Two species produce viable hybrids even though they diverged 150 million years ago

July 14, 2020 • 10:00 am

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.

Here are the two species involved, the Russian sturgeon (Acipenser gueldenstaedtii), a source of meat and caviar, and a member of the fish family Acipenseridae.

From Wikipedia

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.

Source: Encyclopedia Brittanica

 

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).

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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

Read and weep: Trump’s reported nominees for Cabinet posts and other offices

November 11, 2016 • 10:00 am

We all knew that when Trump was elected he was going to fill the executive branch and the courts with extremists, incompetents, and ideologues. That’s pretty much a given unless the Donald undergoes some miraculous personality transformation. Now, according to BuzzFeed, his transition team has aseembled a list of potential nominees for Cabinet posts. It does nothing to calm my fears.

Note that this is tentative; as BuzzFeed notes:

The list of 41 names, obtained by BuzzFeed News, covers 13 departments, the attorney general, Office of Management and Budget, White House chief of staff, and White House counsel.

A source familiar with the list stressed it was not final, and it is unclear whether transition officials have narrowed it down or added more potential candidates for consideration by Trump.

Still, have a look at it. The most egregious is Ben Carson being vetted for Secretary of Education or Health and Human Services; this is a man who is a full-bore Biblical creationist. Chris Christie is entangled with criminal conspiracies, and as for Newt Gingrich and Sarah Palin (seriously, Secretary of the Interior? Shades of James G. Watt!), I have no words.

As I said, the real cabinet may be very different, but I think its ideological slant won’t be too far off from this.

We’re in for a rough four years. Here’s the list from BuzzFeed:

List of Potential Trump Cabinet Nominees:

Attorney General:

Gov. Chris Christie
Attorney General Pam Bondi
Sen. Jeff Sessions
Former Mayor Rudy Giuliani

Secretary of Commerce:

Christie
Former Nucor CEO Dan DiMicco
Businessman Lew Eisenberg
Former Gov. Mike Huckabee
Sen. David Perdue
Former Sen. Jim Talent

Agriculture Secretary:

Gov. Sam Brownback
National Council of Farmer Cooperatives CEO Chuck Conner
Gov. Dave Heineman
Texas Agricultural Commissioner Sid Miller
Former Georgia Gov. Sonny Perdue

Secretary of Education:

Ben Carson
Hoover Institution fellow William Evers

Secretary of Energy:

Venture Capitalist Robert Grady
Businessman Harold Hamm

Secretary of Health and Human Services:

Former New Jersey state Sen. Rich Bagger
Ben Carson
Newt Gingrich
Gov. Rick Scott

Secretary of Homeland Security:

Christie
Sheriff David Clarke

Secretary of the Interior:

Gov. Jan Brewer
Gov. Mary Fallin
Grady
Hamm
Oil Executive Forrest Lucas
Rep. Cynthia Lummis
Former Gov. Sarah Palin

Secretary of Defense:

Former Gen. Mike Flynn
Stephen Hadley
Rep. Duncan Hunter Jr.
Sessions
Former Sen. Jim Talent

Secretary of State:

John Bolton
Sen. Bob Corker
Gingrich

Treasury Secretary:

Rep. Jeb Hensarling
Businessman Carl Icahn
Banker Steven Mnuchin

Chief of Staff:

Reince Priebus

Director of Office of Management and Budget:

Sessions

Secretary of Labor:

EEOC Commissioner Victoria Lipnic

Veterans Affairs:

Rep. Jeff Miller

White House Counsel:

Donald McGahn

To read more about the awful Trump Cabinet that may be in store for us, read New York Magazine’s speculations, though they devalue their article by using Millennial lingo and referring to men by their genitals: “The Trump Cabinet is Shaping Up to be a Total Sausagefest.” (Imagine what people would say if I published a post saying “The Huffington Post List of Editors is Shaping Up to be a Total [insert your favorite term for female genitalia here]Fest!) After all, the vast majority of its editors are women.

Scientific American has already reported that Trump chose a climate-change skeptic, Myron Ebell, to head his EPA (Environmental Protection Agency) team:

Ebell is a well-known and polarizing figure in the energy and environment realm. His participation in the EPA transition signals that the Trump team is looking to drastically reshape the climate policies the agency has pursued under the Obama administration. Ebell’s role is likely to infuriate environmentalists and Democrats but buoy critics of Obama’s climate rules.

Ebell, who was dubbed an “elegant nerd” and a “policy wonk” by Vanity Fair, is known for his prolific writings that question what he calls climate change “alarmism.” He appears frequently in the media and before Congress. He’s also chairman of the Cooler Heads Coalition, a group of nonprofits that “question global warming alarmism and oppose energy-rationing policies.”

Any tears left? The Daily Kos reports that “The architect of the most racist law in American history has been named to Trump’s team.” The man is Kansas’s Secretary of State, Kris Kobach, and the law is this:

Why should this terrify you? Because Donald Trump has just named one of the most racist politicians in all of America to his transition team. Kris Kobach was the architect of the most racist law in modern American history. SB 1070 passed in Arizona in 2010. What did it mean? If you have brown skin or an accent, police had a right to stop you, detain you and demand you prove your citizenship.

Arizona’s S.B. 1070 compels police to ask for papers from anyone they have a reasonable suspicion of being without status. Under this law any person of color, or anyone with a foreign accent, can be required to prove their status and be jailed—regardless of whether they are a citizen or an immigrant—until they can do so. The Supreme Court indicated that prolonged detention would be impermissible, but people’s rights will likely be violated before that limitation can be enforced.

By targeting certain groups of people living within the state, the Arizona law amounts to an ethnically divisive and deeply hostile social policy. It raises the specter of states treating people differently based solely on their appearance rather than on their actions. Every person in Arizona and states that pass S.B. 1070-like legislation will be required to carry proof of their legal status at all times or face the possibility of being detained. In practice it will be people of color that bear the brunt of these policies.

It was nicknamed the “Papers Please” law and thanks to Kris Kobach and the right-wing ALEC (American Legislative Exchange Council), two dozen copycat bills quickly spread to other conservative states. Can you imagine walking down the street and police having the power to stop you and your children and throw you in jail on the suspicion alone that maybe you don’t belong? Because you have brown skin or an accent? Terrifying, right?

Kobach has also spoken before white nationalist and anti-immigration groups. Welcome to Götterdämmerung.