Evolution of apomixis loci in Pilosella and Hieracium (Asteraceae) inferred from the conservation of apomixis-linked markers in natural and experimental populations

Abstract

The Hieracium and Pilosella (Lactuceae, Asteraceae) genera of closely related hawkweeds contain species with two different modes of gametophytic apomixis (asexual seed formation). Both genera contain polyploid species, and in wild populations, sexual and apomictic species co-exist. Apomixis is known to co-exist with sexuality in apomictic Pilosella individuals, however, apomictic Hieracium have been regarded as obligate apomicts. Here, a developmental analysis of apomixis within 16 Hieracium species revealed meiosis and megaspore tetrad formation in 1 to 7% of ovules, for the first time indicating residual sexuality in this genus. Molecular markers linked to the two independent, dominant loci LOSS OF APOMEIOSIS (LOA) and LOSS OF PARTHENOGENESIS (LOP) controlling apomixis in Pilosella piloselloides subsp. praealta were screened across 20 phenotyped Hieracium individuals from natural populations, and 65 phenotyped Pilosella individuals from natural and experimental cross populations, to examine their conservation, inheritance and association with reproductive modes. All of the tested LOA and LOP-linked markers were absent in the 20 Hieracium samples irrespective of their reproductive mode. Within Pilosella, LOA and LOP-linked markers were essentially absent within the sexual plants, although they were not conserved in all apomictic individuals. Both loci appeared to be inherited independently, and evidence for additional genetic factors influencing quantitative expression of LOA and LOP was obtained. Collectively, these data suggest independent evolution of apomixis in Hieracium and Pilosella and are discussed with respect to current knowledge of the evolution of apomixis.

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

“…() who concluded that there is ample evidence for ancient hybridization in the genus, but no indications of recently ongoing hybridization among the extant, likely obligately apomictic species (Mráz and Zdvořák, ); their role as pollen parents remains to be elucidated. While traces of a sexual pathway could be found in embryological investigations in Hieracium (Hand et al., ), there is so far, very little evidence that these embryos actually develop into mature plants (Mráz and Zdvořák, ). In our group, both intermediate species co–occur (at least at a wider scale, not only at the same locality) with their parental species, which can theoretically allow repeated hybridization events.…”

Morphology delimits more species than molecular genetic clusters of invasive Pilosella

“…Hypothesized species relationships of Pilosella sampled, including the postulated parental species for hybrid (intermediate) species (Bräutigam and Greuter, 2007; modified after Hand et al, 2015). …”

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

“…Most apomicts are facultative, with sexuality occurring at low levels alongside apomixis, and Hieracium is no exception (Asker and Jerling 1992; Hand et al 2015; Bicknell et al 2003). Furthermore, many apomictic species produce colourful flowers that attract pollinators and/or they provide pollinator rewards.…”