New Discoveries show Planaria aren’t Individuals, their Stem Cells are.

It’s October, just a day before Halloween! This is the perfect time for a spooky scary article about something terrifying! Like planaria worms! Those are Halloween-y, right? I’ve read a recent paper and became obsessed with them, so search engine optimization be damned! 

So this is an article about adorable little flatworms called planarians. And yes, I do think planaria are cute. I will die on this hill. Anyone who just thinks they’re just gross worms needs to look past the mucus and see them for their little ear flap thingies or their eyespots that look like they have googly eyes. I specifically mean the freshwater planarians. The hammer-headed land planarians are cool, don’t get me wrong (there’s one on my left shoulder for reference) but they’re not as cute.

In the featured image you can see that I’ve made some nice big ones using unholy technology. Normally, most freshwater planaria are shorter than a centimeter. Unless they’re land planarians, those can be a foot long.

• Planarian Regeneration
• Hamilton’s Rule
• Is the Worm or the Neoblast the Individual?
• Why Aren’t Planarians Weird Lopsided Monsters?
• Competition between Sex and Asex
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Planarian Regeneration

You may be familiar with planarians, they are quite a common sight in high school science classrooms. These adorable flatworms, beyond their ability to be adored, are remarkable for their regenerative abilities. Like many worms, cutting one in half will just leave you with two smaller worms with identity crises. Planarians can even regrow their entire body from just a single cell in under two weeks (provided that one cell has some helper cells). They’re like Cell from Dragon Ball Z if you ignore literally everything other than the ability to regrow from a single cell. I guess they do both suck up food through a long tubular mouth attached to the middle of their body.

This is because of the power of stem cells, which are cells that can transform into other kinds of cells. The reason why you can’t regrow a new arm like a lizard is that you are weak and were born to suffer. But also because you don’t have stem cells in your arm. Normally, wounds heal when cells nearby multiply to fill the gap. But if you’ve lost any special cells or tissue structure, that scar tissue can’t replicate it. Turns out an entire arm is a lot more complicated than a nub. But stem cells can replicate all the complexity because they can transform.

Most stem cells can only become a small number of cell types. For instance, you have a small collection of stem cells inside your brain called neural stem cells. Neural stem cells can become neurons and other brain cells, but they can’t fix your skull. 

Planarians have special stem cells called neoblasts scattered throughout the planarian’s whole body ¹hint: these are that one cell that can regrow an entire planarian. Neoblasts are pluripotent stem cells, meaning they can transform into literally any type of cell. This isn’t actually that impressive since planarians only have 40 cell types (humans have ten times that), but still. That’s compared to multipotent stem cells that are limited to just some types of cells. Such as skin stem cells that can make all the different types of cells in the skin but nothing else.

Neoblasts are the only planarian cell type that can divide, all other cells must be replaced by a neoblast. They also self-renew, so they actually clone themself and have this clone (a daughter cell) transform instead, which ensures the worm doesn’t run out. They can also migrate across the body to help repair injuries. On top of all that, neoblasts basically live forever as they don’t age. Pretty cool.

Asexual reproduction

It turns out if you graph badassery against the ability to have sex; the line raises steadily until after the Chuck Noris point where it just drops off a cliff. I call this the Erkon Virginity Phenomenon which describes how extreme badasses stop being able to get any because women fear their size and/or they’ve transcended the very need for sex. This is why I’m single. Totally.

The Planarian’s regenerative abilities are just so badass that many species don’t even bother having sex. Why bother with romance and sexual intercourse when you can just rip yourself in half? The two halves will grow into smaller versions of yourself and you don’t need to worry about child support.

Hamilton’s Rule

Evolution is racist. According to Hamilton’s Rule, evolution generally only favors maximum cooperation among organisms that are very closely related. This is why ants/bees will give their lives to protect the queen that they’re extremely closely related to ²more so than their own fathers interestingly enough but will attack any bees from rival colonies on sight. There are a few species who are willing to accept having brothers from other mothers. But they still have to be their relatives from the same species. 

Evolution selects for the ability to spread your genes to the next generation. But you don’t need to do that by having babies. You share lots of genes with your close relatives, so boosting their ability to reproduce is also advantageous. There comes a point where you’re so closely related that it’s actually just as good to help your relatives survive and reproduce than to ever bother doing it yourself. Hence why bees will sacrifice their own lives to defend their queen. You have nothing to lose if you can’t make kids. If you’re still interested in that idea, here’s another article about the hypothesis that gayness evolved so gay uncles can help take care of their nieces and nephews.

You are not a single creature. You are made up of a family of cooperating single-celled organisms. We’re basically just trillions of protozoans in a trenchcoat. Because they’re all descendants of a single cell they’re closely enough related that they can have maximum cooperation, like ants and bees. At that point, scientists decide it makes more sense to just call the entire collection a single individual. So you still count as a single individual even though you’re made of many living things. And that’s not even getting into mitochondria and the gut microbiome.

But what if they didn’t all come from a single cell? What if your cells were all distantly related to each other? Would evolution still make them cooperate maximally according to Hamilton’s rule?

Are the Planaria or the Neoblasts the Individual?

You might be guessing where I’m going with this, since I spoiled it with the section title, but no. It turns out that distantly related cells don’t cooperate very well.

Planarians who reproduce asexually don’t have that one-cell bottleneck as we do. There are many dozens of different neoblast lineages in a single planarian who will pass on to the next generation together. These lineages may be hundreds of thousands of years old considering that planarians have been around even longer. Neoblasts inside a planarian are often very distantly related to each other.

Scientists have found that neoblasts tend to fight with rival neoblasts for resources. They still get along well enough to keep the body working well, but not as much as you see in sexually reproducing species.

It would be more accurate to describe asexual planarian worms not as animal individuals but as a dynamic environment that groups of neoblasts create and must live in to survive. You can sort of think of neoblasts as farming their daughter cells. Like how queen bees have their daughter worker bees to bring them food and protect them. 

Perhaps the reason why planarians can heal so quickly is because their neoblasts are individuals. Don’t get me wrong, regrowing an entire body in under two weeks is pretty cool, but it’s kind of excessive. Regeneration doesn’t need to be this advanced or quick when you consider that planaria are just tiny worms. In most cases if they are attacked by a predator they’ll be swallowed whole. Are such extreme regenerative abilities really worth the evolutionary cost?

But it does make sense if you consider that the neoblasts are in a reproductive arms race with each other. If the worm is cut in half, neoblasts that grow tissue faster than others make more of their own daughter neoblasts to tend to the new tissue. This increases the odds that your descendants will be in the half of the worm that gets away, not the half that gets eaten. Hence, it may not have been the planarians but the neoblasts that evolved their stellar regenerative ability.

Gray area

That said, the neoblasts are obviously in some weird gray area between being an individual and being cells. 

If we extrapolate that beehive analogy; the worm as a whole would be like if multiple bee hives worked together. Some hives focus entirely on making honey. Some hives focus entirely on protecting everyone from outside threats. Some hives focus on building all the hives. This is like how planaria have specialized organs like eyespots and brains. But, because of Hamilton’s rule, there will still be some political shenanigans. Different hives will try to get a better deal for themselves. Most human countries work well together and even have specializations. As we’ve all been made aware of in this year of 2022; Ukraine specialized in producing a pretty big chuck of the world’s food. Russia made a big chunk of oil and gas. But that doesn’t mean all human countries always cooperate, as we’ve also been made painfully aware.

Bees do not do this (though I wouldn’t be surprised if we discover a species that does). But even if that were the case, the fact that neoblast cannot survive without help is unusual for an individual. 

Remember how I said that planarians can grow back from a single cell as long as that cell has some other helper cells? Neoblasts only have the ability to move and make other cells, they need more specialized cells to feed them. Queen bees can survive briefly on their own. Beehives spread because young queens fly off on their own to found a new hive.

Why Aren’t Planaria Weird Lopsided Monsters?

So if all the cells in planarians have vastly different DNA, why don’t they have more issues? You would think that, since the shape and appearance of an organism come from its DNA, and that planarian cells can have very different DNA, some parts would look different than others? 

But no, this doesn’t happen. Planarians are always perfectly symmetrical and uniform regardless of how genetically heterogeneous their neoblasts are. Some defects do happen, but it doesn’t seem any more likely than in most other animals. No one knows how they coordinate this.

Planarians don’t have an immune system that can reject genetically foreign tissue like how ours can. The brain of one planarian can actually be transplanted into another. That brain will not only gain control but will also retain its memories. You can also give an asexual planarian some cells from a sexual planarian species and it will become sexual, (though it will keep splitting itself in half too.) 

I guess that’s one other thing they have in common with Dragon Ball Z’s Cell. That is if Cell had also gained a penis and vagina after absorbing Android 17 and 18. I apologize if you haven’t watched Dragon Ball, that probably sounds very strange out of context. To be fair, Cell was pretty weird even with context.

Competition between Sex and Asex

As I’ve mentioned, there are both sexual and asexual species of planaria. Sexual species are all hermaphroditic, meaning they have both a dingus and a hoo-ha. Actually, they have weird flatworm equivalents of those things which you are free to look up if you wish. If you’re into that.

Species that can reproduce sexually are generally still able to reproduce asexually if need be. But, if the opportunity presents itself, they can also have sex. Beyond the obvious benefit of sex, this also gives them more genetic diversity. Mixing genes is generally more evolutionarily advantageous than not doing that.

There is the other benefit that sexual planaria don’t have to deal with neoblast infighting. Since they have a single cell bottleneck, all their cells are closely related.

So why don’t all Planaria have Sex? 

Are they just shy? Maybe it’s for religious reasons? While I can’t disprove the existence of a flatworm religion that demonizes sexuality, I highly doubt it. For one, the fact that not every species of planaria have cute ear flaps and googly eyes indisputably proves they have no god. At least not a benevolent one. Also, they’re worms.

There are two hypotheses, besides the silly ones, both of which involve the neoblasts. The first is that planaria could represent what all animals were like during the early transitionary period between single cellularity and multicellularity.  It would also make sense to me that there was a phase of semi-cooperativeness before cells learned to get along perfectly. 

Planarian physiology is simplistic, even for a flatworm, so it’s not entirely unbelievable that they’re an extremely old living fossil. Though I would expect such a species to be closer to our earliest common ancestors with single-celled organisms (fungi, sponges, jellyfish, etc.). It’s far more likely that planarians have just regressed back into this liminal state, not that they’ve always been there.

The second hypothesis relates to how that might have happened. It posits that the sexual planaria were sexual but the neoblasts kicked the sex cells out. Such a cellular mutiny would be conceptually similar to cancer but more organized. 

The sex cells (sperm and eggs) are like the queen bee in sexual organisms. Every other cell forgoes reproducing so that the sex cells can focus on doing that. Sex is at the expense of the neoblast, so it makes sense why they’d evolved to take matters into their own hands. Though, since for planarians asexuality means infighting and no genetic interchange, it’s a bad move in the long term. But they’ve been able to make it work somehow.

As far as I know, neither of those are proven yet. This is all based on fairly recent findings.

Source:

Fields, Chris & Levin, Michael. (2018). Are Planaria Individuals? What Regenerative Biology is Telling Us About the Nature of Multicellularity. Evolutionary Biology. 45. 237-247. 10.1007/s11692-018-9448-9. https://rifters.com/real/articles/Fields-Levin2018_Article_ArePlanariaIndividualsWhatRege.pdf#:~:text=We%20review%20molecular%20and%20developmental%20evidence%20suggesting%20that,genetically%20heterogeneous%2C%20migratory%2C%20effectively%20immortal%2C%20and%20effectively%20autonomous.