Did metamorphosis evolve through hybridization? A scientific cautionary tale.

Written by Joe Ballenger

This question, received through our Facebook page, is one that actually hits close to home for me because it involves one of my scientific idols:

Hybridization question

This dates back to a paper published in 2009, one which appeared in a legitimate scientific journal…and a pretty prestigious one at that. A paper in PNAS is basically a career-maker for most scientists. The citation for the paper is below:

Williamson, D. I. (2009). Caterpillars evolved from onychophorans by hybridogenesis. Proceedings of the National Academy of Sciences, 106(47), 19901-19905.

Onychophora1

Onychophorans, or velvet worms, are distant insect relatives. Image credit: Geoff Gallice, via Wikimedia commons. License info: CC-BY-2.0

The idea that Williamson pushed in this paper, that metamorphosis evolved through hybridization of velvet worms and insects, was something he had been pushing for awhile. His ideas didn’t gain any traction until this particular paper was published in PNAS. It wasn’t well received, because many of his ideas were known to be false before the paper was published. Frankly, it should never have been published…especially not in a journal of this caliber.

The story of how this paper was published is actually really interesting for a lot of reasons, mostly because it’s a very harsh demonstration that your idols might not be perfect. It also gives us a great opportunity to talk about how scientists keep track of all the new discoveries that are constantly happening.

So…how did this idea even get published?

PNAS, peer-review and Williamson

When a scientist makes a discovery, they publish it in a special kind of magazine where volunteer scientists look over their work to make sure it makes sense. The vast majority of the sources we cite here on AaE come from these sorts of publications, usually called journals. The individual articles are usually referred to as papers by scientists in casual conversation.

PNAS, or Proceedings of the National Academy of Sciences, is one such journal. It’s a very highly regarded publication, because it’s run by the National Academy of Sciences. The National Academy is a group of the most respected scientists in the United States. Becoming a National Academy member is just about the highest honor a US scientist can receive, only second to a Nobel Prize.

While virtually all papers given to PNAS went through a standard peer-review process, it used to be that National Academy members could choose reviewers for a certain number of papers. This was intended to be a mechanism by which controversial (but logical-see examples here) ideas could get published. This hasn’t been allowed since 2010, so it’s not an issue anymore, but bad ideas can still get through sometimes.

Williamson paper

Papers which skirted the standard peer-review process were said to be communicated, and this was one such paper. The person who helped this paper get through, Lynn Margulis, is actually a pretty legendary scientist in her own right.

Lynn Margulis and the danger of absolutism

Lynn_Margulis

Lynn Margulis, speaking at a conference in Spain. Image credit: Jpedreira, via Wikimedia commons. License info: CC-BY-SA-2.5

Before I talk about Lynn Margulis, I want to point out that this person is actually one of my scientific idols. Her career played a huge role in forming my ideas of what a scientist should be.

In the 1960s, she put forward the idea that some parts of cells were actually symbionts and that one of the most basic units of life had evolved through cooperation instead of competition. The idea was highly controversial, and she was considered a crackpot for the longest time because the prevailing thought was that competition was the norm in evolution.

She was, however, eventually proven right when it was found that these cell-parts had DNA which didn’t match the rest of the cell. Her endosymbiotic theory is now taught in textbooks, and a lot of breakthroughs have been made because these cell parts resemble bacteria. In fact, I even consider myself to have followed in her footsteps somewhat. In graduate school, I worked on a group of symbiotic viruses used by wasps to turn other insects into incubators for their young.

So Margulis has had a huge impact on my thinking as a scientist, and has directly influenced my career path with her ideas. In many ways, she is one of my most important influences.

Endosymbiosis

Margulis’s big idea: Mitochondria (which power all cells except bacterial cells) and chloroplasts (which harness sunlight for energy in plants) are descended from bacteria which had close relationships with ancient cells. Image credit: Signbrowser. License info: Public Domain

However, not all of her ideas are good. Towards the end of her life, she began to advocate for a lot of things which weren’t true. She was a well-known HIV denialist, who believed that AIDS is caused by spirochaetes.

Another issue that put her at odds with the scientific community were her views on evolution. She viewed evolution as a process which occurs mainly through symbiosis. Instead of little steps, she believed evolution happened by huge leaps and bounds caused by symbiosis. She is what I call an absolutist-she believed that everything happened only one way.

What do we know about the evolution of metamorphosis?

Williamson’s idea is wrong, because we already know a lot about insect genetics. If his idea was right, the genetic markers of velvet worms would be apparent in insects. However, this isn’t the case. You’d also expect the organ systems to be completely replaced during the pupal stage, but again, this isn’t the case. You’d expect different genes to be used to form structures in the larvae and adults, but they use the same genes to form various things. You’d expect flies to have double the genes of velvet worms, but they have about half the genes.

You can see the details here in Nancy’s metamorphosis post. Although it’s not a direct rebuttal to Williamson’s ideas, you can see that Williamson was simply unfamiliar with insect biology in general and the process of metamorphosis in particular. In order for Margulis to feel this paper was worth publishing, she must have had similar misconceptions. Either that, or she felt the need to ignore decades of prior research on the topic.

That doesn’t really get at the crux of this issue, though…

How did metamorphosis evolve?

The best understood part of insect metamorphosis is the larval stage, which I’ll focus on here.

Insects which don’t go through metamorphosis are commonly said to have three stages: egg, nymph, adult. This, however, is a slight over simplification because there’s a stage which is almost always left out: the pronymph. You can see a pronymph in the video below.

In the video above, the cricket sheds a skin just after emerging from the egg. That skin belongs to the pronymph stage, which is the last stage of embryonic development.

Although crickets shed this skin immediately, there are a lot of insect groups where that stage sticks around for a little bit. There’s a lot this stage has in common with the larval stage of things like butterflies. It’s soft, the way the skin forms is the same, neither has wing buds (which are present in insects that don’t have a pupa*), they have similar nervous systems, and the hormonal profiles are pretty similar.

Metamorphosis final

Wingbuds are the signatures of wings being made on bugs which go through incomplete metamorphosis; they’re not there in bugs which go through complete metamorphosis. Image credit: Joe Ballenger

The evolution of the chrysalis, on the other hand, is a lot more mysterious. There are two competing hypothesis about this. One is that the pupa is all the nymphal stages smooshed together, while the competing hypothesis is that the pupa is a modified final nymphal stage. There has been a lot of recent work done on hormonal regulation which has shed some light on this, but the topic is difficult to cover in one blog post.

The one thing which seems to be certain, however, is that the pupa evolved from a mobile developmental stage and seemingly became unable to move relatively late in evolution. We know this because the most ‘primitive’ insects with pupae have pupae that actually get up and walk away:

Most insects with this sort of development can’t do this, but this feature is thought to have been retained from older lineages.

The Bottom Line

In her heyday, Margulis was a brilliant scientist who tenaciously defended an idea that ultimately turned out to be correct. Her endosymbiosis idea also happened to be one of the most important ideas in all of biology. In short, she deserved every bit of praise she received.

However, she is ultimately a flawed hero…and she’s flawed in some ways that are really important. Margulis was brilliant, but she had an ego which got the best of her. Although she deserved every honor she earned, she also deserved every bit of scorn.

This brings me to what I believe is the most important point I’ve ever made on this blog…scientists can be wrong. People who are intelligent can be wrong in some rather spectacular ways. That doesn’t mean that they’re not smart…but being smart doesn’t mean you’re right every time.

Science is all about evidence, and that’s what makes it such a great field. Legendary scientists can be wrong, and newcomers can be completely right. At different points in her career, Lynn Margulis was both.

 Works Cited

  1. Belles, X. (2011). Origin and evolution of insect metamorphosis. eLS.
  2. Hart, M. W., & Grosberg, R. K. (2009). Caterpillars did not evolve from onychophorans by hybridogenesis. Proceedings of the National Academy of Sciences, 106(47), 19906-19909.
  3. Williamson, D. I. (2009). Caterpillars evolved from onychophorans by hybridogenesis. Proceedings of the National Academy of Sciences, 106(47), 19901-19905.
  4. Willis, J. H., & Cox-Foster, D. L. (2010). Insect metamorphosis via hybridogenesis: an evidentiary rebuttal. Journal of insect physiology, 56(4), 333-335.

*Metamorphosis-like processes have evolved in thrips, as well. They have pupae, but that’s another discussion altogether.

This entry was posted in Culture, Developmental Biology, Evolution, Physiology and tagged , , , , . Bookmark the permalink.

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