The Full Breadth of Mendel's Genetics

Abstract

Gregor Mendel's "Experiments on Plant Hybrids" (1865/1866), published 150 years ago, is without doubt one of the most brilliant works in biology. Curiously, Mendel's later studies on Hieracium (hawkweed) are usually seen as a frustrating failure, because it is assumed that they were intended to confirm the segregation ratios he found in Pisum Had this been his intention, such a confirmation would have failed, since, unknown to Mendel, Hieracium species mostly reproduce by means of clonal seeds (apomixis). Here we show that this assumption arises from a misunderstanding that could be explained by a missing page in Mendel's first letter to Carl Nägeli. Mendel's writings clearly indicate his interest in "constant hybrids," hybrids which do not segregate, and which were "essentially different" from "variable hybrids" such as in Pisum After the Pisum studies, Mendel worked mainly on Hieracium for 7 years where he found constant hybrids and some great surprises. He also continued to explore variable hybrids; both variable and constant hybrids were of interest to Mendel with respect to inheritance and to species evolution. Mendel considered that their similarities and differences might provide deep insights and that their differing behaviors were "individual manifestations of a higher more fundamental law."

Keywords: Gregor Mendel; Hieracium; apomixis; constant hybrids; genetics.

Figure 1

Letter I (December 31, 1866). A comparison between Correns’ publication (left) and Mendel’s original handwriting. In Correns’ publication, paragraph four and five are on the same page, but in Mendel’s original letter, paragraph four is at the end of page two and paragraph five is at the top of page three. The handwriting shows that an entire page could be missing. In Correns’ publication a missing page would not be noticed, unless the flow of the content was illogical. Courtesy of the Mendelianum Archives of the Moravian Museum.

Figure 2

Mendel’s 1865/1866 views of inheritance in constant and variable hybrids. Mendel’s interpretation (boxed) of the behavior of determining elements is compared to our current understanding. “Sexual Cross” refers to the specific case of a cross between two homozygotes followed by self-fertilization, and should be compared to “Variable Hybrids” which is classically described in his 1866 article. Mendel’s interpretation of “Constant Hybrids” should be compared to “Apomixis.” Note that Aa has a different meaning in our current understanding from that in Mendel’s scheme; Mendel did not know about meiosis and the distinction between diploid and haploid. Numbers indicate: (1) The union of germinal cells from the female and male (egg and pollen). (2) The primordial cell (zygote): differences between antagonistic elements are mediated (in the mediating cell). (3) Vegetative period, the balance/mediation established in the primordial cell continues. (4) In variable hybrids, at the formation of the reproductive cells (gametes) the antagonistic elements are separated and represent “all constant forms which result from the combination of the characters united in fertilization.” The “arrangement between the conflicting elements is only temporary,” that is, no germinal cells carry the union of conflicting factors. (5) In constant hybrids, at the formation of the reproductive cells (gametes) the antagonistic elements are not separated. The essential difference in the development of constant hybrids is that the union of the factors is permanent. (6) In constant hybrids, the union of germinal cells of identical constitution is proposed (i.e., no parthenogenesis). (7) For comparison, the genetic transmission of apomixis is shown: the unreduced egg cell develops into an embryo by parthenogenesis. Note that in the case of apomixis a breeding system is inherited, which will fix the segregating genetic background of both parents; producing many different apomictic lineages. For simplicity only diploids are shown, but apomixis is often associated with polyploidy. Because apomicts have a simplex dominant genotype (Aaaa) this convenience is used. The type of apomixis shown here is typical for the subgenus Archieracium, which Mendel also used in crosses.

Figure 3

Variation in inflorescence color and size in Hieracium hybrids. Ostenfeld (1910) illustrated 23 H. auricula × aurantiacum hybrids that he obtained. Mendel obtained 84 flowering hybrids from the same cross. The parental species are shown at the top; H. auricula left, with a yellow small inflorescence; and H. aurantiacum right, with a larger orange inflorescence. Next to the inflorescence a single floret is shown. The original image is from the Biodiversity Heritage Library. Digitized by the Mertz Library, New York Botanical Garden (http://www.biodiversitylibrary.org).

Figure 4

Mendel’s first constant Hieracium hybrid (H. praealtum × H. flagellare). Mendel observed no variation in three generations of this artificial hybrid. From herbarium “Hieracia Naegeliana” (Peter 1885). Courtesy of the Museum of Grenoble (H. inops n. hybr., GRM. Arv.-Touv. MHNGr. 191437180).

Figure 5

Four of Mendel’s H. auricula × aurantiacum hybrids from the herbarium Hieracia Naegeliana (Peter 1885). All hybrids were fully fertile. In 1869, 1870, and 1873 Mendel sent material to Nägeli in Munich where they were cultivated in the common garden. Eight of these are described in Peter (1884). Courtesy of Museum de Grenoble (H. pyrrhanthes n. hybr. GRM. Arv.-Touv. MHNGr. 191437163, 191437164, 191437165, and 191437173).