Wednesday, January 23, 2013

An Evolutionary Ploy Employing Polyploidy

Biology concepts – polyploidy, autopolyploidy, allopolyploidy, gigas effect, heterosis

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.
Imagine that there are three different versions of you, each with different strengths and weaknesses, living in different places and surviving in different ways. Sounds like a strange Arnold Schwarzenegger sci-fi movie; maybe one version of you has really big muscles and an accent.

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.

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.
Polyploid plants also tend to have thicker epidermis layers on their leaves, and this, together with the lower density of stomata means that polyploid plants tend to lose less water than diploid version of the same species. That could be helpful in low water environments.

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.

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.
One species being able to live in several habitats is quite the evolutionary advantage. They don’t compete with one another and they can colonize a larger portion of the land. Being polyploid is quite the boon for some plants.

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).

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.
Many crops are polyploid, the results of hybridizations and crosses over many years. These crosses have been meant to increase yields reduce disease susceptibility and expand the environments in which the crops can be grown. For hybrids of two different species, this is called allopolyploidy (allo = different). Using this method, we have developed strong wheat (hexaploid), apple (tetraploid), cotton (tetraploid), and sugar cane (octaploid) crops.

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.

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.
But heterosis could also have unwanted results. In the late 1800’s, hybrids of different spartina bush species were carried out in England in hopes of breeding a species that would better prevent erosion of the tidal mud flats. It turned out that the offspring underwent allopolyploidization and became too strong a species. The new species, Spartina anglica, underwent significant and rapid genetic changes and became invasive in salt marshes. It can crowd out other species and can grow dense enough to prevent some animals from moving from land to water.

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.

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.
But not every polyploid development is so simple. Sometimes the new cytotypes cannot quite overcome the problems inherent in having many more copies of genes all working at once. In corn for instance, some polyploid numbers are better tolerated than others. In 1996, Guo, the same primary researcher involved in the Atriplex work cited above, was working on haploid, diploid, triploid, and tetraploid versions of maize. He found that some gene products (proteins) did increase with increasing gene copy number, but others didn’t.

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.

Hao GY, Lucero ME, Sanderson SC, Zacharias EH, Holbrook NM. (2012). Polyploidy enhances the occupation of heterogeneous environments through hydraulic related trade-offs in Atriplex canescens (Chenopodiaceae). New Phytol.

SALMON, A., AINOUCHE, M., & WENDEL, J. (2005). Genetic and epigenetic consequences of recent hybridization and polyploidy in Spartina (Poaceae) Molecular Ecology, 14 (4), 1163-1175 DOI: 10.1111/j.1365-294X.2005.02488.x

Guo M, Davis D, Birchler JA. (1996). Dosage effects on gene expression in a maize ploidy series Trends in Genetics, 12 (8) DOI: 10.1016/0168-9525(96)81463-6

For more information or classroom activities, see:

Polyploidy in angiosperms –

Polyploidy in crop plants –

Autopolyploidy and allopolyploidy –


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