I don't normally say this but: you're wrong.
It's a quote from the same guy. He'd tried it in the past because they are close enough that he thought it could happen. He didn't expect it to work this time because it hadn't worked with other sturgeon species.
>Both the American paddlefish and the Russian sturgeon are endangered species.[2]
This is where I started getting suspicious about how accurate the article was. The paddlefish is vulnerable, but not endangered. They're doing pretty well in the Missouri, all things considered.
>Researchers in Hungary conducted experiments designed to test if either species could be bred in captivity.
This is where the wiki lost me. Paddlefish are relatively easy to produce in captivity. We've been doing it for decades. It generally doesn't involve sperm from an unrelated species either. I'm curious what the real story is.
> During an experiment to produce gynogenic Russian sturgeon progeny, a negative control was initiated using non-irradiated American paddlefish sperm and eggs from the Russian sturgeon. Unexpectedly, the control cross resulted in viable hybrids.
I'm not a biologist, but I guess it's the "gynogenic" part that's hard to do?
> The initial goal of the study was to encourage the critically endangered sturgeon to reproduce asexually. That isn’t quite how it went.
The focus of the study was the critically endangered sturgeon.
The other species was needed since:
> in gynogenesis, the DNA of the sperm specimen isn’t supposed to transfer to the offspring.
They deliberately picked a distant species (not in the same genus, and not even in the same family) to use as a negative control. But, accidentally, their negative control group "found a way to live" (in their words)!
> American paddlefish are also protected under Appendix II of the Convention on International Trade in Endangered Species (CITES) meaning international trade in the species (including parts and derivatives) is regulated
I suppose if I want to complain about the wiki article being badly written, I should probably think about working on it myself. It's pretty misleading.
As far as these being distantly related species, they are, but they're also as closely related as you can get without being in Acipenser. They're separate families, but those are the only two families within the suborder Acipenseroidei. Super neat fishes. I highly recommend looking up videos of baby paddlefish, as they're adorable. My favorite is when they're just old enough that they start switching to filter feeding. Handling sturgeon just makes you feel like you've got something ancient in your hands, which you kind of do.
What's interesting to me is that the paddlefish shares the Missouri River with both the shovelnose sturgeon and the pallid sturgeon. Also possibly the lake sturgeon if I'm remembering correctly that they sometimes come down the Missouri a bit especially during flood years. Makes me wonder if it's just luck that those sturgeon species aren't tetraploid or if tetraploid individuals are produced and selected against as they hybridize with paddlefish for potentially less well-adapted progeny.
The genetics researcher that claimed humans are pig-monkey hybrids is a professor at UGA. His website has some wild examples of real and possible hybrids: http://www.macroevolution.net/about-me.html
Russian sturgeon is endangered species, because it's hands down the most delicious fish in the world. Especially when grilled or hot-smoked. It is truly in a class of its own, well above the rest. Add to that being the source of the caviar and it will get you eaten to extinction.
You could see about getting in contact with them about that, haha. It does sound like they're considering the potential of the hybrids for aquaculture. The focus seems to be caviar, but if there's continued production, they might consider branching out.
Linking your research to CO2 or climate change helps in getting funded, the parent comment is sarcastic but it brings up a good point IMO.
It doesn't even mean the authors think there is any actual implication for CO2 or that they are being dishonest, in many cases the system has to be gamed like this if you want to be able to do research at all.
It's not that silly of a claim. Some farmed fish require a 20:1 wild fish feed conversation ratio. Modern salmon genetics (and feeds and culture techniques) have brought that down to essentially 1:1. If hybridisation resulted in a caviar producing animal that grew significantly faster and or produced more eggs than the sturgeon currently does...
- From https://www.quantamagazine.org/extra-dna-may-make-unlikely-h...