Hosta Ploidy and Genetics

An explanation of Hosta genetics and differences between diploids and polyploids (triploids and tetraploids).

The rarer, polyploid (non-diploid) hostas often have exceptional plant characteristics gardeners often prefer. Most hostas are diploids.

Common Characteristics

The common characteristics of most tetraploid hostas compared to most diploid hostas:

  • More slug resistance from more leaf thickness (substance). The plant has thicker cell walls and larger stomata cells.  See H. plantaginea ‘Doubled Up’
  • White portions of leaves do not to burn as easily.    See H. ‘Fire and ice’
  • Larger flowers.  See H. ‘Cathedral Windows’
  • More intense fragrance.
  • Wider margins and narrower center variegation.   See H. ‘Liberty’
  • Shorter, stronger and thicker flower scapes with a denser flower arrangement.
  • Leaf surfaces that may have more texture.
  • Plant size that is slightly smaller.
  • Improved plant density.  See H. ‘Touch of Class’
  • Leaves are held more upright creating an upright hosta plant form.   See H. ‘Fireworks’
  • Leaf petioles are shorter and thicker.
  • Pollen grains are generally larger.   Pod production and seed fertilizy is generally lower.
  • Growth rate is generally slower.  See H. ‘Heat Wave’
  • Roots are often slightly shorter.
  • More chloroplasts. Chlorophyll is the green pigment and chloroplasts are the structures where chlorophyll resides and where photosynthesis takes place. There may be up to twice as many green chloroplasts.
hosta ploidy touch of class pp13080_1
H. ‘Touch of Class’
waltersgardens hosta minuteman
H. ‘Minuteman’
waltersgardens hosta revolution
H. ‘Revolution’
liberty 3767_4_4
H. ‘Liberty’

Where can I learn more and select polyploid hostas?

HostasDirect’s Buy Plants – Hostas and HostaSearch™ software allows users to search huge hosta databases with detailed information and thousands of photos for polyploid plants fast. Use the website’s “compare” feature to compare photos and data for each plant you select side by side.  Use the wish list to further select and save your favorite plants.

Testing for ploidy is very expensive and requires special equipment.  Thus, the majority of hostas have NOT been tested for ploidy.   Our search categories note those hostas that are “confirmed” via testing, and those that have NOT been tested but based upon their polyploidy plant characteristics, many are “assumed polyploidy” by hosta experts.

Thus, if you want to search for confirmed (tested) or assumed polyploidy plants, select all of these categories:

  • Assumed Polyploidy
  • Confirmed Triploid 3-3-3
  • Confirmed Tetraploid 4-4-4
  • Confirmed 4-2-2 apical layers
  • Confirmed 4-4-6 apical layers
  • Confirmed 3-3-4 apical layers
  • Confirmed 4-A-4 apical layers

The only way to determine ploidy is to scientifically test.

Although the above are the most common visual ways to tell if a hosta is tetraploid, just because it has these visual characteristics does not mean that it is definitely polyploidy, triploid or tetraploid. A diploid hosta could be less dense than another because of its growing location. Maybe it is not getting enough sunlight or moisture. Perhaps it is not a mature plant yet.  Perhaps its genetics make it a slower grower sending out fewer eyes each season.

Just because a hosta leaf looks or feels thicker and stiffer than others does not mean that it is definitely a triploid, polyploid, or tetraploid. As an example, the thick leafed H. Blue Mouse Ears is a diploid 2-2-2.   The thinner leafed H. Grand Tiara, whose leaves are half that of H. Blue Mouse Ears is a polyploid 4-2-2 becuase it has four sets of chromosomes in layer 1 and two sets in layers two and three  (read more below!   Hosta Sum and Substance is a triploid 3-3-3.  Its leaf thickness is in between the prevoius two hostas.   Hosta Independence, one of the few hostas with spots in its margins, is a tetraploid 4-4-4.  Hosta Stitch in Time, is a very rare 4-A-A (and frankly not vigorous).

Ploidy is the number of chromosomes in each cell of an organism.

The number of chromosomes generally stays the same within each species. Humans, for example, usually have 46 chromosomes made up of two sets of 23 chromosomes. Because we have two sets, we are considered a diploid species. Hostas, along with many other species are mostly diploid in nature, but there are obvious exceptions to this including triploids, tetraploids, and more.. Hostas have 30 chromosomes in the male pollen and female egg parts. When fertilization takes place normally the male and female come together and create a total of 60 chromosomes as a diploid hosta. But, things can happen in nature or through the use of chemicals in the cells of a plant which causes more than two sets of chromosomes to be present.

Polyploid plants contain more than two sets of chromosomes.

Of the 40 or so original species hostas plants that evolved in Japan, eastern China and the islands of South Korea, all are thought to be diploid (but to our knowledge not all species plants have been tested).  The exceptions are species hostas H. clausa, a triploid with apical layers 3-3-3 and species plant H. ventricosa, a tetraploid with apical layers 4-4-4, because they have three and four sets of chromosomes respectively. If you cross a tetraploid with a diploid, you could end up with a triploid hosta. This would mean that each cell in the hosta would have three sets of chromosomes.

Ploidy chimera have different apical layers in all three layers of meristem tissue.

There are three layers of meristem tissue can be genetically different.  They must all be converted to be a tetraploid. If just one or two layers are converted it is said to be a chimera.

  1. The outermost layers is the protective covering of the plant (called L1).
  2. The second layer gives rise to the egg cells and pollen grains (called L2)
  3. The innermost layer gives rise to the leaf, flower and stem tissues (called L3).

The chimera plant may revert back to a diploid or remain a chimera that could produce sports or seed variations.  DNA testing can specify ploidy chimeras that have different ploidy in the three apical layers found within the meristem tissue of a hosta.

What are aneuploid hostas?

These plants have extra or missing chromomes in the apical layers.   An example is Hosta Stitich in Time with apical layers 4-A-A. meaning it has abnormal or missing chromosomes in apical layers L2 and L3.  This is a very interesting hosta that was on the market but proven to be not a consistent grower over time.

How do plants with a new ploidy develop?

In nature perhaps mutations are caused by ultraviolet light, x-rays, cosmic rays, ionizing radiation or other things. Chemically, herbicides such as Surflan (orzalin) and Treflan (trifluralin) have caused polyploids. Plant hormones used during tissue culture production can also create polyploids. Daylily hybridizers have developed tetraploids for decades using a chemical called colchicine but it appears that chemical does not have the same impact on hostas.

Other means of measuring plant ploidy

While visual differences provide a good indication to differentiate polyploidy, triploid and tetraploid hostas from diploids, you can never be certain. The only way to know your hosta’s ploidy is to do DNA testing at a laboratory with expensive special equipment. One method of testing is called flow cytometry, where scientists can measure the amount of DNA in the nucleus. This research has been conducted primarily by Zonnerveld and van Irene at Leiden University in The Netherlands. This will also tell you if your hostas ploidy in all parts of the plant and not just in one area. Only mature plants can be tested. Keep in mind that you will need several results saying that your hosta is polyploidy before you know for sure.

To learn more…

Note that the above information is from various sources including Don Rawson and is only meant to be a brief, non-technical overview of plant ploidy for general gardeners, not plant scientists or hybridizers.   To learn more, we suggest you:

Join the American Hosta Society

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Join your local hosta society or start a new one

You can find your local hosta society on our Hosta Society Links page.  You will make wonderful new friends, find out about local garden tours, gardeners,hybridizers, conventions, cut leaf shows and more.

For more detailed information…

http://www.hostalibrary.org/misc/hybrid/polyploidy.pdf

http://en.wikipedia.org/wiki/Polyploid

http://en.wikipedia.org/wiki/Apical_dominance