Using karyotyping and achene morphology to accurately identify Sonchus oleraceus (Asteraceae) in the context of its biological control

In weed biocontrol, accurate identification of the target weed is essential to select effective and host-specific biocontrol agents. This study focuses on the biocontrol of an invasive weed species in Australia, native from Europe, the common sowthistle, Sonchus oleraceus L. (Asteraceae). During field surveys, the distinction between S. oleraceus and a morphologically related species (S. asper L.) was difficult because of specimens bearing intermediate morphological features. These observations raised questions about the reliability the morphological characters used for distinguishing between these species and the identity of the intermediate phenotypes. First, cytological analyses coupled with morphological comparisons were carried out on specimens collected in Europe and Australia. Results showed that specimens morphologically described as S. oleraceus and S. asper possessed, in accordance with literature 32 and 18 chromosomes, respectively. Specimens with intermediate morphotypes had 32 chromosomes, showing that they belong to S. oleraceus species. The variability of characters used for diagnosis is discussed and for a particular feature, achene ornamentation, an inquiry among 30 people was carried out to determine how this character might be relevant for distinguishing the two species herein considered. The successful identification rate was 92.2% (SE {+/-} 0.77) showing the practical interest of this feature for diagnosis.