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Vol. 9(10), pp. 2-5The McAllen International Orchid Society JournalOctober 2008

Dendrobium alexandrae--A Rare Orchid (and possibly a problem child!)

R J Ferry

Fig. 1. Dendrobium alexandrae. Scan 02 Oct 2008; Isobyl La Croix, 2008, The New Encyclopedia of Orchids.

Fig. 2. Dendrobium spectabile. Scan 02 Oct 2008; Isobyl La Croix, 2008, The New Encyclopedia of Orchids.

In their monumental work on dendrobiums, Margaret and Charles Baker (1996) begin by referring to this species as: "a very rare plant that Cribb suspects may be a hybrid, with D. spectabile (Blume) Miquel as one of its parents." OK, so right off the starting line, we're confronted with a problem that the one we have in the greenhouse (Dendrobium alexandrae) (Fig. 1) may be a natural hybrid or it may be a species. Cribb "suspects" it may be a hybrid, but he doesn't elaborate on what the other parent of D. alexandrae might be if, indeed, it is as he suspects. For the present, a search of the Kew database shows both as accepted species.(Fig. 2) However, supporting the natural hybrid hypothesis, both species are known from Papua, New Guinea, and both flower (in nature) in November-December, although D. spectabile also flowers in January, February, and March. They're also about the same size. Is there some grassroots method by which, as serious orchid grower-hobbyists, we might determine whether D. alexandrae is a "good" species or a natural hybrid? Is there any way we might find out which of the two it is? Yes, there is, but it's going to take some resources, money, and time! In this article we'll explore a few of the avenues and some of the problems to doing so. As we do, remember that this problem isn't unique to just this orchid species; some of the avenues are useful and even necessary for many other species and hybrids.

To begin, we may obtain a clue or two if we utilize an academic tool known as the punnett square (something we had to work with in high school biology eons ago when we studied Mendelian genetics). The following explanation is a bit simplistic, so all of you graduate level genetics majors kindly grant some leeway on this one! We know it's not all this simple, but consider this is a starting point!

Let's assume we have two bona fide species, and we cross them. In our first punnett square, we see that each side of each species' DNA passes the same characteristic: A or B (Tables 1 & 1a). In short, whenever a species breeds with itself or another member of the same species, we can normally expect the offspring to be the same species.

Table 1. Punnett square of Species A.

Species ASpecies A
Species AAAAA
Species AAAAA

Table 1a. Punnett square of Species B.

Species BSpecies B
Species BBBBB
Species BBBBB

Assuming all the above is valid, what happens when we hybridize one species with another? In table 2, we see that all offspring are part A and part B:

Table 2. Hybridization of species A and B (producing the hybrid AB).

Species BSpecies B
Species AABAB
Species AABAB

Still not lost? OK, now let's get back to the problem we may have started with. If Dendrobium alexandrae is, in fact, a "good species," then it ought to breed true (i.e. make plantlets that all flower pretty much identically). If, however, it's a man-made hybrid, or--in this case--a natural hybrid (one that "just happened" in nature, possibly by a confused insect or one that wasn't choosey whether it looked for a reward in this or that species), then we have a new ballgame to consider! What happens if we cross the natural hybrid with itself?

We can't do this with animals because we can't really mate an animal with itself, but with plants we can do this two ways. Either we take pollen from a flower and put it on the stigmatic surface of the same flower (called "selfing"), or we take pollen from one flower and put it on the stigmatic surface of another flower on the same plant. This is called a sibling-crossing, usually referred to simply as a "sib-crossing." As a matter of fact, we can take two flowers on the same inflorescence and use the pollen of flower A on flower B, and then use the pollen from flower B on flower A. This sib-crossing is termed a "cross" and a "back-cross." Doing this ought to yield the same thing, but DNA is a funny chromosomal strand with all sorts of hang-ups, and sometimes the results will work one way but not the other. We'll do both, but theoretically, either way, the result ought to look like what we see in Table 3.

Let's assume we actually do the hybridization and raise--say--a thousand seedlings. The kicker here is that we have to raise them until they're big enough for us to see what flowers they actually produce, and that can take a while! Look closely, again, at the punnett square in table 3. Assuming, however, we've done that, what we ought to see in our thousand flowering plants is that about 25% will look like (actually, not just "look like," but will be!) one parent; 25% will be the other parent; and 50% will look like (and be!) our original hybrid!

Table 3. Hybridization of Hybrid AB with itself.

Hybrid AB, crossed with itselfDNA ADNA B
DNA AAA (species A!)AB (hybrid AB)
DNA BAB (hybrid AB)BB (species B!)

Notice: we gave the results in percentages, not concrete "set-in-stone" numbers. Suppose we raised 3,278 seedlings to flowering. About half of them ought to be like our original hybrid parent, and the other half would be split between the two species-parents. Remember, these numbers are all percentage approximations, and the actual results might vary by a few dozen or even a hundred in any direction.

Now we are faced with the problem of whether Dendrobium alexandrae is a species or a natural hybrid. So we have three choices:

  1. Do nothing at all other than culture the plant and enjoy its flowers;
  2. Do nothing, and hope somebody else will solve the problem for us.; or
  3. We can cross and back-cross flowers, harvest the seed in due time, flask it, and raise however many plantlets for which we can afford time and space.

The third choice obviously involves attention, effort, resources, and a lot of time! Obviously to the die-hard orchiologist, it has to be a worthwhile project! On one hand, one may be proliferating a very rare orchid species to share with other individuals. Given enough plants, one may even be able to conquer the governmental red tape and convey some back to Papua, New Guinea to be put back into the natural habitat of the species. If it turns out to be a hybrid, the persistent orchidologist may discover what the unknown parent actually is (remember: Cribb hypothesizes one parent might be Dendrobium spectabile).

However, the decision to propagate this (or any) orchid plant needs to consider several factors. Does one know how to make the pollination, and--if so--does one have the facilities to harvest and successfully flask the seed? Will there be sufficient space to house the flask(s) properly, and--in due time--replate plantlets into flasks where they'll have more growing space? Assuming all these conditions are met, will the orchidist-hybridizer be able to supply the proper environmental conditions to "grow on" dozens, even hundreds, of plantlets to flowering sizes? Finally, one must realize that this probably will take two to five years from the date of placing the pollen to that day one actually begins to see flowers! In short, when one asks a simple question about orchids, one might expect a long-term project before getting answers!

Another question needs to be asked. What will one do with--say--994 plants of Dendrobium alexandrae if, in fact, after all the tme, effort, and expense, it does turn out to breed true as a "good species?" If they're offered for sale (to help defray some of the long-term expenses of raising them) will there be much of a market demand for the plants? This, and other questions, need to come to mind as one looks to embark on the project, not after one is well into it! Of course if one's doing all this as a "labor of love," then that can sweep a lot of questions under the proverbial rug!

Fig. 3. Pl#130808-23. Dendrobium alexandrae. Digital photo DSC_2559, 02 October 2008.

As for the particular plant, Pl#130808-23 (Fig. 3), that now sits in your editor's greenhouse, when the plant flowers, it'll be sib-crossed, and that will be the beginning of a long and very involved process. We'll keep you posted.

Copyright © 2008 R J Ferry