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.