Not To Be Mythed
David G. Leach, Brookville, Pa.
When many of us first became interested in rhododendrons nearly twenty years ago every novice breeder was solemnly assured both verbally and in print (1) that the species do not cross in nature to produce hybrid populations in the wild; (2) that primary crosses only, between species, produce hybrids of value for the breeder and that all of the promising combinations had already been explored; and (3) that it makes no difference whatever which of the two parents in a cross bears the seeds.
No evidence to support these assertions was ever offered. This was the gospel, the truth through the repetition, and almost everyone believed it.
The legend concerning the rectitude of the un-chaperoned species in nature apparently arose at the time some of the plant hunters in Asia received cash bonuses for each species newly discovered and their reputations were earned to a considerable extent on the number of unknown species introduced to cultivation through their efforts. Naturally, they were filled with under enthusiasm for the proposition that there are in the mountain wildernesses of Asia, as elsewhere, hybrid swarms of Rhododendrons resulting from the introgressive hybridization which takes place in nature with almost all plants. Without in the least doubting their sincerity or deprecating the contributions made by these resolute plant hunters, their views seem incomprehensible in the face of the massive scientific studies of other wild plant populations.
It would be extraordinary indeed if Rhododendrons did not cross in the wilderness, as do other plants, and ample evidence now exists that they do.
The genetic proof for hybridity on a large scale in nature is, in fact, overwhelming. The proof includes an account by this author, published in the "Bulletin" several years ago, of the recreation in cultivation of " R. furbishii " from a controlled cross of arborescens with bakeri .
The assertion that only the primary crosses, between species, produce hybrids of value for the breeder appears to have arisen early in the twentieth century with the British amateur hybridists who bad an astounding influx of newly introduced species at their disposal and not a very good idea of the operation of segregating factors in the second and subsequent generations in plant breeding. Again, without in the least minimizing their enormous contributions, the evidence is fatally against this view. But legends die hard, and slowly.
Even as late as 1949 the talented breeder, F. J. Rose, who produced so many notable hybrids, wrote "The theory is sound that for the best results one species should be used as a parent." This one-species theory was a natural rear guard action in retreat from the two-species law. But it was a skirmish in a lost cause. F. C. Puddle at Bodnant in Wales was among the most systematic of the pioneer breeders, the most scientific in approach and exceptionally keen in making useful deductions from his results. Writing in the R. H. S. "Rhododendron yearbook" for 1948 he bucked the universal view and continued to maintain his previously expressed opinion that the "primary hybrids...merely represent...a foretaste of what may be achieved in successive generations."
Another thirteen years have fully borne out Mr. Puddle's view and indeed it was reasonable to suppose that the passage of time would do exactly that, since both in fact with other cross-breeding plants, and in scientific theory, the segregating generations which follow the first mating often produce the desired results if sufficient progeny are grown from such crosses. The final proof is, further, that the vast majority of the newest introductions and those which the nurserymen are now industriously propagating for release are the result of crossing hybrid with hybrid.
Coming now to the theory that it makes no difference which of the two parents in a cross bears the seed, the literature is filled with such witching assertions. Only the Dutch have dissented, and they only to the extent of saying that the hardier of the parents should bear the seed.
There is no evidence offered to support the popular view which would mean, as in the foregoing, that Rhododendrons are unique in the plant world in yet another respect. The fact is, however, that maternal inheritance has been proved to operate in the twenty different plant genera which have been carefully investigated for it.
The ovule produced in female generation is large and contains much more cytoplasm than the sperm, which is almost devoid of cytoplasm, the living substance outside of the nucleus of the cell. In Mirabilis jalapa , the familiar Four O' Clock, the cytoplasm of the ovule carries primordia which determine what type of plastid will develop in the plant grown from the seed.
Some Four O'Clocks have normal green branches whereas others are white in whole or in part. Seeds taken from branches which are entirely green and seeds taken from a branch which is wholly white produce plants without chlorophyll, which of course die. It makes no difference whether the pollen comes from a branch that is wholly green or wholly white. The leaf color of the branch that bears the seeds solely indicates the leaf pigment of the offspring. Thus we have a genuine transmission of a characteristic through self-perpetuating bodies in the cytoplasm, unlike as it may be the inheritance which occurs through genes in the nucleus. The cytoplasmic bodies are often called plasma-genes and are usually thought to work in conjunction with the genes in the nucleus to produce their effects.
Almost every Rhododendron breeder has noticed that a cross which failed may be successfully achieved by reversing the parents in some cases. This is vaguely explained as cytoplasmic incompatibility to the breeder who looks for an answer, but it may well be in fact an example of maternal inheritance which produces male sterility. The phenomenon has been thoroughly explored in corn at the University of Illinois in a classic case where each chromosome in a male--sterile strain was painstakingly replaced in a series of crosses, one by one, with a chromosome from a male fertile strain. In spite of the complete replacement of the chromosomes in the course of the investigation, the males remained sterile and the trait was transmitted only through the female. Finally, a few fertile grains of pollen were found in the male-sterile strain and the reciprocal cross was made but the male-sterility was not transmitted at all through the male parent.
I report now an example of maternal inheritance in rhododendrons of an extensive character and offer evidence that it does indeed make a difference which of two parents serves as the seed parent.
I have at my trial grounds a few plants from the cross catawbiense var. album Glass x discolor, and also a few plants grown from the reciprocal cross, discolor x catawbiense var. album Glass. The two lots of progeny are conspicuously different, not in one respect but in a variety of ways.
The flowers of the hybrids resulting from the cross, catawbiense var. album Glass x discolor are approximately one-third larger than those on the hybrids which came from the reciprocal cross. The lobes are broader and the blossoms are almost white, with only a faint pink flush. There is a pronounced red blotch deep in the throat and the stigma is green. The much smaller flowers on the hybrids which resulted from the reciprocal cross, discolor x catawbiense var. album Glass, are pale lavender with a greenish yellow dorsal blotch and a red stigma.
Other differences between the hybrids from the two crosses are as follows:
|Catawbiense var. album x discolor||Discolor x catawbiense var. album|
|Leaf size||1¾" x 5"||2⅛" x 6"|
|Pedicel||1⅝" long, dull red dorsally, ventrally green||1¼" long, bright red dorsally; ventrally light green|
|Number of flowers to truss||14||12|
In every respect the hybrids which came from
are superior ornamentally to those that resulted from the reciprocal mating. The difference is that of a modestly successful cross compared with one which produced seedlings not worth keeping.
This is, of course, an extreme example but it serves to illustrate that maternal inheritance does exist in Rhododendrons. The leaf size, branching habit, foliage quantity, hardiness, number of flowers to the truss and the truss formation in each case favor the parent which bore the seeds. It is reasonable to assume that the plasmagenes in the larger amount of cytoplasm in the ovules, interacting with genes in the nucleus, affected the characteristics which favored the female parent in each of the two crosses.
So we are ready now to discard the last of the three misconceptions which have existed widely for more than forty years. It is not a matter of indifference which parent in a cross bears the seeds.
Since all scientific breeding is guided by the laws of probability, the prudent hybridist will be quick to note now that the mathematical odds favor a predominant influence of the female parent in his crosses. If a choice can be made between the two parents, the plant with the majority of good characteristics might well be chosen to bear the seeds; and any plant with a particularly undesirable trait which is to be eliminated in the progeny from the cross might wisely be used as the pollen parent. Over a period of time and a number of crosses, the chances are that such care in selecting the seed parents will give superior results in the breeding program.