The NYT oversells a new report of a “virgin birth” in crocodiles

This is an example of science reporting that’s misleading—not because it gets the facts wrong, but because it oversells a rather mundane finding as a potentially important insight into the life of the extinct dinosaurs.  And no, there’s nothing in the original paper—about a single “virgin birth” in a crocodile (really a “stillbirth”)—to suggest the Big Sell: that dinosaurs could have reproduced via “virgin births”, too. Dinosaur Jesus probably didn’t exist.

So here we have one new paper from the Royal Society’s Biology Letters; click on first screenshot below to read (the pdf is here) reporting the occurrence of parthenogenesis in one crocodile in one zoo. The NYT Trilobite reports about it in the second screenshot below (I found it archived here).

Parthenogenesis is a form of asexual reproduction that occurs without the fusion of a sperm and an egg. It occurs across various animal taxa, and can arise in different ways.  Here’s what Wikipedia says about its distribution (note that it doesn’t occur naturally in mammals).

Parthenogenesis occurs naturally in some plants, algae, invertebrate animal species (including nematodes, some tardigrades, water fleas, some scorpions, aphids, some mites, some bees, some Phasmatodea and parasitic wasps) and a few vertebrates (such as some fish, amphibians, reptiles and birds).

Parthenogenesis can occur in a variety of ways, often after hybridization between species that produces an individual whose chromosomes can’t pair properly during meiosis (cell division producing sexual gametes). That mispairing increases the likelihood that an unfertilized egg in a hybrid can have a full chromosome complement and develop into an offspring. But in most other cases, like this one, asexual reproduction occurs as a modification of non-hybrid meiosis.  That’s what apparently happened in this case.

The authors report that an 18-year-old American crocodile (Crocodylus acutus) in a reptile park in Costa Rica laid a clutch of 14 eggs. Candling of the eggs (holding them up to the light) revealed that seven apparently contained embryos (or dense spots), but none developed to hatching. One fetus, however, almost made it, and when the egg was dissected it contained a single female. Here’s the photo from the paper: mom on top and the fetus at the bottom. There was clearly something wrong with the baby as it failed to hatch, and I suspect it was simply screwed up.

Genetic analysis of the fetal DNA compared to mom’s showed that they were pretty much genetically identical, with the fetus being, in effect, a clone of the mother.

How did this happen? If you don’t need the details, skip this next part between the lines.

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DNA results from the paper suggest that the fetus came from fusion of two of the products of meiosis. Here’s a diagram of how a female produces an egg (or a male produces a sperm) during meiosis (figure from Wikipedia). It entails doubling of each of the chromosomes in a sperm or egg precursor cell followed by two bouts of cell division.  The circle on the extreme left is of the precursor gamete cell in a species having four chromosomes, with two copies of each of two different chromosomes (remember, we have two pair of every chromosome).  Then each pair doubles, which happens during normal cell division (mitosis) as well as in meiosis (second circle).

But then meiosis begins in gonadal cells: instead of each doubled chromosome splitting and going to a new cell, producing two genetically identical cells (this is normal cell division), the chromosome twins of each pair, now doubled (and having reshuffled bits from meiotic “recombination”), go into separate cells (third circle from left).  This is the first phase of meiosis, or “meiosis I”.  Now we have two daughter cells, each containing two chromosomes that are doubled but connected by the centromere. The chromosome number is really two, not four, because these doubled chromosomes (called “chromatids”) are going to go to different cells.

During “meiosis II”, the doubled chromosomes of each pair split, with each going to a new cell, so instead of two pair of joined chromosomes, we simply get two single chromosomes.  The chromosome halves can sort out independently of each other, which is called “segregation”, and is one way to recombine existing genes. In the end, a cell with four chromosomes has, though doubling and then two  cell divisions, produced a “haploid” cell with only two chromosomes. These final cells have only one instead of two copies of each chromosome pair. They are the gametes: eggs and sperm, and are shown on the extreme right.  This happens in eggs and sperm, and when they fuse, the normal chromosome number is restored. You can see that when this happens in both sexes, you get novel combinations of parental genes—one of the likely reasons why sex evolved.

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Now, how did this produce the parthenogenic crocodile fetus?  In females, usually only one of the four products of meiosis goes on to form the haploid egg, with the other three cells being segregated into “polar bodies” that eventually die. But sometimes a polar body will fuse with the egg cell, restoring the normal chromosome complement and producing a diploid individual having two copies of each chromosome.  It’s a bit more complicated than this , but the result is that a female can produce a normal diploid offspring without being fertilized. This is simply because two of the halved cells that would normally become egg cells fuse with each other.

Using DNA sequencing, the authors confirmed that this is what happened (the formal process is called “automictic parthenogensis with terminal fusion”). But the diploid fetus, which would normally go on to form a crocodile that would hatch, was somehow inviable.

Note that this is the first report of parthenogenesis in a crocodile. (Since the mother was 18 and had been alone since she was two, it was extremely unlikely that the fetus came from her storing sperm after being inseminated sixteen years before, as some animals can store sperm for a substantial period.)

So far so good. And the NYT article below gets the details and facts right. Where it goes off the rails is instantiated in the sub-headline.  (The big headline is wrong, too: this is NOT a “virgin birth” but a virgin stillbirth. No new croc entered the world. 

Because crocodiles are related to the extinct dinosaurs, author Greenwood (and the authors of the original paper), suggest that this is a case of “facultative parthenogesis that could also have occurred in dinosaurs! Dinosaurs could have reproduced asexually! T. rex could have produced baby T. rex copies without having to mate! Note the subheadline above, mentioning “Jurassic Park”.)

The article also says this:

So, did dinosaurs do it, as the discovery of parthenogenesis in crocodiles suggests? Parthenogenesis is best confirmed with DNA analysis, a process that has allowed scientists to tell it apart from delayed conception, where a female stores sperm for as long as six years before using it to fertilize eggs. Without the ability to retrieve dinosaurs’ and pterosaurs’ DNA, which does not persist in fossils, certainty is not available.

“We’ll never be able to prove they could do it,” Dr. Booth said. “But it suggests they had the ability.”

My response is “no, it doesn’t.”

But at least both the authors and the journalist say we can’t prove that there were Jesus-saurs, but I’d go further and argue that this is speculation far beyond what’s warranted from the data—speculation prompted by the desire to jazz up what is a decent but not terribly exciting result. In fact, I’d say that because the single dead offspring represents a bug and not a feature of croc reproduction, it says absolutely nothing about the likelihood that dinosaurs reproduced asexually, much less that they did so “facultatively”—as a regular evolved feature of their lifestyle.

First of all, this isn’t really “facultative parthenogesis,” which refers to species that can produce normal offspring sometimes via sexual reproduction and sometimes asexually. (That might be an evolved property, allowing an individual to pass on its genes when populations are sparse and there is nobody around to mate with.)

This isn’t what’s going on with crocs.  What we see here is probably a rare developmental screwup (it hasn’t been reported before, despite there being many captive crocs), an anomaly that is an evolutionary dead end.  Rare parthenogensis has been seen in other species too, including birds (see below), but in those species results in largely inviable or infertile offspring.  True “facultative parthenogenesis” isn’t common, but does occur in some creatures like lizards. And it’s not a bug, but an evolved feature.

Below I show the family tree of some vertebrates, including the dinosaurs, birds, lizards (one of the “squamates” along with snakes), and the crocs. (This slide was created by my friend Phil Ward, who teaches evolution at UC Davis.) You see that the crocs are related to dinosaurs, but not as closely related as are living birds, which are a group (“clade”) phylogenetically embedded within the now-extinct dinosaurs.

The upshot is that a rare occurrence of asexual reproduction, especially when it leads to a dead or sterile offspring, says nothing about the likelihood of facultative parthenogenesis in a relative. After all, birds are more closely related to dinosaurs than are crocs, and a couple of birds have reproduced asexually (turkeys can produce fertile offspring this way, though rarely, and California condors have also done it). But nobody goes shouting from the rooftops “dinosaurs could have reproduced asexually” because of a few rare cases in birds.  Quite a few lizards reproduce normally by parthenogenesis, but you don’t hear people extrapolating from lizards to dinosaurs, either.  Even some fish reproduce parthenogenetically, so why not write that “because fish are related to dinosaurs, dinos could also have reproduced parthenogenically.”?  It’s all pilpul.

The facts in the NYT report are accurate, including the caveat that we can’t test the asexual-dino hypothesis, but the author couldn’t resist bringing in the dinosaur angle—without any good reason to do so.  What will happen is that people will ignore the fact that the single parthenogenic crocodile fetus was stillborn (or “stillhatched”), was probably just a rare developmental anomaly, and go away instead with the lesson that “DINOSAURS COULD HAVE REPRODUCED BY CLONING THEMSELVES!!!”.