In the Crevice Gardening thread, John Husbands mentioned older cultivars not shpowing their original vigour and asked why. As this is sort of my area of expertise, as I have been hybridizing for some 20 years in Europe, I thought we may well discuss it. There are lots of myths and misunderstandings out there.
When we are talking about hybrid vigour, we are essentially refering to the results of a particular hybrid that either 1) shows increased vigour as a strain or 2) a particular selection of a cross that is particularly vigourous. On a genetic level we are differentiating between a cross between X and Y which either gives a very large percentage of superior off-spring (F1 hybrid strains) or simply that particular off-spring that showed excellence and was thus selected for further clonal propogation. The difference is important to understand, as a selection is clonal, while an F1 strain is not!
There are plants on the market that we purchase as a strain. This included many marigolds, zinnias, ext., which are traditionally propagated per seed. We expect some variation. Clonal plants are those reproduced by cuttings or, often, clonal cellular propgation (often refered to as meristem culture). Penstemons and Pelargonium are good examples of root cutting propogation. All cultivars of japanese maples fall into this category, as do may foliage plants and roses (they are grafted or, less commonly, rooted).
To go back to hybrid vigour, this is a somewhat antiquated term, as it comes from a time when genetics were poorly understood. What we are seeing is a particular genetic harmony that we perceive as positive. Bigger flowers, better branching, more growth, less disease, etc. It is simply a crap shoot and we have won! It is the same with animals, such as cows, sheep, canaries, etc. We have learned that X mated to Y gives desirable off-spring in most cases. Very often, this vigour is lost over generations due to genetic drift, an accumulation of poor genetics over the generations. The root may be as simple as one of the mates for the strain becoming extinct or infertile. Thus we loose the original strain. When we attempt to line-breed (which is mating closely related genomes), we may save the strain for a while, but, ultimately, to maintain genetic vigour, we must out cross to non-related genomes in an attempt to recover what has been lost.
When we purchase a particular plant cultivar we may not know, if it is actually clonal or a seed strain. What was once always clonal may have been substituted over the years by a similar plant or even a seed strain. One of the reasons we see a 'loss of vigour'. It is simply not 100% the original. Add to this that mutation takes place in literally every cell division, a 50 year old cultivar, propagated only per cuttings/division, will have had some genetic drift. Again, we may see loss of vigour. Also, the more poular a cultivar is, the more chances of seeing mutation/change.
Cell culture (meristem) does not produce absolutely identical off-spring, but they are pretty close. The reason is, again, mutation, which, in this case, where we are promoting massive simple cell division to create the clones, we will see much more mutation in the finished product. As the young plants are culled for vigour, what typically reaches the market is changes in the visual (flowers and leaves). This can be a real plus. Just look at orchids and Hosta. Many of the current favs are the result of cell propagation mutations.
Nothing is forever and, although commercial interests often damage or destroy any given product, there is always something new on the horizon. Why we still hybridize and one of the very good reason for the private gardener to hybridize, select and pass-on worthy creations. As gardeners, we tend not to be swayed by commercial fashion, rather by that which brings joy to ourselves...and gardeners love to share.
