サシキ品種の成立についての考察

抄録

In Japan, there are many native races of forest trees, which are customarily propagated by cuttings. They are acting the same rôle as these of the agricultural crops, but are not considered as clones. They are clone complexes, i. e. the populations consist of resembling clones.
Some one says that these races formed themselves at first as clones. Then, thoughtless treatments of most foresters spoilt them and brought them into much complicated constitution as they are showing at present.
This opinion, however, is very hardly acceptable, because cuttings are customarily collected in a very few number from a single tree, and forest plantation, on the other hand, requires usually large amount of planting materials. It seems to be necessary for growing up a clone, overcoming the above mentioned difficulties, that the genetic conception of clones has already been firmly established.
The following explanation seems, therefore, to be more reasonable that these races have naturally become evident in the course of successive vegetative propagation, and they substantially include many resembling clones.
In a population originated from seeds, each tree has a genotype different from any other tree, and genetic character can take any possible combination each other. When people start to propagate the trees by cuttings refusing the sexual means, they cannot collect cuttings from all the trees included in the population. The number of genotypes becomes, therefore, much decreased, and no more genotypes can be introduced into the population. As the vegetative generation progresses, the included genotypes become smaller and smaller in their number, number of individuals of the same genotype becomes larger and larger, till at last, foresters can easily recognize that some morphological characters are closely relating to some sylvicultural characteristics. Now it is possible to select desirable trees by means of their morphological characters. After several repeated selection, the races are established as those provided with the desirable properties and certain morphological characters.
Two or more clones very closely resembling can not easily be separated in the course of the race formation. It is evident that in the sexual population there are many closely resembling genotypes. It is reasonable to suppose that some of these have been brought into the vegetative population together, when the correlation has become evident between certain morphological and sylvicultural characters. Then, these resembling clones will be all equally selected and included in the same race.
These two phenomena can be clearly demonstrated on the paper using the model population (Figs. 1 & 2).
It is possible, among our native races of Cryptomeria and Thujopsis, there may be some real clones. In some districts, the races have originated from the small number of seedlings or cuttings, which was introduced from the other districts. In these cases, it is highly possible that the races are real clones. We have, however, no means to ascertain it, so we must treat them also as possible clone complexes.
The circumstances are quite different from the above in the case of horticultural trees. In these, it is much easier that the races have been established as the clones from the beginning.
From the stand point of the racial improvement, we must immediately start to separate the most desirable genotypes out of these native races to establish real clones.