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Maybe you’ve strolled past one of these plants
on a hike somewhere in North America.
Maybe it was their beautiful,
bell-like flowers that caught your eye.
But these brightly-colored blooms hold a scientific secret.
Although their flowers might look different close up,
they’re actually all the same species…
well, sort of.
And that’s the rub.
They kind of mess with the whole idea
of what a species is.
These unassuming mountain flowers are straining
scientists’ understanding of genetics to the breaking point.
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These flowers are all Beardtongues,
or members of the Penstemon genus.
The purple ones are the New Mexico Beardtongue
and the Upright Blue Beardtongue,
while the red ones are just called Beardtongues.
But more than that, they’re all part of what’s called
a species complex,
referring to two or more species
that are so closely related that they almost count as one.
As a quick reminder,
generally we define a species as groups of organisms
that can breed with one another in nature
but can’t breed with similar organisms of another group,
or species.
The members of a species
might look pretty similar to one another,
but they don’t have to.
What differentiates these beardtongues
is that each flower is specially adapted
to suit the thing that pollinates it.
The New Mexico and Upright Beardtongues
have wide, purpley-blue flowers –
perfect for attracting insects like bees
who are great at seeing blue-to-ultraviolet light
and want a nice little landing pad to plop down on
when they gather nectar.
The red Beardtongues, on the other hand,
are mostly pollinated by hummingbirds,
who prefer red colors and don’t need a landing pad,
since they can just hover there to suck out the loads of nectar
sitting deep in the flower’s bell.
Since the hummingbirds and bees don’t cross over to the flowers they don’t like,
red flower pollen rarely reaches blue flowers and vice versa.
That drives the species apart.
But that doesn’t necessarily mean
they can’t reproduce with one another.
Despite our definitions,
genes may continue to be exchanged between recently diverged species.
But the way that’s happening in beardtongues is weird.
Like really weird.
To understand why,
we need to talk about the law of independent assortment.
This goes all the way back to Mendel –
the pea plant guy.
For the most part,
the different flavors of a gene –
also called alleles –
will be split up into reproductive cells
independent from the alleles of another gene.
If gene A and gene B are on two different chromosomes,
those two chromosomes are gonna go their separate ways.
In fact, if they’re far apart on the same chromosome,
that can happen too,
because chromosomes love
getting together and swapping arms
as reproductive cells are made.
This means, say,
the genes for hair color probably
won’t influence the genes for some other feature,
like height.
Except independent assortment also says
that some genes aren’t all that independent
from one another.
They’re actually passed on or inherited together.
Such genes are said to be linked.
Usually when this happens,
the genes are physically close to each other
on the same chromosome.
Think about it like this:
If you’re traveling in a car,
you wouldn’t expect the other cars
to end up in the same place you’re going.
They’re all moving independently.
But if you’re on a bus,
some of the other passengers
will probably also get off at your the same stop.
In the case of flowers, you might have genes for shape,
color or size that are all very close to one another
and passed down to baby flowers in a group.
This is what actually happens in various species of petunias.
There are genes for color,
scent and size that make one petunia species different
from another and travel together in a group.
And It would make sense
if beardtongues were doing that.
They are not.
In fact, they are bending those rules almost to the point of snapping.
When researchers in 2023
peeked under the genetic hood of these bell-shaped beauties,
they found around 21 loci,
or places, on 8 chromosomes
that were all traveling together and being inherited as a group.
And here’s the part where this episode
becomes an unexpected plea for more people
to do PhDs in plant genetics,
because the language in the paper is couched in molecular biology hedges
since no one’s sat down
and proved the function of each of these genetic loci,
so we can’t actually say what they do.
However, it seems like the genes in these places
are key to defining the bird and bee features of each flower.
But, and here’s the rub, instead of those genes sitting close to each other
on one particular chromosome,
they were spread out all over over all the whole genome.
And all of the DNA in between was not traveling together.
Think about how weird this is.
Like, imagine you're on the interstate
and there's a bunch of cars and there's 21 of them
that are all taking the same route
and ending up in the same place every time,
even though they are not planned to go to the same place.
Like there’s nothing making them stick together.
So that… is asking for an explanation.
The researchers think it’s because
there’s strong selective pressure
to keep the groups of genes that make
the hummingbird-pollinated and bee-pollinated flowers.
So they’ll be passed down through generations sticking together –
or getting off at the same exit –
even if that means bending the usual inheritance rules.
Because there’s good reason for hummingbird flowers
to stay hummingbird flowers
and bee flowers to stay bee flowers.
Hybrid flowers, between the two,
seem to be are really, really, rare.
So you might get a hybrid purple baby flower.
And that flower could breed with another hybrid
and to keep making these intermediate purple flowers…
but it won’t.
Because the birds and the bees are both gonna see it
and go like “meh I don’t actually care that much about that flower”
and move on to a flower they actually like. then move on.
That means that the hybrids
are more likely to backcross
with one of the parent species.
Really quickly,
the offspring of the hybrids
are going to end up
with all blue flower traits or all red flower traits again –
because that’s what the pollinators will pollinate.
But the hybrid still happen,
so genes were still swapped
between the reds and the blues.
Meaning our these 21 cars
are selectively pressured to stay together,
despite not being physically connected,
but everything in between them can move.
So it looks like the reds and blues
are breaking the law of independent assortment,
with these 21 unconnected loci staying together.
But natural selection beats out everything else,
and forces those traits to stay in a group.
The big question that still remains
is how much of the shared DNA between reds and blues
is because the flowers recently split into different species,
and how much of it is gene flow happening afterward.
Figuring this out could not only explain
what’s going on with Beardtongues,
but also help geneticists understand the early stages
of how species split off from one another.
These flowers could even help uncover
how species evolve new traits quickly
to adapt to environmental changes,
like those stemming from the climate crisis.
So rather than a freak of nature,
Beardtongues might just be the next pea plant,
revealing brand new truths about genetics.
Hi folks, Reid here.
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