Seeds of doubt: Mendel's choice of Hieracium to study inheritance, a case of right plant, wrong trait

Abstract

In this review, we explore Gregor Mendel's hybridization experiments with Hieracium , update current knowledge on apomictic reproduction and describe approaches now being used to develop true-breeding hybrid crops. From our perspective, it is easy to conclude that Gregor Mendel's work on pea was insightful, but his peers clearly did not regard it as being either very convincing or of much importance. One apparent criticism was that his findings only applied to pea. We know from a letter he wrote to Carl von Nägeli, a leading botanist, that he believed he needed to "verify, with other plants, the results obtained with Pisum". For this purpose, Mendel adopted Hieracium subgenus Pilosella, a phenotypically diverse taxon under botanical study at the time. What Mendel could not have known, however, is that the majority of these plants are not sexual plants like pea, but instead are facultatively apomictic. In these forms, the majority of seed arises asexually, and such progeny are, therefore, clones of the maternal parent. Mendel obtained very few hybrids in his Hieracium crosses, yet we calculate that he probably emasculated in excess of 5000 Hieracium florets to even obtain the numbers he did. Despite that effort, he was perplexed by the results, and they ultimately led him to conclude that "the hybrids of Hieracium show a behaviour exactly opposite to those of Pisum". Apomixis is now a topic of intense research interest, and in an ironic twist of history, Hieracium subgenus Pilosella has been developed as a molecular model to study this trait. In this paper, we explore further Mendel's hybridization experiments with Hieracium, update current knowledge on apomictic reproduction and describe approaches now being used to develop true-breeding hybrid crops.

Fig. 1

A figure reproduced from Ostenfeld and Rosenberg (1906) illustrating the parents used and the progeny derived from their hybridization experiments with Hieracium. 1 and 2: Hieracium excellens, 3: H. pilosella, 4; H. aurantiacum, 5: H. excellens × aurantiacum, 6: H. excellens × pilosella, 7: H. pilosella × aurantiacum, 8: H. pilosella. (Ostenfeld and Rosenberg (1906) Experimental and cytological studies in the Hieracia. I. Castration and hybridization experiments with some species of Hieracia. Botanisk Tidsskrift 27:225–248)