Biology concepts – polyploidy, autopolyploidy,
allopolyploidy, gigas effect, heterosis
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The
Sixth Day was a cheesy science fiction thriller
about
cloning. But plants do have different versions
of
themselves in many cases, produced not by
cloning,
but by polyploidization. I have no idea
what
cloning Arnold had to do with the funky
lights
in the eyes; maybe it is a vitamin D thing.
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For some organisms, this isn’t science fiction, it is
science fact. In the last two weeks we discussed how one mammal manages to
survive while being polyploidy in all its cells. We have also discussed how our
bodies have discrete sets of polyploidy cell types. While these cells are
crucial for human development, they are tightly regulated; indiscriminate
polyploidy in humans is deadly- it's called cancer.
Now we can talk about whole groups of organisms that use
polyploidy as a key to their evolution. Not only can they survive as polyploidy
beings, they thrive on it.
A study from late 2012 highlights the importance of
polyploidy in plants. It turns out that plants can tolerate being polyploid
much better than most animals can. In fact, being polyploid is the reason for
much of their success in colonizing different habitats.
The researchers in the 2012 study were looking at a plant called Atriplex canescens, a drought resistant shrub that lives in
the Chihuahuan Desert of the American Southwest. A. canescens has three versions of itself, called cytotypes. One is diploid in all its
cells (except the ovule and pollen sperm of course). Another is tetraploid
(4n), and the third is hexaploid (6n). It turns out that each cytotype lives in
a slightly different habitat in the desert, depending on how much water is
available.
The hexaploid version lives in the clay, the type of soil
that is most likely to be water-poor. The diploid cytotype lives in the sandy
soil nearest the regular sources of water, and the 4n shrub lives in between.
Therefore, it was hypothesized that the different ploidys result in different
physiologic and structural characteristics. This turns out to be so.
When plants have more than two copies of each chromosome, it
changes the structures of their leaves and stems. Polyploid plants tend to have
larger, but less densely packed pores in their leaves. We talked about these pores, called stomata, in an earlier post. They are responsible for releasing water and oxygen to the
outside world. This regulates the movement of water in the plant. As more water
evaporates from the stomata, more is drawn up from the roots by negative
pressure, called transpiration.
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Embolisms
are bad for plants in the xylem and for human
in the
arteries. Divers have to ascend slowly from
deep dive so that
the gases in their blood has time
to adjust to the pressure
change. If the rise too quickly,
the gases come out of
solution and forms bubbles
in their vessels. This is called
decompression sickness
or the bends, and it can kill.
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Polyploid plants also have changes in their xylem. The xylem is the vessel-like tissue that
moves sugars and nutrients throughout the plant. In time of drought, low water
levels can cause an air pocket to form in the xylem. This stops the xylem flow,
much like an air or solid object embolus can stop the flow of blood when it
gets stuck in a blood vessel. You wonder why the nurse takes such care to
remove the air from the syringe when she gives you a shot? An air bubble
getting stuck in an artery in your heart, lung, or brain could very well kill
you.
Emboli formation is less likely in polyploid xylem, because
the channels are bigger. This is good for safety and remaining alive in drought
conditions, but it is not good for growing fast when more water is available.
Therefore, the diploid versions of a species are more likely to live where there
is more water, and the polyploid versions where there is less water.
This is exactly what the researchers found out. The
hexaploid cytotype had the high measured water resistance, with the largest
stomata, thickest leaves and widest xylem channels. The opposite was true for
the diploid version, and the 4n cytotype was in the middle. Therefore, they
show that water conservation and movement is different in the different ploidy
plants and this accounts for their different habitats.
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The
gigas effect isn’t just seen in the watermelon fruit.
The
leaves and flowers of the tetraploid version (on
the
right) or bigger than those of the diploid watermelon
plant.
More DNA means a bigger nucleus, and a bigger
nucleus
needs a bigger cell. So all the structures get
bigger as
well.
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The
advantages all seem to come from size; bigger stomata, thicker epidermal cells,
wider xylem. If a cell has more DNA to house, the cell is necessarily going to
be bigger. This leads to the bigger plant structures, and their size leads to
less water loss. If the conditions arise where water is not available in a
certain area, these characteristics will be advantageous and selected for by
evolution.
But larger cells are supposed to be one of the disadvantages
of polyploidization. Called the gigas effect, larger cells leads to higher energy needs and
altered surface area to volume ratios. These change can inhibit interactions
between the plasma membrane proteins and cytoplasmic elements can be
disadvantageous, even lethal. However, for some things in plants, like fruits,
huge increases in DNA, up to 126n or more work just fine.
Do you like watermelon? More watermelon is better then,
right? Melons grow large because of the gigas effect. Many watermelon species
are triploid or higher. The strawberries that come coated in chocolate and are
as big as your palm are very likely to be octaploid (8n).
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Autopolyploids
can arise from genome duplications, or from
hybridizations
between a diploid gamete and a haploid
gamete,
with later stabilization of the genome by
polyploidization.
But all the genes come from one version of
the
organism. Allopolyploidization (on the right) come from
hybridization
of two different organisms, often with a sterile
first
generation (F1), and polyploidization to return fertility.
This
plant has several copies of different genes, so it has quite
the
chance to become a new species.
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Many crop hybrids are often sterile in first generation,
especially if they come about from autopolyploidy
hybridizations. “Auto” means same, so these are crosses between variants of the
same species, and are often associated with endoreplication events (see When Too Much Is Just Enough)
giving a diploid gamete mating with a haploid gamete to give a triploid
organism. Triploids are often sterile. This is how you have things like
seedless watermelons and you know those little black dots in your banana, those
are the undeveloped seeds. You have to propagate these plants by cuttings
(called vegetative reproduction), not by seeds.
When you induce polyploidy in the triploid hybrids, they
become fertile again, and they (and allopolyploids) also display another
feature, called heterosis, also
known as hybridization vigor. This heterosis is another reason why most of the
cash crops of the world are polyploid. While the crosses are meant to alter
traits, the resulting polyploidization increases heartiness. Still think GM
crops are a bad idea – you’ve been eating them your entire life.
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When
plants undergo polyploidization, they have more
copies
of each gene, called redundancy. This represented
by
the green circles, only showing two here for convenience.
The
plant may get rid of some copies, as in the left panel, or
may
compartmentalize some of the functions in each allele
(subfunctionalization,
on the right). In other cases, some alleles
may
drift genetically, until they have new functions –
neofunctionalization,
as show in the middle panel. New functions
could
lead to a new species, if environmental changes make
them
advantageous.
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The new talents of S. anglica are related to its polyploidization. When plants become polyploid, they may have lots of DNA
with the same functions; therefore they tend to try and reduce their genetic
load. This can occur by getting rid of some gene copies, or letting mutations
run wild in some alleles, as others will still be around to perform the needed
function.
This can lead to subfunctionalization
(altered functions) or neofunctionalization
(new functions) in the changing genes. New functions + change in environment
can lead to new species, ie, speciation.
Speciation due to polyploidy is apparent in 15% of angiosperms and 31% of
ferns. In fact, 40-100% of flowering plants have some polyploidy in their past.
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The
sweet corn in your low country boil is a direct
effect
of polyploidization. The sucrose produced in
polyploids
is higher than in diploid corn, so
naturally
we can’t get enough of the polyploidy
versions
of corn. Sweet corn with shrimp, sausage,
and
potatoes – can’t beat it.
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For
example, sucrose synthase levels were twice as high in the 4n version as
in the 2n version of maize as expected, but mRNA levels were 3x higher in the
haploid plants and 6x higher in the triploid versions! Obviously, some
regulatory pathways were not controlled as well at some of the polyploidy
levels. In these plants, fully 10% of the genes had an “odd-ploidy” effect. This
leads to less than stable cytotypes and poor endurance in the environment.
Next time, we will see that fish are one of the
exceptions of the animal world. They tolerate polyploidy well, and we have even
used that fact to increase our harvests, but also our headaches.
For
more information or classroom activities, see:
Polyploidy
in angiosperms –
Polyploidy
in crop plants –
Autopolyploidy
and allopolyploidy –
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