QBARS - v23n1 Variability in Hybrid Populations

Variability In Hybrid Populations
W. L. Tolstead - Davis and Elkins College, Elkins, West Virginia

Few of us realize how variable hybrid populations of Rhododendrons - or anything else - can be. To illustrate this I have summarized in the accompanying table the theoretical mathematical combinations that can occur in the diploid and the several kinds of polyploid populations. This variation is due to too reasons, the random combining of the maternal and paternal chromosomes during meiosis (reduction division) and the new combinations which occur when the eggs and sperms are combined in fertilization.

The basic chromosome number in Rhododendron is 13, or to put it otherwise, 13 chromosomes must be present to carry the full mass of the hereditary code. If a part is absent the resulting offspring may be abnormal, if not dead, or, if functional, they might be sterile. Most species of Rhododendron are diploids. These plants have two sets of chromosomes (26), one originating from the sperm and the other, from the egg of their parents. This is also the primitive number. Examples of species with this number are Rhododendron maximum , R. catawbiense , R. fortunei , R. arboreum and R. nudiflorum . Polyploids are found especially in the Lapponicum and Triflorum Series.

While wild species might be relatively uniform when compared with the hybrid crosses, they are nevertheless quite variable, and this seems to be especially true in Rhododendron species. To this all species-fans will readily attest.

You will note that odd numbers do not occur in functioning populations. They occur as even sets only. If, for example, a cross between a 3n and a 4n gamete should occur, the resulting plant (7n) would probably be sterile and never enter the two breeding populations of their parents. They may be valuable horticulturally as individuals, but are not worth the trouble for breeding future generations.

The relative simplicity of diploids can be complicated greatly by polyploidy, multiplying the number of chromosomes in the body cells and gametes, 4 x 13, 6 x 13, etc. As you can see by examining the table the number of combinations can be terrifically numerous. In fact, so much so that every organism is unique. It is highly improbable that any two plants would be exactly alike. There never was in the past, nor never will there be another plant exactly like the one you select as "superior." If there would be, it would be very unusual indeed.

This brings out the fact that, if a breeder raises one or two flats of a cross (F1) between two different species, or another generation (F2) he is actually making a very small sampling of the total potentiality of his crosses. He can afford to give a flat to his competitor because the two would never be able to obtain identical plants. But there are practical limitations to this idea because the neighbor might come up with a better combination than you were able to obtain by your limited sampling.

How do plant breeders treat most of this variability? They ignore it and confine their interest and selections to a few traits. Ten would be a large number to work with at one time.

A - A visual symbolization of the number of chromosomes in each body cell, except on the haploid line where the number 13 is found, the number of chromosomes occurring in the sperms and eggs of diploids.

B - The number of possible combinations formed by meiosis, and thus the number of possible combinations found in the sperms and eggs.

C - The number of possible combinations formed in the zygote as a result of fertilization of the egg by the sperm, = B ² . This represents the possible number of kinds of adult plants.