Science (1956) 124: 684-685
Graft-Induced Transmission to Progeny of Cytoplasmic Male Sterility in Petunia
Department of Vegetable Crops,
University of California, Davis

Many different manifestations have been revealed of hereditary properties of the cytoplasm. Thus, such different phenomena as maternal characters determined in the egg prior to fertilization, temporary environmentally induced, cytoplasmically determined characters, and interactions between nuclear genes and cytoplasmic factors have been demonstrated (1, 2). In a few instances (3), hereditary elements in the cytoplasm seem to be independent of nuclear control; one of these is the cytoplasmic male sterility in Petunia — the subject of this report (4).

The main avenue of attack on the nature of cytoplasmic inheritance has been to transfer by breeding different genomes into different cytoplasms. Despite the advantages of this method in demonstrating the matroclinous products, it neither proves that the cytoplasm as a whole by itself (a genuine "plasmon") determines hereditary traits in the same sense as nuclear genes do, nor can it prove that certain loci ("plasmagenes") are responsible for hereditary properties. Goldschmidt (2), while reviewing the body of evidence, tends to reject the plasmagene concept as suggestive, unnecessary, and misleading and chooses to find alternative interpretations to the facts. With certain assumptions Michaelis (5) formulated an interesting statistical model for hereditary units in the cytoplasm. Although his assumptions are reasonable, they are arbitrary; and the method is useful only for characteristics that express themselves in measurable gradations. Only in the case of the killer effect in Paramecium (6) has a self-reproducing hereditary particle of the cytoplasm ("kappa") been demonstrated. The behavior and chemical nature of this particle does not reveal whether it is an intimate part of the cytoplasm or a self-reproducing foreign inclusion producing deleterious effects in certain genotypes. Significant gaps thus exist in our knowledge about hereditary elements of the cytoplasm.

This preliminary report deals with an attempt to obtain evidence on the nature of cytoplasmic male sterility in Petunia by means of grafting. The following lines were used (7) : (i) Northern Star, a fertile variety having a lavender corolla with a large white central star, and (ii) P-431-54 (ms), a completely male-sterile line having a dark garnet corolla and violet throat color. Anthers of the latter are greatly reduced in size and are devoid of functional pollen.

Self- and sib-pollinations of the Northern Star for two generations yielded only fertile progeny. By crossing the male-sterile line with Northern Star and three other unrelated varieties for two generations, exclusively male-sterile progeny were obtained (Fig. 1). These results, which conform to the much wider experience of other workers, suggest that this sterility factor is independent of nuclear genes.

Reciprocal grafts between fertile and male-sterile lines were made in two separate series of experiments in 1954 and 1955. No changes were noticed in the fertility or sterility of the graft components, but observations were limited by the short survival of the scions. All scions of 15 combinations of male-sterile and fertile died shortly after producing a few flowers; consequently, seeds could not be obtained from pollinations with fertile pollen. Scions of the fertile variety grafted on male-sterile stocks survived slightly longer than the reciprocal grafts. Seed was produced from two scions in each of the two series, from both sib- and self-crosses (the latter with pollen from the same flower as well as that from flowers of the donor plant). The germination rate of the seed in petri dishes was approximately 45 percent. All the work was conducted in a greenhouse in complete isolation from other lines of Petunia.

Progenies were grown from seeds produced by the scions and were subjected to additional test crosses to test the inheritance of male sterility. The results of these tests are summarized diagrammatically in Fig. 1. Progeny of the fertile scions grafted on male sterile stocks consistently included both fertile and sterile plants, whether from self- or sib-crosses. The number of mature plants obtained was small because the seeds germinated poorly in soil and many seedlings died. In the first series from selfing the scions with pollen of the same flower, three fertile and three male-sterile plants were obtained. Two of the male-sterile plants recovered some fertility after the third or fourth flower, and only one plant remained entirely sterile for the 12 months of its life. Seeds obtained from scion flowers pollinated with pollen from the donor plant produced eight fertile and two male-sterile plants. In the second series, 11 plants of a total of 38 remained completely male-sterile. All plants in both series showed the characteristic flower color of the Northern Star and no resemblance to the color of the male-sterile line or that of the F1's.

Fig. 1. Diagram of pedigrees. Graft combination indicated in upper right by fraction symbol, the scion being above, the stock below, the line.

Backcrosses between the first-generation male-sterile plants in the progeny of selfed scions and the Northern Star donor gave only male-sterile progeny, but the maximum number of plants tested per backcross was only 28. Only fertile progeny were secured from the selfs and backcrosses with the Northern Star donor of the fertile first-generation segregants in the progeny of selfed scions. Selfing the Northern Star donor likewise yielded only fertile progeny.

For several reasons these data do not permit extensive speculation about the nature of this cytoplasmic sterility. High heterozygosity of the material is likely because it is moderately self-incompatible. Large numbers of offspring of the F2 are needed to establish independence of the cytoplasmic factor from nuclear genes. Low germination rates and high seedling mortality might have distorted segregation ratios. It would also be of interest to know whether longer lived scions would continue to maintain the same phenotype. The apparently autonomous behavior in the scions might be the result of a certain threshold requirement obscured by the short life of the scions. No explanation can be offered now for the better survival of fertile/male-sterile than reciprocal grafts. Experiments will be undertaken to study these and related problems.

Whatever doubts may be raised by these factors of uncertainty, the fact remains that the grafting induced changes in the fertile scion that resulted in the appearance of cytoplasmic sterility in its progeny. Although it seems most likely that this change was induced by movement of cytoplasmic sterility determinants from stock to scion, the tests do not entirely rule out other explanations. There is a remote possibility, for instance, that nutritional deficiency, which might have been induced by grafting, might cause a disturbance in cytoplasmic enzyme activity in such a way as to lead to an increase or decrease of sterility-determining entities of the cytoplasm.

11 July 1956

References and Notes

  1. E. Caspari, Advances in Genet. 2, 1 (1943).
  2. R. B. Goldschmidt, Theoretical Genetics (Univ. of California Press, Berkeley, 1955), pp. 193-244.
  3. F. von Wettstein, Biol. Zbl. 65, 149 (1946); P. Michaelis, Advances in Genet. 6, 287 (1954).
  4. I am indebted to C. M. Rick of the department of vegetable crops, University of California, Davis, for collecting some of the data and for giving helpful criticism and advice in carrying out the experiments and in writing the present report.
  5. P. Michaelis, Modellversuche zur Plastiden- und Plasmavererbung. Züchter 25, 209 (1955).
  6. T. M. Sonneborn, Advances in Genet. 1, 263 (1947).
  7. The lines were kindly furnished by W. Atlee Burpee Co.