How Hawkweed Confounded the Father of Genetics for 7 Years
Gregor Johann Mendel was an Austrian monk who gained posthumous recognition as the Father of Modern Genetics. He came from a family with limited financial means, so in order to pursue his education he opted to become a novitiate of the Augustinian order in 1843, moving to Brünn in Moravia (now part of modern day Czech Republic). (Ref 1)
Peas and Australian sheep
Abbot Cyril Franz Napp was sweating bullets. The monastery made a tidy profit by spinning wool from sheep reared on the premises. But this profit margin was threatened by imported merino wool from Australia which was high-quality and easy on the pockets. Although it was known that breeding introduced traits into offspring (i.e. progeny), the exact question of “what was inherited and how” (In German: Was vererbt und wie?) remained unanswered. So when Mendel, proposed experimental hybridization in pea plants, the abbot agreed, in hopes that takeaways from Mendel’s study would cross over for the sheep. (Ref 2)
Mendel selected the pea plant (Pisum sativum) primarily because he could control the source of pollen with a paintbrush. It was self-fertilization if the pollen came from the same flower, or cross-fertilization if it came from another flower. Pea plants also had observable properties that could be easily catalogued. He chose 7 distinctive properties to measure: pea shape (round or wrinkled), pea color (green or yellow), pod shape (constricted or inflated), pod color (green or yellow), flower color (purple or white), plant size (tall or dwarf), and position of flowers (axial or terminal). (Refs 3–5)
Peas have sex too
Mendel began by crossing purebred pea plants with purple-red and white flower color. The filial or F1 generation were all purple-red, contrary to the existing notion at the time that a filial generation inherited “blended characteristics”. When Mendel crossed the F1 with each other, he noticed a white flower for every 3 red flowers. This 3:1 red:white proportion indicated a hidden, “recessive” trait among the “dominant” trait. But what happened when two pea plants that are each hybrid for two traits are crossed?
Dubbed as a dihybrid cross, he crossbred purebred pea plants with yellow and green seed colors, and round and wrinkled seed shape. Then he crossed the filial F1 with one another as he did before.
The result of this dihybrid cross were pea plants that had seeds that were (round, yellow), (round, green), (wrinkled, yellow) and (wrinkled, green) in a 9:3:3:1 ratio. But when Mendel analyzed each characteristic independently as a proportion of the total, the ratio was ~3:1 for seed color and seed shape. So even though there were 2 characteristics in a single cross, the traits had segregated independently.
Curiosity amidst frustration
The mark of any good experiment is the reproduction of results. Is independent segregation seen in other plants? So, after consulting with Swiss botanist Carl Nägeli, Mendel tried verifying his segregation ratios in Hieracium (subgenus Pilosella), aka hawkweed.
Mendel noted, in 1869:
“The condition of the Hieracium hybrids in the range we are concerned with must necessarily be determined by experiments; for we do not possess a complete theory of hybridisation, and we may be led into erroneous conclusions if we take rules deduced from observation of certain other hybrids to be Laws of hybridisation, and try to apply them to Hieracium without further consideration. If by the experimental method we can obtain a sufficient insight into the phenomenon of hybridisation in Hieracium, then by the help of the experience which has been collected respecting the structural relations of the wild forms, a satisfactory judgment in regard to this question may become possible.” (Ref 6)
Seven years worth of hybridization experiments ensued, but the hawkweed refused to play ball. Did this mean that the laws of trait inheritance were invalidated? Not quite, because genetics is a complicated matter and there are instances of non-Mendelian inheritance. So what really happened here with hawkweed?
Some plants don’t have sex.
Facultative apomixis was the culprit. Apomixis is an asexual seed formation that results in clones of the maternal parent (i.e. mom clones). Facultative apomixis is a mix of sexual and asexual processes, where about 2% of the time, the seed formed is by sexual means. So far, we were looking at a purely sexual picture in the case of the pea plant. Mendel removed the male organs called stamens with a pair of tweezers to prevent self-pollination. He then invoked cross-pollination by carefully dusting pollen from a ripe flower on the pistil (the female organ). Despite conducting this process on over 5000 Hieracium florets through the years of 1866–1873, Mendel produced several hundred hybrids (probably owing to that 2% statistic) and the enterprise left him perplexed. He probably thought that the small size of the flower and imperfection in his stamen removal process were to blame. (Refs 7,8)
It is no small wonder that Carl Correns, Nägeli’s student, noted this quote from Mendel’s correspondence with Nägeli :
“On this occasion I cannot resist remarking how striking it is that the hybrids of Hieracium show a behaviour exactly opposite to those of Pisum. Evidently we are here dealing only with individual phenomena, which are the manifestation of a higher, more universal law”. (Ref 9)
Although the humble hawkweed tripped up Mendel and caused him a fair degree of stress, the history books say that his scientific temper remained unchanged. Mendel was a physicist, a meteorologist, and a brilliant administrator.
The matter with the hawkweed was an unfortunate tryst, but apomixis is being researched as a serious candidate for farmers who want to maintain and propagate crops with desirable traits. (Ref 10)
Counterpoint: An alternative view
Other scientists think Mendel chose to work with hawkweed knowing very well he expected the progeny to “remain constant” and “propagate like pure strains”. Peter van Dijik and Noel Ellis believe that Mendel wanted to test the generalities of his observations in the pea plant. They believe a purported missing page in Mendel’s communication to Nägeli might have led to the misreading of the letter, thus contributing to the traditional view of Mendel’s frustration. (Ref 11)
Regardless, Mendel’s work paved the way for modern genetics and continues to inspire a new generation of biologists.
References:
- Britannica. https://www.britannica.com/biography/Gregor-Mendel#ref4790
- Wood RJ, Orel V. Scientific Breeding in Central Europe during the Early Nineteenth Century: Background to Mendel’s Later Work. J Hist Biol. 2005;38:239–272.
- Miko I. Gregor Mendel and the principles of inheritance. Nature Education. 2008;1(1):134.
- Mendel G. Experiments in a Monastery Garden. Am Zool. 1986;26(3):749–752.
- Mendel G. Versuche über Plflanzenhybriden. Verhand-
lungen des naturforschenden Vereines in Brünn, Bd. IV für das Jahr
1865, Abhandlungen, 3–47. Translated into the English, available here. - Mendel G. Ueber einige aus künstlichen Befruchtung
gewonnen Hieracium-Bastarde. Verhandlungen des naturforschenden
Vereines, Abhandlungen, Brünn, Bd. VIII für das Jahr 1869, 26–31. Translated into the English, available here. - Bicknell R, Catanach A, Hand M, Koltunow A. Seeds of doubt: Mendel’s choice of Hieracium to study inheritance, a case of right plant, wrong trait. Theor Appl Genet. 2016;129(12):2253–2266.
- Bicknell RA, Lambie SC, Butler RC. Quantification of progeny classes in two facultatively apomictic accessions of Hieracium. Hereditas. 2003;138:11–20.
- Mendel G. Briefe an Carl Nägeli. 1866–1873. Translated by Leonie Kellen Piternick and George Piternick. Available here.
- Adam D. In findings that elucidate Mendel’s 19th C work, gene triggers asexual births in dandelion. National Academy of Sciences. http://blog.pnas.org/2022/01/in-findings-that-elucidate-mendels-19th-c-work-gene-triggers-asexual-births-in-dandelion/. Published January 18, 2022. Accessed February 8, 2022.
- van Dijk PJ, Ellis TH. The Full Breadth of Mendel’s Genetics. Genetics. 2016;204(4):1327–1336.
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