Double or Just Trouble?

In a previous blog we talked about doubling the chromosome number in Siberians to produce tetraploids. Now I’d like to talk about another kind of doubling - doubling the petal number. When we use “double” as a description of a flower this is actually not very precise. Generally it would mean more than usual number of petals and not a strict doubling of all the flower parts. However, at least three different effects can cause this increase in petal count in Siberians (I'm using "petals" here to include both petals (standards) and sepals (falls)). The resulting flowers look quite different from each other. Probably you could see the same effects in other irises, but we’re into Siberians here. The three routes to “doubling” are:

1. The reproductive parts of the plant (stamens, stylearms, ovary) are converted to petal--like structures (petalloids). Example 1 shows a seedling where this is the predominant effect.
2. Flowers are hose-in-hose where a second (or more) flower grows up inside the first one. This can be seen in example 2 with a second complete flower growing up inside the first. Incidentally, this iris was introduced recently as Double Play.
3. The flower has six falls through the conversion of the normal three standards into falls and with the sexual parts of flower unchanged. Example 4 shows this effect (introduced as Six Love) but we won’t be talking about it further here.

Example 1

Example 2

Example 3

Example 4

The first two effects can be created by the same mutation – a master gene that turns on the machinery to produce ovary, styles and stamens loses its activity and the master gene controling petal formation in the same cell comes alive and takes over, so we get lots of extra petal-like structures at the expense of the reproductive parts. The same master gene is also responsible for making sure that only one flower is produced from each terminal flower meristem. When it loses its function this means that more than one meristem can produce flowers, hence the hose-in-hose, multiple flower effect.  The 6- fall trait is caused by a different mutation that acts specifically in controlling production of the standards (petals). These effects can occur together complicating things further in terms of flower structure.

So that’s the basics of how it happens, but why get excited about it? Well, everyone likes variety and mutipetal flowers are very popular in many of the major ornamental groups e.g. Japanese iris, peonies, dahlias, lilacs, roses, petunias, and daffodils. More petals more flower power. Oh, by the way did you spot that the photo in example 3 is a daffodil and not an iris? The control of flower form is similar across most, if not all, flowering plants and the same kinds of mutations occur so that we see these related multipetal effects everywhere.

In addition to the extra fullness, several special effects are possible with multipetals including repeating expression of falls with strong blaze signals as in Kaboom (Bauer-Coble, 2001: Example 5), or alternating layers of color when you have a hose-in-hose structure in a bicolor or amoena iris (color/white/color/white sequence) as shown with the seedlings in examples 6 (not fully open) and 7.  These need further development but the effects already make some novel and rather beautiful flowers.

Example 5

Example 6

Example 7

Mulipetal (double) flowers have long been known but have not always been appreciated. The famous French naturalist Rousseau said in his “Letters on the Elements of Botany” (1787, 2^nd Ed.)   “Whenever you find them double do not meddle with them, they are disfigured; or if you like, dressed in our fashion: nature will no longer be found among them; she refuses to reproduce anything from monsters thus mutilated, for if the more brilliant parts of the flower, namely the corol, be multiplied, it is at the expense of the more essential parts, which disappear under this addition of brilliancy”

Rousseau has hit on a real problem here. At least with the current multipetal Siberians, it would be wonderful if every flower looked the same and was as well organized as the ones in the photos, but sadly, that isn’t so. Every flower on the plant may be different, and some can be less organized in form and quite untidy. Why is this? The probable answer is that there are two copies of the genes for each trait in diploid plants. In the case of multipetal Siberians only one of these two genes is in the mutated state and the other is “normal”, so these two control systems fight it out in each flower – sometimes the mutant form predominates and the flower is almost fully doubled and seems symmetrical; in other cases the normal gene tends to compete better (for unknown reasons) and the flower is less double and quite mixed up in its structure. Only by finding a way to get both these genes in the mutated (inactive) form can one put this battle to rest and expect to get fully double flowers every time. And this is not so easy to do when the reproductive parts of the flower, especially the ovaries, are heavily modified or not present! There is some pollen on the sides of the petalloids at times which is active and allows crosses to be made quite readily with standard Siberians, but, so far, crosses with mutipetals as the pod parents have been unsuccessful – and these plants never seem to produce bee pods either. So should we redouble our efforts to get these potentially perfect (and perfectly sterile) multipetal forms or is it more trouble than it’s worth?