Ralph E. Cleland

The Cytogenetics of Oenothera

Publisher Summary This chapter discusses Euoenothera (Onagra) as found in North America or as represented by European derivatives of North American origin. Oenothera comprises fifteen subgenera, of which only the largest has been extensively studied cytogenetically. The genus is native to the western hemisphere but has been extensively introduced into other areas of the world. The first reference to Oenothera in Europe is that of Prosperus Alpinus of Padua. It has become widely distributed throughout Europe where it constitutes an important element in the flora. To summarize this situation, most Oenotheras are “complex-heterozygotes.” Each race has two different complexes or permanent genomes. This results from the inclusion of all genes within a single linkage group for which the plant is heterozygous, the presence of one or more lethals in each genome and self pollination. Hybrids derived from the crossing of true-breeding complex-heterozygotes have two or more linkage groups. The prevalence of translocations that have brought about the widespread occurrence of structural hybridity naturally tends to focus interest upon chromosome structure in Oenothera. One of the striking features of Oenothera genetics is the relative paucity of detectable crossing-over. The genes that show crossing-over in the races lie in the transitional zone between the regularly pairing end segments and the proximal segments in which the genes that differentiate the complexes lie.

Some aspects of the cyto-genetics of Oenothera

Oenothera has long been known as one of Natures most consistent non-conformists. In many features of its genetical and cytological behavior it has shown a wide departure from the rules which prevail in other organisms. It occupies, therefore, a position of major interest in the cyto-genetic field. It was de Vries who first discovered that Oenothera is peculiar. His attention was called to the genus in 1886 when he found Oe. lamarckiana growing in great numbers in an abandoned field near Hilversum, in Holland. He found, among thousands of typical plants, occasional aberrant or exceptional individuals which seemed to have arisen as sports from the prevailing type. Collecting seed from typical plants, he grew many thousands of individuals in his experimental garden over a period of years and found the same aberrant types appearing from year to year in small percentages that had been present in the field, some of which bred true to their new characters. All of this seemed to de Vries to furnish a clue to the method by which new species are formed; and it was principally on the basis of his extensive study of Oenothera that he was led to formulate the celebrated Mutation Theory of Evolution (101) which assumed that species spring into ekistence suddenly, as a result of genetical modifications of major importance, rather than through gradual accumulation of many small variations. But de Vries soon found that Oenothera is peculiar, not only because it produces sports, but also because it shows in certain aspects of its ordinary breeding behavior an anomalous condition, setting it apart from other organisms. de Vries was already becoming familiar, through his own experiments on a wide variety of material (98, 99, 100, 101), with the basic principles of heredity which he was later to find had already been set forth in Mendels work. Thus, he was able to appreciate the uniqueness of Oenothera in certain aspects of its genetical behavior. Briefly, the anomaly which he found in Oenothera consists in the fact that a species which, when inbred, behaves like a pure species in that it breeds

New Evidence Bearing Upon the Problem of the Cytological Basis for Genetical Peculiarities in the Oenotheras

THIS is a preliminary report of a joint cytological and genetical study of a number of species and specieshybrids of Oenothera, which was made in the hope of obtaining experimental evidence upon the interesting problem of linkage presented by this genus. The material was grown during the summers of 1927 and 1928 in the botanical garden at TUbingen, Germany, with support provided by the Notgemeinschaft der deutschen Wissenschaft. The cytological study was begun in the Botanical Institute at TUbingen, continued in the Botanical Institute at Jena and completed at Goucher College, Baltimore. Our procedure was to grow during the first summer the F1 s of reciprocally made crosses involving seven species as parents, together with the parents themselves, making both cytological and genetical studies; and during the following summer to grow and analyze the F2s and backcrosses, where these were not already sufficiently known from the literature. Thus a direct comparison was made between the cytological and genetical behavior of a large number of plants belonging to a considerable range of hybrid combinations. The chief responsibility for the cytological results falls upon the first author, who is also largely responsible for this preliminary account; the genetical results have been obtained by the second author. I Read before the Joint Genetics Sections of the Botanical Society of America and the American Society of Zoologists, on December 27, 1928.

Cytological Study of Meiosis in Anthers of Oenothera muricata

1. Early stages in the heterotypic prophase in every way resemble those found in previously described species. They give evidence of the presence of telosynapsis as opposed to parasynapsis. 2. The stage known in most plants as diakinesis is absent in this species. There is no pairing of homologous chromosomes, but instead all of the fourteen univalent chromosomes are found to be united end to end to form a large closed circle, or occasionally an open chain. This configuration appears to be normally present and typical of the species. 3. The circle remains intact throughout the heterotypic metaphase, and spindle fiber attachments are made in such a way that adjacent chromosomes are pulled toward opposite poles. In general, the position of the circle in the spindle is horizontal, but since the chromosomes are alternately pulled to upper and lower poles, it takes on a regular zigzag appearance as seen from the side. 4. Irregularities occur in the zigzag arrangement in approximately 20 per cent of the cells. 5. The circle breaks up with the pulling apart of the chromosomes in anaphase. Subsequent stages in every respect are like those previously described for other species. 6. The chromosomes are without much doubt arranged within the circle according to a fixed scheme, for otherwise a certain amount of non-disjunction would occur almost every time that a reduction division took place. 7. If it be true that the chromosomes are arranged in a definite fashion within the circle in all cells, then it naturally follows that as a result of the normal separation of adjacent chromosomes in heterotypic anaphase the same chromosome complexes will in every case be formed at the poles. 8. There will then be but two kinds of complex formed, and two kinds of gamete; and the presence of balanced lethals probably accounts for the fact that neither can function in the homozygous condition, but only a combination of the two can be successful. 9. Irregularities in the zigzag arrangement doubtless lead to abnormal distribution of the chromosomes, which may result in functionless spores or gametes when the resultant complexes lack a representative of one or more of the chromosome pairs; but in many cases irregularities merely cause an exchange of the members of one or more homologous pairs from one complex to the other, and the complexes thus formed being complete, the cells containing them may be capable of functioning. 10. Such complexes, although complete, are abnormal. If they function in crosses with other species, they may result in the appearance of cross-over individuals. In selfed lines the union of such complexes with normal ones containing the proper lethal may result in a plant deviating more or less from the usual type.

EXTRA, DIMINUTIVE CHROMOSOMES IN OENOTHERA

Although Euoenotherca has proved to be unusual -in many features of its cytogenetic behavior, it has shown very little tendency toward alteration in chromosome number. Most large genera of plants possess a considerable degree of aneuploidy or polyploidy, and this is true of some of the subgenera of Oenothera, such as Kneiffia or Hartmannia (Hecht, 1942). In Euoenothera, however, naturally-occurring aneuploids or polyploids have been conspicuous by their absence. The few plants with aberrant chromosome numbers which have been found (2n + 1, 3n, 4n) have been picked up in pedigreed cultures in experimental gardens. The cases to be reported here are the first cases of naturally-occurring euoenotheras with aneuploid chromosome numbers. The two strains under consideration are both characterized by the presence of extra, diminutive chromosomes. Both have come from California, but from widely separated localities within the state. One strain (Mono) was derived from seed collected in 1930 by Dr. D. A. Johansen in Mono Co., on the eastern slope of the Sierra Nevadas, at an elevation of 9,000 ft. The second strain (Mataguey) was collected in 1934 by Dr. P. A. Munz, along Mataguey Creek, in the Volcan Mts., eastern San Diego Co., at an elevation of 3,700 ft. These localities are about 500 miles apart. Both strains belong to the hookeri alliance, but they differ materially in foliage characters and

The Problem of Species in Oenothera

Except for those forms growing in the southwestern portion of the United States and contiguous areas, the North American euoenotheras do not show a clear-cut segregation into readily recognizable species. Instead, the population consists of a multitude of geographical races which because of balanced lethals breed true, and because of a self-pollinating habit maintain a considerable degree of isolation from each other. Each race is a complex-heterozygote, whose two complexes differ from each other both in segmental arrangement and in genic composition. These races are of hybrid origin, owing their origin to the infrequent occasions upon which the reproductive barriers have been transgressed; or they have been derived from such hybrids through subsequent alteration in segmental arrangement or in genetic composition. One complex in a race often masks the phenotypic effect of the other complex, so that the presence of the latter, and the relationships which this race bears through the latter, can only be shown by outcrosses. These races fall on the basis of cytogenetic behavior into larger groups. These are the comparatively narrow-leaved forms of the western plains area, the middle-western broad-leaved forms, the north-eastern broad-leaved forms with difficulty distinguishable phenotypically from the middle-western ones, a third broad-leaved group found in Virginia and North Carolina, the grandifloras of the southeast coastal plains area and the parvifloras of the north-east. These groups can not always be distinguished from each other with any degree of certainty on the basis of phenotypic characters and, hence, are of little value to the systematist. They show distinctive cytogenetical behavior, however, and constitute the nearest approach to species in the usual sense of the word to be found in the assemblage. The major factors which have brought about this situation have been (in addition to gene mutation) segmental interchange leading to circle formation, the development of balanced lethals, the acquisition of a self-pollinating habit resulting in the erection of reproductive barriers between races, and the occasional transgression of these barriers through outcrossing leading to the origin of new races. Evolutionary progress in this group has, therefore, followed a rather different pattern from that found in other organisms. The cytogenetic situation in the South American oenotheras, so far as this is known, is briefly summarized and compared with the situation in the North American euoenotheras.

Plastid behavior in North AmericanEuoenotheras

SummaryExtensive records of plastid behavior in the North American euoenotheras by the writer and students support in general the conclusions of Stubbe regarding the number of plastid and genome classes and their behavior relative to each other, except that our findings do not make it necessary to distinguish between classes II and III as defined byStubbe.Clarification of the plastid situation in the North American euoenotheras and their European derivatives makes it easier to visualize the precise manner in which the present groups of races in North America have come into existence. Parviflora I and parviflora II no doubt arose as a result of crosses between population 1 as female and populations 2 and 3 respectively as male.Biennis arose through hybridization between population 2 as female and population 3 as male. It has not been determined which population served as the female and which as the male parent in the crosses between populations 3 and 4 that gave rise to the strigosas.

Chromosome Configurations in Oenothera (Grandiflora x Lamarckiana)

The experiments herein reported seem to indicate that parasitism of Lym)nnaea stagnalis appressa by the larvae of trematode worms causes the snails to generate and to lose more heat to the surrounding medium than do healthy ones. On the average, they raise the temperature of a small quantity of water about 2.7 times -as high as do the healthy snails. C. T. HURST C. R.. WALKER WESTERN STATE COLLEGE 0F COLORADO

FURTHER EVIDENCE BEARING UPON THE ORIGIN OF EXTRA DIMINUTIVE CHROMOSOMES IN OENOTHERA HOOKERI

Extra diminutive chromosomes are present in a number of plant genera. They are of considerable cytogenetic importance and their origin is a matter of phylogenetic interest. In earlier papers (Cleland, 1951; Cleland and Hyde, 1963) it has been shown that two races of Oenothera hookeri (Mono and Mataguey), growing 500 miles apart in California, have such diminutives, and that these are homologous, at least in part, and therefore able to synapse with each other, although incapable of synapsing with ordinary Euoenothera chromosomes. In the latter paper, alternative suggestions have been offered regarding their origin: (1) they are the result of deletion of pairing ends, and represent regions proximal to the pairing ends of the chromosomes from which they have been derived; (2) they are intact chromosomes derived from another subgenus as the result of intersubgeneric crossing and backcrossing. The chromosomes of most of the other subgenera are smaller than those of Euoenothera to which hookeri belongs. The second hypothesis has been considered the more likely of the two, and its likelihood is enhanced by the facts to be presented in this paper. The opportunity to test the plausibility of this hypothesis came in 1963 when I had the hybrid Mono X Mataguey, with 7 pairs of chromosomes plus a pair of diminutives, growing in the same greenhouse with several species of Raimannia. Although Mono X Mataguey is a hybrid, it can still be regarded as a typical hookeri.

The S-factor situation in a small sample of anOenothera (Raimannia) heterophylla population

SummarySeed obtained from two or three individuals in an extensiveHeterophylla population produced plants with a total of six S-factors. The number of different S-factors in the population as a whole is probably very large, and this increases very greatly the likelihood of successful pollinations among the individuals of this self-incompatible race.