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. 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.
Highlights
Gregor Mendel is justifiably referred to as the ‘Father of genetics’ due to his pioneering work on inheritance using the garden pea (Pisum sativum) as his model system
He acknowledged that the principles of inheritance he described for pea could not be applied to the data he obtained using Hieracium
Backcrossing to a sexual species indicated that the trait of autonomous endosperm formation, denoted AutE, was a dominant locus. These analyses suggest that perhaps both processes of autonomous endosperm formation and parthenogenesis are in close linkage at LOSS OF PARTHENOGENESIS (LOP) as identified in H. praealtum
Summary
Gregor Mendel is justifiably referred to as the ‘Father of genetics’ due to his pioneering work on inheritance using the garden pea (Pisum sativum) as his model system. The genetic dominance of apomixis in Hieracium species was initially established using conventional experimental crosses, whereby apomictic H. aurantiacum and H. piloselloides were used as pollen parents in crosses with a sexual accession of H. pilosella which revealed dominant loci that independently and, respectively, co-segregated with apomeiosis and parthenogenesis (Bicknell et al 2000). When MiMe plants were crossed to the haploid inducer, low numbers of viable seed were formed; 98 % of the diploid progeny had chromosomes only from the MiMe parent, and heterozygosity of this parent was retained in the diploid progeny This breakthrough proof-of-principle study highlights a potential strategy for engineering apomixis into normally sexually reproducing crop species. Any plant expressing either apomeiosis or parthenogenesis alone could, “bide its time” producing millions of seeds until a chance mutation or hybridization event completed the base requirements for apomixis, and selection at the level of the genotype, as described above, was established
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