DIGITALNA ARHIVA ŠUMARSKOG LISTA
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| ŠUMARSKI LIST 1-2/1966 str. 108 <-- 108 --> PDF |
HYBRIDS BETWEEN SPECIES PRODUCED SPONTANEOUSLY IN NATURE A fairly large number of interspecific hybrids have been described wich belong to a common genus, and which originated spontaneously. Of these hybrids the greatest number belong to the genera Picea, Pinus, Larix, Abies, Populus, Quercus, Aesculus and Acer. Many spontaneous hybrids came into being in arboreta, parks and tree alleys. If in these places there is only one tree of a species, it crosses with the other species. Spontaneous hybrids can also originate in the natural range of two species, in places where these two species are in contact. In this country Fukare k and his co-workers (17, 18) found spontaneous hybrids between Acer obtusatum and A. pseudoplatanus and between A. Heldreichii and A. pseudoplatanus. In hybridization between species taking place in nature there can occur either introgression or hybrid swarms. Introgression is found in a number of interspecific hybrids of the genus Picea, or in Populus nigra in cases when P. deltoides was introduced. Hybrid swarms, as stated by Wright , are an evident example in the American species of the genus Aesculus. Hybridization between species sometimes leads to only a small number of isolated intermediate types or to introgression or hybrid swarms. This may be the result of limited crossability, differences in flowering time, or nature and amount of selection pressure. Hybrids originating spontaneously have promted tree breeders to attempt to produce them artificially. Observations of growth rate and habit of such spontaneous hybrids can greatly help the tree breeder in planning and carrying out hybridization. An example is Populus nigra in Europe. It appears that Salix alba presents a similar case. Spontaneous hybrids between this species and Salix fragilis appear to have a better growth than the pure parental species. Thus improvement of willows in this sense is presently in progress. PRODUCTION OF HYBRIDS DISPLAYING HETEROSIS One of the most important tasks in interspecific hybridization is obtaining the Fi generation displaying heterosis. Therefore no financial means should be spared if one is to succeed in this. When planning such a project it is necessary to bear in mind all the previous knowledge about the species crossability, so that we may not experience total failure and disappointment. Therefore the work program should be developed gradually according to plan. According to Wrigh t (61) there are 4 stages of work leading to the final goal. First comes the determination of the species which are mutually crossable, then what relationship exists between the species to be crossed. It is fairly well established that success in crossing related species is usually comparatively good, while in distant ones it is usually negative. In the crossing of related species whose geographical distributions are separated, heterosis can be expected in the progeny. Valuable contributions concerning problems of relationship and interspecific hybridization in the genus Pinus were given by D u f f i e 1 d (12) and C r i t c h f i e 1 d (9). Also, the work cf S a y 1 o r (50) is important because it explains the relationships of individual pine species on the basis of karyotype analysis. On the basis of an analysis of spontaneous |