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If you have an oak tree,
and that oak tree drops an acorn,
and that acorn sprouts into a seedling,
that seedling is not the child of the oak tree…
it’s the grandchild.
The parents lived out their entire lives
in a way you never even saw.
And that happens in every single land plant.
Plants may seem simple,
but their life cycles have a whole extra multicellular organism
that’s rarely ever seen.
Here’s what they get out of this whole scheme –
and where their “invisible” generations can be found.
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So to talk about plants
it might be helpful to do a quick reminder
about how animals work.
Animals have two copies of their genome in each cell,
which makes them diploid.
During reproduction,
animals make sperm and egg cells,
which each only have one set of chromosomes,
making them haploid.
These individual cells never really grow on their own,
but they do combine
to form a diploid cell again during fertilization,
which then grows into a new animal.
Plants saw that system and went, that’s way too easy.
In plants, the haploid phase does grow on its own.
In fact, all land plants alternate between
being diploid and haploid organisms
in a process called alternation of generations.
So here’s a quick breakdown of the cycle:
Let’s start with a diploid plant,
roughly equivalent to you and me,
with two copies of its genome per cell.
Instead of making sperm and egg cells,
this plant makes haploid spores.
We call this, appropriately
the sporophyte.
Literally the “spore-plant”.
These haploid spores
then grow into a new multicellular plant,
with every cell having one copy of its genome.
Eventually it produces haploid gametes –
the plant version of sperm and egg cells.
If sporophytes are spore-plants
, this gamete-plant is,
appropriately again, the gametophyte.
Sometimes scientists are good at naming things.
Two gametes then find each other, fuse,
and produce a new diploid organism,
a new sporophyte, and the cycle begins again.
Imagine that in people.
Instead of producing sperm or eggs,
a diploid person would just kind of spontaneously,
asexually produce haploid kids all by themselves.
But then those kids would do things the normal way.
How could this ever evolve?
It’s actually not unique to plants.
Alternation of generations has evolved
a couple of different times in the tree of life.
But all plants seem to have inherited it
from a common ancestor.
Early in evolution,
the sporophyte and gametophyte shared
roughly equal halves of the life cycle,
and may have looked pretty similar to each other.
In extant plants, however,
one of the two forms is dominant over the other.
In bryophytes such as mosses,
the haploid gametophyte is the dominant form.
That is, it has one copy of its genome
and will make gametes, not spores.
All that green stuff you see is likely the gametophyte.
The sporophyte appears only occasionally
as a kind of flower-bud looking thing on a tall stalk
attached to the rest of the moss.
It’ll make spores
and once again produce the moss you’re used to.
On the other hand,
vascular plants are sporophyte dominant.
That’s most land plants
like trees and flowers and shrubs – and ferns.
Fern gametophytes look like little green splats.
They’re free-living enough to be noticeable,
if you’re looking hard enough.
However, flowering plants,
as well as conifers, gingkos, and the like,
have become so lopsided that you have to essentially
dissect plant bits to find the gametophyte.
And this is where the invisible generations come in.
The fully-grown gametophyte is just a couple of cells
producing the pollen cell or ovum,
embedded in different structures in the flower
and dependent on its quote-unquote parent to survive.
It’s still there, as its own multicellular thing.
Though we’re getting to pretty charitable definitions
of “multicellular,”
like, single digits.
And it produces the pollen and egg cells
the plant needs to continue the alternation of generations.
That’s why our oak seedling from the beginning
is the grandchild of the big oak tree.
Its immediate parent lived and died
unnoticed in the oak flowers.
But…
why though?
What advantage does this give to plants?
It might allow plants, and other similar organisms,
to take advantage of both the reliability of asexual reproduction
and the diversity-generating process of sexual reproduction.
When you reproduce asexually,
you don’t need a partner to get the job done.
But when you recombine your genes through sexual reproduction,
you’re a lot more resistant to any surprises
evolution might throw your way.
It also allows plants to double-dip
into the advantages of both diploid and haploid genomes.
Diploid plants may have
an easier time compensating mutations
because they’ve essentially got a backup copy.
At the same time, for the same reason,
the haploid phase might be really good
at weeding out problem mutations.
As to why plants grew to favor one or the other so dramatically,
instead of having a more even split,
it looks like this is something scientists
are still trying to figure out and debating over,
but we think that with vascular plants at least
it might have been a key part of how plants took over dry land.
One idea is that, in early plants,
like in modern mosses and bryophytes,
the gametes needed abundant water
to survive and find each other.
This limited where they could grow on land.
But the haploid spores
didn’t need to find other spores –
they could just grow on their own, even during drier times.
The diploid genome –
with its diversity and ability to have back-up genes –
may have had an easier time adapting to dry land.
Finally, some studies have suggested
there may have actually been something genetic
about the sporophyte stage that enabled plants
to evolve vascular tissue in the first place.
That’s the nutrient-carrying tissue
that allows plants to grow up and away from the water.
This means gametophytes
would have always have been doomed
to stay low and water-dependent:
they never co-opted their genetic machinery
the same way their children did.
If that’s true,
it’d make sense
why the sporophyte-dominant plants
took over the land.
But like I said,
it’s still an open question.
We, that is to say, humans,
can also take advantage of these generational shenanigans.
There’s a way that scientists can take pollen
and stress it out until it actually starts
to grow into an embryo without ever meeting an egg cell.
Chemicals can then be used to force it to double its chromosomes.
Basically, you they skip a generation
and create a quick pseudo-clone of the parent plant.
Which is really neat if you’re into plant breeding
because it means you can develop new breeds very quickly
without having to mess around with all the pedigree stuff
if it had to reproduce sexually.
So there you have it.
Unlike animals,
plants go through half their life cycles
with half as much DNA,
and they swap the way
they reproduce every time.
Evolution has reduced the role of some of those halves,
but it hasn’t gone back on the process entirely,
maybe just because evolution
doesn’t really do that kind of thing.
But while we can turn the system
to our advantage,
it’s also just really interesting to think
about these whole generations of plants that we,
for the most part, never notice.
So much of life happens
kind of behind-the-scenes or in ways that we don’t know about.
So I hope you’ve enjoyed this little peek at the invisible.
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