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

Abstract

Key message 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. AbstractFrom 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.

The Full Breadth of Mendel’s Genetics

“…His only subsequent publication on plants is a preliminary communication on artificial Hieracium hybrids (Mendel 1870). The usual supposition about Mendel’s Hieracium experiments, which were carried out over 7 years, is that they were intended to verify the results he obtained with his Pisum experiments (Nogler 2006; Bicknell et al 2016). Hawkweeds are related to dandelions and, like them, often reproduce by a peculiar and rare breeding system called apomixis.…”

Evolutionary history and genetic diversity of apomictic allopolyploids in Hieracium s.str.: morphological versus genomic features

“…Moreover, the diversification of parental repeatomes contrasts with the phylogenetic patterns and the evolutionary history of the genus. Once again, Hieracium behaves as puzzling as it once did in Mendel's experience (Bicknell et al., ).…”

Genetic mapping in polyploids

“…There are numerous reports of the association of gametophytic apomixis with polyploidy (Wet and Harlan, 1970;Whitton et al, 2008), with a number of possible theories proposed (both mechanistic and evolutionary) but no consensus on the precise reason why this should be (Quarin et al, 2001;Comai, 2005;Zielinski and Scheid, 2012). Whatever the connection, the potential attractiveness of programmable apomixis for breeding has widely been acknowledged (Spillane et al, 2004;Abdi et al, 2016;Bicknell et al, 2016;Bradshaw, 2016), and not just at the polyploid level. Just like clonal propagation, apomixis would allow the fixation of heterosis and has been suggested as a superior method to produce F1 hybrids by immortalising the F1 rather than recreating it each season through repeated crossing of inbred parental lines.…”