It sometimes annoys me that people will defend or condemn a theory they have not bothered to read, let alone understand. For example:
New Phytol. 37: 72-81. 1938
PHYLOGENY AND POLYPLOIDY IN ROSA
By EILEEN W. ERLANSON, D.Sc.
“In 1925 Hurst put forward what Boulenger designates as ‘sa bizarre classification’ based on the hypothesis of five differential septets of chromosomes in Rosa which were represented in nature by ‘five fundamental diploid species’. It is evident that the number five was adopted because the author already accepted the hypothesis of an extinct ancestral decaploid. The geographic distribution of polyploid and diploid species led Hurst to conclude that modern rose species are all descended from this arctic decaploid.”
http://bulbnrose.x10.mx/Roses/breeding/Erlanson/ErlansonPhylogeny1938.html
She was quite wrong.
Experiments in Genetics (1925)
Charles Chamberlain Hurst
“One looked upon the septets of chromosomes in the spontaneous species of Rosa as sets of seven chromosomes which had in some way been duplicated and reduplicated. But one day when comparing the taxonomic characters of the species in the living collection at Kew, one was struck by the fact that the tetraploid species showed the combined characters of two distinct diploid species, while the hexaploid species showed the combined characters of three distinct diploid species and the octoploid species showed the combined characters of four distinct diploid species.”
“Taxonomic analyses of the 400 forms of Rosa examined cytologically by Tackholm (1922) and myself, from material collected at Kew, Paris, Cambridge and Burbage, fully confirmed the previous observations, except that the characters in the polyploid species were found to be represented in five differential diploid species instead of four, which number (five) corresponds with the five septets of chromosomes found in Rosa.”
http://bulbnrose.x10.mx/Roses/Hurst/HURST2.HTM
In other words, Hurst observed five large and distinct sets of characters in Rosa, which (not coincidentally) correspond with the five sets of chromosomes found in some irregular hexaploids.
I admire what little I have read of Boulenger’s work (mostly written in French, which I can’t read), in large part because he was a zoologist who brought “fresh eyes” to the study of Rosa after his retirement. But as a zoologist studying mostly fish, reptiles, and amphibians, he was perhaps somewhat unprepared to consider the implications of polyploidy.
Hurst also brought “fresh eyes”, considering that some of his earlier work in genetics involved horses, rabbits and Paphiopedilum hybrids. Unlike all the earlier botanists I have read, Hurst took an objective approach by looking at a large collection of species and varieties, examining each for a list of “about one hundred” traits, tabulating the results and then analyzing the data for patterns. He did not assume (like John Lindley) that “shining leaves” vs. “opaque leaves” was a clear distinction between species, whereas the character, number and position of prickles was not. Rather, Hurst saw groups of about 50 traits (half of his original list) that generally hung together in species distributed around the world.
Rosa woodsii, R. foliolosa and R. cinnamomea are not the same, of course, but they are all very much more similar to each other than they are to R. moschata, R. xanthina, R. rugosa, or R. macrophylla.
Hurst’s theory is no longer quite acceptable for Rosa, though it works out very nicely for wheat and its relatives. Hexaploid bread wheat, for example, was built up from three distinct species: one Triticum and two Aegilops. It has also been demonstrated that a form of Prunus domestica (hexaploid) can be synthesized by crossing the Sloe (P. spinosa, tetraploid) with the diploid Cherry-plum (P. cerasifera).
There is some differential pairing of chromosomes in hybrids of Rosa species, but this usually does not involve complete sets of seven, which seemed necessary to Hurst back in the 1920s. He (and everyone else at the time) assumed that the traits that distinguished species must be distributed among all the chromosomes. There was already some evidence of gene linkage, but no one yet imagined that large sets of traits could be tied to the genes of one chromosome, and even to a segment of that chromosome which could behave as a “unit” of heredity (supergene).
In discussing Hurst’s “Septet Scheme”, it is important to distinguish between his theory of differential pairing, and his observation of large sets of traits that remain together in species of Rosa distributed around the world.
