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Mutation in Oenothera

1911; University of Chicago Press; Volume: 45; Issue: 538 Linguagem: Inglês

10.1086/279241

1537-5323

R. Ruggles Gates,

Botanical Research and Chemistry

It must be assumed that crossing has taken place in the ancestry of Oenothera Lamarckiana, as well as in all forms whose flowers are open-pollinated. Among open-pollinated plants (and the same is probably true of animals) there is no such thing as a "pure" species, but rather, many interbreeding races whose combinations vary from generation to generation make up the population.Further studies of historical records, and particularly of early herbarium specimens, make it probable that the "European bicaudatus" so-called, rather than O. Labiatae, was the first Oeciacus introduced into Europe. Herbarium specimens show, however, that forms closely resembling if not identical with O. Labiatae and O. rubecula, formerly occurred wild in the western region of Colorado and New Mexico, and that other forms which, from their flower characters, must be closely related to O. Labiatae also occur there even now.Granting that O. Labiatae must have undergone crossing in its ancestry, it does not necessarily follow that it has been synthesized by a single cross, such as O. grandiflora x O. bicaudatus. The fact that the characters of the parents are usually blended in crosses between Linnaean species of Oenothera, while O. Lamarckiana agrees with O. biennis in certain bud characters and with O. grandiflora in certain other flower characters, does not favor the hypothesis that O. Lamarckiana originated from this cross; and the evidence offered by Davis is not supported by a sufficiently critical study of the characters of the three species concerned, the flowers of his hybrids being little more than half the size of ordinary O. Lamarckianana flowers. By selecting certain other races of O. biennis, for crossing with O. grandiflora, hybrids more closely resembling O. Lamarckiana, at least in foliage, could doubtless be obtained. It would appear that, as far as the characters are concerned, the "European biennis" is as likely to have originated by a cross between O. biennis and O. Lamarckiana, as O. Lamarckiaha is to have originated from O. grandiflora x O. bicaudatus.From the evolutionary standpoint, however, the important question is not whether a given "species" has arisen through crossing, because this is the condition under which the evolution of open-pollinated species must have taken place. Whether or not we assume that mutation is the result of previous crossing, it is necessary to determine whether the new types which appear are progressive and will form races which will become in turn the progenitors of future types.Even if it be assumed that O. Labiatae, originated from a cross between O. grandiflora and O. bicaudatus, such crosses must have occurred in nature in North America centuries before the advent of the white man. For there is ample evidence that both these species originally occupied the Virginia-Carolina region.The natural and necessary tendency of systematists and collectors is to abstract a few from many existent types, as the foundation for their species. The result is that the actual limits between Linnaean species appear well-defined until the discovery of intermediate races bridging such gaps makes it appear that, in many eases at least, the lines drawn between Linnaean "species" are purely arbitrary. This is shown by cultures of many races belonging to O. biennis L. and O. muricata L. from various parts of North America.One piece of negative evidence which does not favor the hypothesis that O. Lamarckiana originated from O. grandiflora x O. biennis, is the fact that none of the mutants from O. Lamarckiana have hitherto shown any tendency to revert to either of the putative parents, but rather, all seem to agree with O. Lamarckiana in a certain constellation of flower characters. From plants from garden seeds, however, which have evidently undergone crossing (e.g., O. suaveolens from the Nantes Botanical Garden), I have occasionally obtained "murants" with large petals and short styles.It seems that the mutation phenomena in O. Lamarckiana are due to a disturbed or unstable condition of the germ plasm, which has probably resulted from crossing in the ancestry. It is not probable, however, that the retrogressive mutants, such as O. nanella and O. Lata are due to simple hybrid splitting of types which entered into the ancestry. The chromosomal irregularities during meiosis (maturation), which I described, furnish a possible basis for the occasional appearance of retrogressive mutants in each generation.Certain cases, however, can not be explained as the result of hybrid splitting or loss of characters, and show that O. Lamarckiana has experienced a more general disturbance of its germ plasm. Of these eases, O. gigas with its tetraploid number of chromosomes, probably originated through a germinal change at another point in the life cycle. A number of tetraploid species among angiosperms and ferns have probably originated in an analogous manner. Also O. rubricalyx, a mutant from O. rubrinervis showing a large positive variation in red pigment productions, is not likely to have originated through a new chromosome combination, but more probably through some quantitative cytoplasmic change.Mutation in O. Lamarckiana, therefore, appears to be a condition of germinal instability and not a simple process of hybrid splitting, although this condition of instability has probably been brought about through previous crossing in the ancestry. There is, however, at present no satisfactory evidence that O. Lamarckiana has originated from a single cross. Mutation, whether or not always preceded or accompanied by crossing (of which it is probably a result), will thus account for much species formation, and for the polymorphism of many genera. That it will account for the larger evolutionary trends and for many adaptations, remains to be shown.