This week on SciShow News, scientists have taken some steps to solve a couple genetic
mysteries: why birds lay eggs, and the biology of addiction.
Giving birth to live young, called viviparity, is a pretty sweet evolutionary strategy.
It seems to be helpful in colder environments, and predators might gobble up eggs more easily
than little scampering offspring.
Lots of vertebrates give birth to live animals, including most mammals and a number of snakes
and lizards.
But there’s one huge exception: birds, and the larger group they belong to, called archosauromorphs,
which also includes crocodiles and some dinosaurs.
Until this week, we thought all these creatures had always laid eggs.
Maybe because of some genetic reason that prevented viviparity.
But according to a new study published in the journal Nature Communications, we were wrong
The study is about an archosauromorph called Dinocephalosaurus, a marine reptile dating
back to the Triassic period.
It’s a primitive member of the group, only distantly related to crocodiles or birds.
But these researchers think that it gave birth to live young.
As evidence, they present a fossil from around 245 million years ago with an embryo clearly
preserved inside of it.
The tiny bones look like a mini-adult.
There was no sign of fossilized calcium material from an eggshell,
so it probably wasn’t in an egg.
And the researchers know it’s not something the adult specimen ate, because it’s in
the wrong position to be food.
It’s facing with its head forward, while most marine predators swallow their prey head-first,
so that it’s preserved tail-forward.
Plus, the baby is curled up like a typical vertebrate embryo.
It’s in the fetal position, or as close as you can get for a reptile
whose neck is longer than its body.
So whatever’s stopping birds and crocodiles from giving birth to live young didn’t stop
their earliest relatives.
Maybe flight makes it impossible for birds, since they’d have to carry around those
heavy, growing offspring.
Or maybe there was some other evolutionary pressure for archosauromorphs to lay eggs.
Scientists don’t know for sure yet, but this study means they are making some progress.
Meanwhile, scientists do know that addiction is at least partly genetic, and some people
are more predisposed to form drug habits.
And in a paper published this week, researchers from the University of British Columbia have
genetically engineered mice that can resist cocaine addiction.
Their study involved a group of proteins called cadherins, which help your cells stick together.
Cadherin also helps with the formation and stability of synapses, the gaps between your
nerve cells, which your brain uses to do everything from sending messages through your body to
storing memories.
Addiction generally involves intense memories.
Cocaine molecules, for example, cause a buildup of extra dopamine, a neurotransmitter associated
with feel-good reward systems in the brain.
So cocaine addiction changes synapses in that reward system, to build memories of those
euphoric highs, which can make people change their behavior and keep seeking out that feeling.
In this study, researchers genetically engineered mice to produce extra cadherin, which they
predicted would mean more stable synapses, stronger memories,
and more tendency towards addiction.
But what they found was the opposite: mice that were producing more cadherin showed less
tendency toward cocaine addiction.
Their experiments involved a well-established method called conditioned place preference,
where a box is divided into rooms that are different enough that mice can tell them apart.
The walls of each room can be decorated differently, the floors can have different textures, there
might be different smells, and – in this case –
one of them was associated with a dose of cocaine.
In this experiment, the mice were placed in a room with plain walls while they were high
on cocaine, and the other room had stripey walls.
After a couple days of this, the mice were put in the box without any drug and allowed
to wander.
The normal mice spent more time in the plain-walled room, possibly because their reward systems
had been tricked into remembering that this was an awesome place to be.
Not that cocaine is awesome.
But the mice with extra cadherin spent the time in both rooms.
And the researchers took this to mean the genetically-engineered mice weren’t forming
strong memories related to addiction.
It’s possible that, instead of helping create or strengthen synapses with related memories
to cocaine, the extra cadherin prevented existing synapses from being changed.
Now, the researchers caution that this finding isn’t very ready to apply to humans yet.
Our synapses need to get stronger and weaker, or reform completely for our memories to work
properly, so flooding a human brain with extra cadherins
isn’t something we would want to do.
What it does help with is understanding which genes might play a part in addiction.
That can help scientists identify people who might be more predisposed to addiction, or
others who might be more resilient, which could eventually help us develop biochemical
treatments.
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