Hurst's decaploid theory for the origin of rose species: wrong but not forgotten

C.C. Hurst (1870-1947) was the brilliant amateur geneticist who did more that anyone to elucidate the tangled history of the rose. He confirmed that the basic chromosome number of roses was 7 and he distinguished 5 rose genomes, A, B, C, D and E, each associated with a set of seven rose chromosomes. Diploid roses have chromosomes in pairs so they may be represented as AA or BB etc.

The basic building blocks of rose evolution are these diploids:

• the AA diploids: the odorata group (R. odorata, R. brunonii, R. moschata, R. multiflora etc)
• the BB diploids: the hugonis group (R. hugonis, R. wilmottiae, R. sericea)
• the CC diploids: the rugosa group (R. rugosa)
• the DD diploids: the woodsii group (R. woodsii, R. nitida, R. blanda etc)
• the EE diploids: the macrophylla group (R. macrophylla)

Roses exist not only as diploids but as tetraploids, pentaploids, hexaploids and octoploids. These can all be seen to be derived from the diploids by appropriate duplication and hybridisation. Thus Rosa nutkana and Rosa moyesii are both hexaploids but Rosa moyesii is (according to Hurst) AABBEE and Rosa nutkana is AADDEE. This is the accepted "upwards" theory of rose origins in which the polyploids derive from the diploids by chromosome doubling and hybridisation.

However, somewhat incredibly Hurst did not believe this. He put forward another theory: the decaploid theory. In this "downwards" theory roses derive from a hypothetical arctic decaploid AABBCCDDEE by progressive loss of genomes, ending up with diploids that have lost all their genomes except one!

No-one believed Hurst then and no-one believes him now. The upwards theory is based on mechanisms that are well attested in hundreds of groups of organisms, and is based on the diploids, all of which still exist. The downwards theory is based on a hypothetical decaploid that no-one has ever found and requires a mechanism that has never been demonstrated in any group.

So why did Hurst put forward his bizarre theory? He gives a variety of reasons:

1. the doubling of a sterile AB hybrid to give a AABB tetraploid would result in a homozygous plant whereas the roses are heterozygous.
2. the diploids occur in different habitats and regions and are not geographically available in the precise pairs necessary for hybridisation.
3. Hurst worked on the idea that polyploids were adapted to higher altitudes or higher latitudes than their diploid progenitors, therefore the polyploids would have to migrate northwards or up mountains very quickly in order to find habitat to which they were fitted.
4. Hurst also worked under the now defunct theory that the flora of the northern hemisphere had an arctic origin. Therefore it made more sense for arctic polyploids to become tropical diploids rather than vice versa.
5. Hurst thought that a single origin from one decaploid progenitor species was more parsimonious that multiple origins from multiple diploid species.

Today we can see that all these arguments are based on false assumptions. However, I don't mean to belittle Hurst because his theory proved to be false: he was a giant of genetics, who came up with a brilliant if erroneous theory: wrong but not forgotten.

[For a full list of Hurst's rose genomes see this link]