Editor’s note: You may have noticed that there are often a few lilies on the bulb sale list that are marked as “tetra”. The following article will provide a brief explanation of what that means and why it is important both from a gardener’s and a hybridizer’s point of view.
Every live organism has cells. Growth results from the division of these cells. Cells contain chromosomes that carry the genes that convey the hereditary characteristics of the organism. Each species has a certain number of chromosomes in each of its cells. For lilies that number is 24, 12 of which come from the female parent and 12 from the male parent. In nature this number is always constant and we say that lilies are diploids when they have 24 chromosomes. This is expressed as 2n.
Sometimes, through an accident in nature or through intervention by man, lilies appear which have 36 chromosomes in each cell instead of 24 and we refer to them as triploid (3n). Sometimes they have 48 instead of 24 and they are known as tetraploids (4n). If they have 36 or 48 chromosomes we say they are polyploid.
Triploids and tetraploids can be produced by the use of chemicals at various stages in the lily’s development or through breeding. The methods are not new; tetraploids were produced 50 years ago by scientists working for the US Department of Agriculture and by others around the world. At the present time there are a number of different lilies which are available on the market which are triploid or tetraploid.
There are several ways to determine whether a certain lily bulb is a diploid, triploid or tetraploid. One which Dutch growers use extensively is called a Flow Cytometer, an expensive machine. Another method is to stain the root tip with a dye, photograph it with a camera and actually count the chromosomes with the help of a microscope.
Other clues are if the pollen grains are especially large or if the breathing pores (stomata) on the undersides of the leaves are larger than usual or if a suspected lily will cross (that is, mate) with another lily that is known to be a polyploid. These clues should then lead to an actual count of the chromosomes to be certain.
What are the benefits of a triploid (3n) or tetraploid (4n)? First, since there are more chromosomes (more mass) in each cell than normal, the plant may be larger, stronger; the flowers may be larger and brighter, though sometimes fewer in number, the tepals may be thicker and the root systems may be larger. From a gardeners viewpoint, these plants are usually large, sturdy and long-lasting. From a hybridizer’s standpoint, polyploids are valuable because lilies which refuse to cross with other lilies when they are at the diplioid (2n) level will cross at the tetraploid (4n) level. A good example is ‘Black Beauty’. This lily resulted from a cross between L.henryi (Division VI Trumpet) and L.speciosum var.rubrum (Division VII Oriental). At the diploid level it refused to mate with either trumpets or orientals, but after it was converted to the tetraploid level it has mated with both and resulted in beautiful lilies, for example ‘Leslie Woodriff’.
There are a few rules to follow when hybridizing with polyploids. With few exceptions, you can cross a diploid (2n) with another diploid in the same division without regard to which of the parents will be the pod parent and which the pollen parent. If one lily you wish to use is triploid (3n) then it must be the pod parent (female) because triploid pollen (the male) is seldom fertile. The pollen parent in that example should be a tetraploid (4n). If both lilies you are working with are tetraploid, then either can be the pollen or pod parent.
An easy rule to remember is a always use the lily with the lower chromosome count as the pod parent, that is, always use tetraploid pollen (4n) on a triploid (3n) or a diploid (2n).
- If you cross two diploids (2n), the resulting seedlings with most likely be diploid.
- If you cross a triploid (3n) and a tetraploid (4n), you may get more triploids.
- If you cross two tetraploids (4n), you should get more tetraploids.
- If you cross a diploid (2n) with a tetraploid (4n) you may get either triploids or tetraploids.
Many diploid lilies have been converted to polyploids by the use of chemicals after they were marketed as diploids so in some cases there are both diploid and polyploid forms of the same cultivar available. When listed for sale, the polyploid form of a cultivar will be noted. For example, if the diploid form of ‘Black Beauty’ is being offered for sale, it will just be listed as ‘Black Beauty’. If the polyploid form is offered, it will be listed as Tetra ‘Black Beauty’. If a cultivar only exists in a polyploid form, then it is listed by name with tetraploid or triploid listed in the description.
Pictured above are Pearl Jessica (left) and Pearl White (right), growing in Barbara's garden in Hepburn. Both are Griesbach Tetraploid Asiatics.
from (CPLS Newsletter September 2017)
This article was reprinted in part from the Iowa Regional Lily Society Newsletter May 1996, and was previously reprinted in the December 1996 CPLS newsletter.