MICHAELIS, PETER

(b. Munich, Germany, 28 May 1900; d. Cologne, Germany, 3 August 1975)

botany, genetics.

Michaelis was the only son of Oskar Michaelis, a portrait painter, and his wife, Thusnelda Jaeger, the daughter of a professor of philosophy at Tübingen. His parents’ home was a place where artists and other prominent persons met in his mother’s salon; many of them had their portraits painted by his father. Meeting these people considerably influenced Michaelis’ education. He was talented in music and drawing, and very interested in living things. By the age of seventeen he had an extensive knowledge of the flora around Munich, including algae, mosses, ferns, and fungi.

Michaelis’ education at the Realgymnasium was interrupted when he entered the army as a cadet in May 1918. He was discharged in January 1919 without having seen action. In October 1919 Michaelis enrolled at the University of Munich to study botany, geology, geography, and zoology. He received a Ph.D. in July 1923. His dissertation, done under Karl von Goebel, dealt with the flower morphology of the Euphorbiaceae with special regard to the angiospermous flower’s phylogeny. This work, based on the investigation of 130 genera out of nearly 300, was awarded a prize by the university.

In April 1923 Otto Renner had appointed Michaelis his assistant at Jena. Michaelis remembered this period with ambivalence. Not only did he have to share the position with a colleague, he also fell ill, and an extended stay in the hospital toppled him into financial ruin. On the other hand, at Jena he began his lifelong genetic work on cytoplasmic inheritance in Epilobium (the willow herb), which in some ways paralleled Renner’s work on plastid inheritance in Oenothera. The phenotypic difference of reciprocal hybrids in Epilobium, which earlier had attracted the attention of other geneticists, could be attributed to genetic differences in the cytoplasm apart from the chromosomes. Fritz von Wettstein, Correns’ former assistant, created the term “plasmone” for them. At that time the debate on the role of the genome and the plasmone in heredity and their bearing upon development was in full swing. Michaelis’ first contribution to this question was published in 1929.

In 1927 Michaelis moved to Stuttgart, where he was chief assistant to Richard Harder until 1933. He lectured and taught courses on various botanical subjects. During the winters of 1931 to 1932 and 1932 to 1933, he carried out ecological research in the Alps, connecting this work with his hobbies of mountain climbing and skiing. Assisted by his fiancee, Gertrud Aichele, he studied the influence of wintry climatic conditions on the timberline. They were married in 1932 and had two daughters and one son.

The departure of Harder, whose field was cryptogamic botany, to become professor at the University of Göttingen (1933) influenced Michaelis’ career. His consequent pursuit of the experiments on plasmic inheritance in Epilobium, documented by a series of publications, had brought him a reputation in the field of non-Mendelian genetics, Thereupon Erwin Baur, director of the Kaiser Wilhelm Institute for Breeding Research in Müncheberg/Mark (not far from Berlin), offered him a staff position, which Michaelis accepted. From April 1933 on, he could concentrate on his work on Epilobium, Baur, who supported him personally and professionally, died in the fall of 1933. His successor, Wilhelm Rudorf, gave Michaelis his own department of plasmic inheritance research. In 1941 he was appointed a scientist member of the Kaiser-Wilhelm-Gesellschaft. In 1944 the prosperous period of Müncheberg eventually was ended. The approaching eastern battlefront necessitated the transfer of equipment, livestock, and seed material to various locations in the interior of Germany.

From 1946 through 1955 the Kaiser Wilhelm Institut—now a part of the Max-Planck-Gesellschaft—was provisionally reestablished at Voldagsen, near Hameln (not far from Hannover), and in 1956 it was moved to Cologne. In both places Michaelis continued his work on cytoplasmic inheritance, until he retired in 1968. He became an internationally known geneticist in the still controversial field of plasmic inheritance, to which he contributed more than 100 publications. He had shown that the plasmone may differ not only between different species but also between races of the same species. By 1947 he had investigated about 500 strains from 25 species of Epilobium. In most cases plasmone differences appeared as an absence of the necessary cooperation between nucleus and cytoplasm, resulting in flower malformation or inhibition of vegetative development and growth. In general these disturbances were nonspecific.

When it was observed that the inhibited plants produce phenotypic alterations with the tendency to better growth and eventual normalization in certain areas, Michaelis tried to use these alterations as a tool in the analysis of the plasmone. Altered shoots could be propagated vegetatively and regenerated to whole plants. In part Michaelis recognized similarities with dauermodifications; on the other hand, the alterations showed maternal inheritance. Michaelis regarded them as different plasmotypes. He created the hypothesis of intraindividual recombination of the plasmone constituents under certain conditions by segregation and selective multiplication of these genetic elements, a great number of which should be present in the cytoplasm. In order to test this new hypothesis, he elaborated the method of variegation pattern analysis. Even though the method could be applied successfully to the plastid segregation, the cytological difficulties concerning the plasmotypes could not be overcome. It was not yet possible to solve the problems of localization and of the mode of action of the cytoplasmic determinants.

At the end of his scientific career Michaelis was invited by the Japan Society for the Promotion of Science to lecture for two months in Japanese universities and research institutes. In November 1967 the University of Strasbourg awarded him an honorary doctorate.

BIBLIOGRAPHY

I. Original Works. Michaelis’ dissertation, “Blüten-morphologische Untersuchungen an den Euphorbiaceen unter besonderer Berücksichtigung der Phylogenie der Angiospermenblüte,” is in Botanische Abhandlungen, 3 (1924), 1–150. His ecological work yielded five publications, the last being “Ökologische Studien an der alpinen Baumgrenze. V. Osmotischer Wert und Wassergehald während des Winters in den verschiedenen Höhenlagen,” in Jahrbücher für wissenschaftliche Botanik80 (1934), 337–362. A comprehensive review of his genetic work on Epilobium is “Cytoplasmic Inheritance in Epilobium and its Theoretical Significance,” in Advances in Genetics6 (1954), 287–401, with a detailed bibliography. Of Michaelis’ later publications the following are noteworthy: “Über Gesetzmässigkeiten der Plasmon-Umkombination und über eine Methode zur Trennung einer Plastiden-, Chrondriosomen- resp. Sphaerosomen-, (Mikrosomen)-und einer Zytoplasmavererbung,” in Cytologia, 20 (1955), 315–338; “Cytoplasmic Inheritance on Epilobium (a Survey),” Nucleus, 8 (1966), 83–92; and “Über Plastiden-Restitutionen (Rückmutationen),” in Cytologia, 34 supp. no. (1969), 1–115.

Wilfried Stubbe