Overview
Here we look at the “single-cell bottleneck”. We uncover why most multicellular organisms begin life as a single cell and examine the benefits and risks of this biological process. We look at the concept of “genetic clonality”, an aspect crucial to the single-cell bottleneck, as we discuss its significance in cell behavior and its role in managing internal cellular conflicts.
Finally, we’ll address the ongoing research by scientists like Ashleigh Griffin and Jack Howe of Oxford University who are investigating why some species, such as certain flatworms, don’t utilize single-cell bottlenecks.
Further Reading
The following paper (open access) by Jack Howe, Jochen C. Rink, Bo Wang, and Ashleigh S. Griffin; details the role of the single-cell bottleneck in biology, how somatic and germline tissues segregate, how this is similar to relatedness in colonial insects, and the mysteries surrounding the absence of clonality in flatworms.
Transcript
Imagine your surprise if, while flying a kite on a warm spring afternoon, a sudden gust of wind ripped the string from your hand, slicing off your pinky finger.
Gross.
Now imagine your surprise if several days later you started growing a new finger and your severed finger started growing a new you!
This would be front-page news all around the world, but why? The normal way humans and other animals reproduce is actually far more impressive.
You started life as a single cell! Once that cell was fertilized, it started copying its DNA, separating those copies and then splitting in two. 2 become 4, 4 become 8, until eventually there were trillions.
If a single cell can grow into a full fledged human, why can’t a finger? Afterall, the chance that a single cell will die, or possess some sort of genetic defect is huge but a finger contains millions of cells. If one or many cells in that finger are defective, the others would be there to take over, right? At first glance this seems like a better way to reproduce.
It turns out that many plants and some animals actually can reproduce through a process similar to finger shedding. Biologists call it “budding”. Hydra, for example, are little multi-celled animals found in pond scum. One way they reproduce is by simply sprouting and then shedding a little copy of themselves after a good meal.
Coral polyps and some flatworms are able to reproduce through fission! Like us, they are multi-celled animals. Like us, each cell has its own entire copy of DNA. Like us, different cell types (skin cells vs stomach cells, for example) are produced by turning specific genes off and others on within those cells; but unlike us, when these animals want to reproduce they rip themselves in half! Both chunks regenerate any missing body parts, and voila: Reproduction has occurred!
If this is possible, why is it that in humans and so many other animals, evolution bets it all on one single cell every generation? Is this an example of reckless gambling or did the mindless process of evolution stumble upon a brilliant, little-known trick?
Stated Clearly Presents: Evolution’s hidden wildcard: The Single-Cell Bottleneck
When biologists talk about the single-cell bottleneck, they’re referring to the strange fact that so many multi-celled organisms start out as a single cell. In sexual reproducers, of course, that single cell (the zygote) is a fusion between what you could call half-cells (gametes, most commonly a sperm cell and an egg cell) that come from each parent.
Pretty much every animal you can find at a zoo – all vertebrates from tiny fish to giant elephants and whales, all insects, and even cephalopods, use a single-cell bottleneck every time they reproduce. Even most species of flatworm, hydra and other animals that can reproduce through budding and fission, will occasionally mate and use a single-cell bottleneck instead. Why? Restarting each generation as a single-cell is an extremely complex, risky process.
Whenever we find a complex trait or process in biology this usually means the trait is doing something important. If it wasn’t, the relentless filter of natural selection, likely would have stopped the process or trait from evolving.
If you stop to think carefully about it, you’ll realize that starting fully fresh each generation, risky as it might be, does several extremely helpful things:
First it corrects any defects an adult might have that were caused by bad conditions during development. Charlie here has great genetics, his “DNA recipe” seems to be nearly flawless, but while developing in the womb, his umbilical cord happened to wrap around his hind leg, dramatically stunting its growth. By the time he was born, the cake had been baked – he now must live his entire life as a tripod! If he ever fathers kittens, however, that developmental error will be automatically corrected in them.
Likewise, a fresh start can easily correct any injuries and epigenetic glitches that accumulate through an animal’s lifetime, errors that may be too complex to fully heal or reverse.
Most importantly, the single-cell bottleneck resets a state that scientists call “genetic clonality”. What is genetic clonality and why does it matter?
If you’ve ever studied wildlife conservation, you probably know that at the population level, genetic diversity is key to the survival of a group or species. When each individual has a unique DNA sequence resulting in different traits, this ensures that at least some animals in the group will survive, no matter what challenge comes their way. The opposite is true, however, when you compare DNA sequences between cells of a single individual. Here, too much genetic diversity can lead to trouble.
The behavior of each cell in your body is dictated by many things, including conditions in the environment around you, chemicals and even parasites that might get inside you, signals from neighboring cells, and the influence of epigenetic markers. But at its core, a cell’s behavior stems from its DNA sequence. As cells replicate, mutations can occur in DNA, some of which can change how a mutated cell will behave.
Two cells are considered perfect genetic clones if they have identical DNA sequences. Early on in development, all or most of your cells were perfect genetic clones of the original, but by the time you’re an adult, mutations accumulate meaning the cells of your left hand have slightly different DNA sequences than the cells of your right hand. Some genetic differences can cause conflict between cells.
In an extreme example, a lung cell might mutate in a way that causes it to rapidly reproduce out of control. In most cases, neighboring cells will eventually force it to stop, but if they fail, cancer is born!
A milder example would be mutations that simply cause a cell to misbehave. Liver cells do dangerous work, helping filter toxins from your blood. Conditions can be so harsh that it’s common for a liver cell to die and be replaced before it’s even one year old. Imagine a mutation in one of your liver cells that causes that cell to stop helping filter toxins. This might allow that cell to live a longer, healthier life than its neighboring cells, but now it’s just taking up space and resources without providing any service in return. The cell colony as a whole (in this example that colony is “you”) will be slightly weaker as a result.
Our bodies go to drastic measures to stop internal conflicts between cells. In extreme cases, immune cells seek out and destroy individuals behaving as if their DNA has mutated too far from the norm. But an easier, far less violent way to ensure that each cell shares the same DNA, is to start each generation through a single-cell bottleneck.
Though it might seem wild to gamble all hope on a single cell, the reward often outweighs the risk. If that single cell happens to have a negative mutation, the new organism will often fail to develop early on, allowing the parents to simply try again. If that single cell happens to have a beneficial mutation, that mutation will be in every single one of the new organism’s cells!
To hedge their bets even further, in many animal species, individuals carefully choose who they’ll mate with, preferring partners who are healthy, or otherwise doing well in the struggle for existence. On top of this, each male in many species produces millions of sperm that must race each other to fertilize a single egg cell. The genes inside a particular sperm cell are the genes that built that particular sperm cell. This means a competition between sperm, is a competition between genes. Only those with the best cell-building genes are likely to fertilize the egg.
Biologists who study the mathematics of natural selection have found that single-cell bottlenecks are so useful, it’s not clear how some animals get by without them. A collaborative project between researchers at Oxford, Stanford and the Max Plank Institute in Go-ting-in are currently studying a population of flatworms that, so far as we can tell, never use single-cell bottlenecks, they strictly reproduce through fission. Is this population doomed to slowly degrade through mutational meltdown, or have flatworms evolved ways to effectively work with or even benefit from genetically diverse cells? Finding the answers to mysteries like this not only helps us better understand the world around us, but could lead to breakthroughs in the treatment of cancer and aging.
In summary:
At first glance, reproducing through a single-cell bottleneck seems like a dangerous, wild gamble, but it actually does many things for us. Most importantly, it prevents conflicts from popping up between cells by resetting genetic clonality each generation. This reset is so essential that scientists are currently trying to understand how some flatworms and other animal populations seem to survive without it.
