Here’s an amazing paper—though the striking part isn’t sufficiently emphasized—reporting artificial hybridization between two long-diverged species, and the resulting appearance of viable hybrids despite what must be substantial genetic divergence between the components of a hybrid genome.
The other species is the American paddlefish (Polyodon spathula), a member of the fish family Polyodontidae, and the only living species in that genus (the Chinese paddlefish, Psephurus gladius, which is in a different genus, has been pronounced extinct). Note the different morphology: the “paddle” on the fish below, which it uses for filter feeding (the Russian sturgeon is a predator on crustaceans, molluscs, and smaller fish), and the lack of bumpy protuberances on the paddlefish’s back. The fin shapes and placement also differ. These species are strikingly divergent in morphology, behavior, and, of course, in time.
How long since these two species had a common ancestor? If you go to the great site Timetree, which gives divergence times of many species calculated from the literature (you can enter any pair and will usually get an answer), you find a median divergence time of 136 million years and an estimated time of about 158 million years. That is a very long time! (The paper below, using older data, says the divergence time is longer: about 184 million years.)
For comparison, the divergence between our species and the Virginia opossum occurred about 160 million years ago, so the sturgeon/paddlefish hybrids are equivalent, with respect to divergence time, to getting a viable hybrid between a human and a possum!
The hybridization is reported in the new paper below from the journal Genes (click on screenshot to go to paper and get the pdf; reference at bottom).
The results can be stated briefly. During an experiment designed to get Russian sturgeon offspring without a father’s genes (“gynogenesis”), the researchers did a control cross between female Russian sturgeon and male American paddlefish (sturgeon eggs were put in a solution of diluted paddlefish sperm). There were five such crosses. They showed a hatching rate of 78-85%, and a survival rate of hybrids at 6 months from 49-68%—comparable to survival in artificial fertilization experiments in pure species.
One complication: Russian sturgeon are “polyploid”, that is, their ancestors underwent a complete duplication of their genomes, so they have four copies of each chromosome (i.e., they are “tetraploid”). Most modern fish are polyploid in this way. Paddlefish, however, diverged so long ago that they are “diploid,” that is, like most modern vertebrate species, they have only two copies of each chromosome.
Because of this difference, two kinds of hybrids were produced. “SH hybrids” were “triploids”, with three sets of each chromosome: two from the sturgeon egg and one from the paddlefish egg (total chromosomes around 165). They also obtained “pentaploid” hybrids, called LH, apparently resulting from the fusion of a sturgeon egg that had a full set of maternal chromosomes plus one set of paddlefish chromosomes (total chromosomes around 300).
You can see the two types of hybrids below. (a) is the Russian sturgeon, and (the caption is wonky), the rest of the pictures show hybrids, without a picture of the paddlefish itself). (c) is the LH hybrid with a full sturgeon genome (four copies of each chromosome) and half a paddlefish genome. That’s why it looks mostly sturgeon-y, with a bit of a paddle in front.
(d) is the “LH” triploid hybrid, which has half a sturgeon genome less than (b). That’s why it looks more paddlefish-y.
I’m assuming that these pictures are labeled correctly in the paper; but it could be that (b), unlabeled in the graph, is an SH hybrid, (c) the LH hybrid (it has a longer “paddle”), and (d) the pure paddlefish. If there’s a labeling error, it should be fixed.
Regardless, the surprising thing is that viable and apparently vigorous hybrids were produced. Were they fertile? No data on that are reported, but, given the difference in ploidy level between the parents, I doubt it: the different numbers of chromosomes from each species would play hob with meiosis—the chromosome-pairing process that is required for the formation of sperm and eggs.
But still, despite about 150 million years apart, the genomes can still work together to produce a viable and functional individual. That is simply amazing; like I said, it’s sort of like getting a viable offspring between a human and a possum. (What a monstrosity that would be: a furry anthropoid with huge teeth and a prehensile tail!)
But why could such long-diverged species still have their genes function fairly harmoniously in a single individual? Well, we don’t know, but there are several possibilities.
1.) While the two species could be long diverged, the function of the diverged genes hasn’t changed much, so they don’t screw up development. In other words, adaptive evolution in these species has been slow since they shared their common ancestor. (This isn’t true for non-coding bits of the genome, for that’s the way they estimated the great divergence time of the species.) I’m not that keen to embrace this possibility since the fish are so different in appearance and behavior, and in ways that seem adaptive.
2.) The species have an “open” developmental system that will tolerate errors or disharmonies more readily, compensating for them by producing a functional individual. This used to be the explanation for why animals like frogs could produce viable hybrids between long-diverged species, while mammals couldn’t. But saying the developmental system is “open” is just another way of saying that “long-diverged genes still function together”, and is thus a bit tautological. To really test whether the individual species are more tolerant of screwups, one could, I suppose, look at the effects of mutations in these species that usually wreck development in fish. If sturgeon and/or paddlefish are less disrupted by such mutations than are other fish, one might have something to go on.
3.) The polyploidy of the sturgeon genome somehow buffers development against screwup. That is, having two or four copies of the sturgeon genome in hybrids can somehow override the detrimental effects of genome incompatibility. I’m not sure exactly how this would work, but perhaps it would be through a “dosage” effect: if you have a full dose (at least two copies) of one species’ genes, you can override any effects of other-species’ genes that act to “poison” development.
Well, we don’t know the answer. What is remarkable is the resilience of the genome, and the development it induces, in the face of ancient divergence. Remember, this is about 22 times the temporal divergence between humans and chimpanzees, and we cannot produce hybrids with chimps (it’s been tried; see Why Evolution is True).
Káldy, J., A. Mozsár, G. Fazekas, M. Farkas, D. L. Fazekas, G. L. Fazekas, K. Goda, Z. Gyöngy, B. Kovács, K. Semmens, M. Bercsényi, M. Molnár, and E. Patakiné Várkonyi. 2020. Hybridization of Russian Sturgeon (Acipenser gueldenstaedtii, Brandt and Ratzeberg, 1833) and American Paddlefish (Polyodon spathula, Walbaum 1792) and Evaluation of Their Progeny. Genes 11(7), 753; https://doi.org/10.3390/genes11070753