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Imagine you’re a scientist peering down the microscope at some bird cells.
But your view, unfortunately, is blocked by little tiny flecks of lint.
Well that is exactly what happened when scientists first discovered the
little genetic mysteries that we’re going to be talking about today.
Except with some careful cleaning, scientists realised it was not dust at all,
they were actually miniscule chromosomes.
And as they looked more and more into it, and picked up better genetic
sequencing techniques over the years, scientists would come to discover just
how crucial these tiny chromosomes are.
In fact, they might just be the building blocks for entire genomes.
So, chromosomes, right. The little X’s we learn about in high school bio.
All our DNA is contained in those long stretches, which can condense down to
those convenient X shapes when the cell is duplicating them to divide.
They’ve got caps on the ends, called telomeres, and joining points somewhere
near the middle of the X, called centromeres.
And that’s just how cells store and handle their DNA, right?
No! Wrong!
Because the instant you look at the cells of any vertebrate that’s not a mammal,
you see much smaller lengths of DNA: microchromosomes.
Typically, microchromosomes are around 20 million rungs of the DNA ladder.
That might sound like a lot, but it’s less than one and a half micrometers in size.
Compare that to regular chromosomes, which can be
20 micrometers long, or longer.
Which might explain why scientists initially thought they were lint.
See, early cell biology was all about grabbing a sample of cells, staining them and
carefully looking at them under the microscope.
So a little hard to tell what was what.
But the more researchers looked, the more they saw these so-called specks in
all sorts of vertebrates: birds, fishes, amphibians, turtles.
Even in some insects, like water bugs.
Just not mammals.
Then, as methods got better, scientists noticed that the specks not only
looked like regular, big, chromosomes. They acted like them, too.
They replicate when the cell divides, and they’re
transcribed to make proteins. It’s just very small.
But the researchers still couldn’t figure out how these
microchromosomes got there, or what they actually did.
One of the first ideas was that they weren’t really chromosomes,
but instead a dense jumble of junk DNA that didn’t really do anything.
Which did make sense as a theory, if you consider how
mammal-centric a lot of biology can be.
I mean, if mammals don’t have them, they couldn’t possibly be real chromosomes.
But in recent years, scientists have been able to sequence
smaller chunks of DNA in more detail, and that’s helped us understand
exactly what’s going on with microchromosomes.
Early studies in chickens found that microchromosomes
actually contain a lot of genes.
And not just any old genes, but about half the chicken genome.
This told researchers that microchromosomes are
important parts of the bird genome.
Then, in a 2021 study, researchers lined up and compared the DNA sequences of
both microchromosomes and macrochromosomes for different animals.
The study included 7 species of bird, 10 snakes and lizards, three mammals, and a
modern relative of early vertebrates: the lancelet.
By comparing the DNA of different groups of animals, scientists could look at
where microchromosomes came from, evolutionarily speaking.
And they could map how the sequences of microchromosomes changed as
different groups branched off and evolved into new ones.
When they compared the microchromosomes of birds, turtles, snakes, and lizards,
researchers saw that they were all pretty similar overall.
But different reptile or bird groups shared
different sections of their microchromosomes.
Like, even though emu microchromosomes had 21 regions that
were similar to either some lizards or some turtles, it only shared eight with both.
And some of those microchromosomes were found in
different bird or reptile macrochromosomes too.
What this tells scientists is that microchromosomes probably joined up at some
point along their evolutionary journey to form the big chromosomes we know today.
And how they joined up and shuffled around was
slightly different in different birds or reptiles.
The kicker here is where the lancelet comes in.
The researchers found that almost all bird and reptile microchromosomes
lined up with one or two of the lancelet’s macrochromosomes.
Finding that, in an early offshoot of all vertebrates,
means that these microchromosomes must have been present in an ancient
ancestor of vertebrates that lived more than 684 million years ago.
And then those microchromosomes were passed
down to birds, lancelets, and everything in between.
So it’s not microchromosomes that are weird.
It’s us mammals, with our giant, chunky mega-chromosomes.
And the researchers proposed a reason for that, as well.
Although our chromosomes don’t look like they’ve been
built by fusing together smaller ones, that’s just because their DNA has become so
jumbled up that you can’t even recognize the microchromosomes anymore.
How do we know this? Why, from looking at the world’s
weirdest mammal, of course: the platypus.
The platypus represents one of the oldest branches of the mammal group.
Platypuses don’t have microchromosomes, but their macrochromosomes are made
up entirely of microchromosomes that have stuck together.
Scientists think other mammals might have started out
with chromosomes that look like the platypus’s.
But over time, their DNA got shuffled around enough
that the microchromosomes aren’t recognizable any more.
And all that adding in and jumbling might explain
why our chromosomes today have so many repeats and junk DNA.
It’s like continuously buying new clothes, and never donating the old ones.
Your wardrobe gets bigger and bigger and you end
up with a couple of the same shirt in there.
But aside from building the big chromosomes we’re most familiar with,
microchromosomes might have some other function.
It’s not clear exactly why, but scientists have noticed that
microchromosomes will tend to huddle together in the middle of the cell.
It could be that bits of genes from one microchromosome somehow regulate bits
from another microchromosome, so they need to work together.
Or that the microchromosomes form a compartment
in the cell that helps genes to stay active.
These are all things geneticists desperately want to understand better in the future.
Needless to say though, microchromosomes are doing a lot more
than just dusting up microscope slides.
If you want to learn more about how the way we study DNA is changing,
you might enjoy Brilliant’s course Computational Biology.
The field of genomics generates so much data that the people who study DNA
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