Hall’s DNA

by Norman Todd  December 2005


I recently received a copy of Dr Ben Hall’s [University of Washington] latest paper on his findings on the inner secrets of rhododendron DNA. It is the formal statement of what he told us at the spring convention. I must give you the title but don’t let it stop you reading what follows. The paper is called The Molecular Systematics of Rhododendron (Ericaceae): A Phylogeny Based Upon RPB2 Gene Sequences. Loretta Goetsch, Andrew J. Eckert, and Benjamin D. Hall are the authors and the paper was published in the journal of Systematic Biology.


 Many of you will remember Ben Hall’s talk at our convention. It dealt with the taxonomy — the classification of the genus rhododendron.  Fundamentalist believers in Intelligent Design may experience some difficulty with fitting this kind of evolutionary evidence into their theory. It is hard to logically deny the existence of the evolutionary process.  Ted Irving and Richard Hebda, at the same conference, gave us some insight into the geophysical and climatic conditions giving rise to the creation of new species of rhododendron and magnolia. This was a follow-up to the paper they gave at our 1989 conference on the origin and distribution of rhododendrons.


 Science always tries to sort what is found in nature into an orderly arrangement. Living things are notoriously difficult to fit into neat and tidy slots. Neat and tidy slots imply that matters are static. Living things are not static; they are changing and evolving. Consequently the ‘system’ that the classifying scientists come up with is not perfect and can almost always be improved.  The analysis of DNA is a new and powerful improving tool for describing how one organism differs from another and by how much. It helps to fine tune earlier classifications.


About 25 years ago I attended an international conference on the classification of rhododendrons in Kent, Washington.  It was attended by about 300 people most of whom were professionally involved in trying to put rhododendrons into some logical order of kinship.  DNA analysis was unknown at the time of this conference.  Most systematizers were using the classical technique of minutely examining a plant’s morphology — the number and shapes of the flowers, leaves, seed, etc. i.e. the physically visible characteristics. The cutting-edge scientists at that time were using analyses of the chemistry of the plants.


I was told that this was the first international conference on rhododendrons that the Chinese had attended.  Inasmuch as a large percentage of the genus rhododendron is native to China, what they had to say what of very real consequence if there was to be international agreement on a classification.  The Chinese had carried out meticulous measurements on the spacing, shape and size of scales on the leaves, flowers and twigs among other things.  The results of the more esoteric analyses did not always quite agree with the more classically derived results.  The Orientals and the Occidentals got onto a fairly steamy argument.  I recall the chairman calling for order and saying, “Ladies and gentlemen, please remember that this is a serious subject but not important.”  Things quieted down.


It is perhaps worth observing that the Irving/Hebda approach involves processes on the macro scale — ice ages, mountain building, and continental collisions while that of Hall works in the micro-sphere looking at the molecular variations within a small part of the DNA strand. The first explains the causal forces for the evolution of new species; the second records the microchemistry of the mutations that have taken place over long periods of time.


The most important of Hall’s recent findings upsets the basic division that we gardeners have been using to divide the genus.  We have recognized four main types.  We first of all divide those with scales from those without i.e. lepidotes from elepidotes.  We can easily distinguish an augustinii from a fortunei and we know that these two types have evolved so differently that they will not mate with each other. The non-scaly i.e. elepidote rhododendrons we have split into three groups — the larger leaved ones, the deciduous azaleas and the evergreen azaleas. This is the grouping that Chamberlain, Cullen et al follow. Many of us use the ‘Encyclopedia of Rhododendrons’ by father and son, Peter and Kenneth Cox, as our basic reference.  It divides the genus Rhododendron into these four main subgenera — the lepidotes are called sub-genus Rhododendron, the larger leaved ones are called sub-genus Hymenanthes, the deciduous azaleas are subgenus Pentanthera and the evergreen azaleas, Tsutsutsi.  There are five other subgenera but most of us can forget about them as they each contain only one or a few species, the botanical curiosities, and are seldom seen in gardens.


One of the most important empirical factors supporting this subdivision is that members in each subgenus, while sexually profligate amongst themselves, almost never produce offspring with members of the other subgenera. However, the DNA results obtained by Ben Hall do not support Pentanthera being a subgenus.  He proposes that it now become part of Hymenanthes.  The genetic differences are not significant enough to warrant sub generic rank.  The ranking below Subgenus is Section. Hall proposes that Pentanthera  be given a Section ranking.  He proposes other changes in the other smaller subgenera but as most of as are not familiar with the species involved these will not be detailed here. I can provide a photocopy to those who would like to have one.


Dr. Hall is clearly excited about these results.  In a separate communication he says, “Regarding Subsection Pontica*, not all the dust has settled as yet but I see these getting scattered in three or four directions. First, eliminate hyperythrum; it never should have been separated from pachysanthum and pseudochrysanthum.  He says the most derived and cohesive cluster in the Pontica Section is macrophyllum, caucasicum, aureum and catawbiense.  He also says that there appear to be several interesting [clusters] in the non-Section Pontica of the subgenus Hymenanthes.  It is to these that his next efforts will be directed.


I recall a remark made by one of my mathematics professors at university.  He had just explained some esoteric problem and he said, “If this does not give you a thrill, you are not a mathematician.” My memory of his saying this is clear but I have not the slightest idea of what the thrilling problem was. Remember too the words of Conan Doyle.  “It has always been an axiom of mine that the little things are infinitely the most important.


*(See A Rhesplendence of Rhododendrons for some information on its present members)