Trichomes in the tribe Lactuceae (Asteraceae) -: taxonomic implications

Although the infratribal classification of the Lactuceae has been the focus of several studies, relationships between the genera are not yet fully resolved. While trichome characters have been shown to be an important tool in taxa delimitation in many plant families, a detailed study on trichomes within the Lactuceae was still lacking. We assess here how trichome types can contribute to the clarification of generic relationships in this tribe. Trichome diversity was investigated on stems and leaves of 135 representatives of 53 genera by light and scanning electron microscopy. According to their morphology and anatomy, trichomes were classified into eight types and several subtypes. Phenetic analysis based on the presence or absence of particular trichome types was performed. The previously supposed monophyly of exclusively American genera and affinities between several closely related genera were confirmed. A new circumscription of the subtribe Hieraciinae is proposed, in accordance with recent molecular data. According to the present study, this subtribe is characterized by a unique combination of two trichome types and comprises the following genera: Andryala, Hieracium, Hispidella, Pilosella, Stenotheca. The previously included genera Arnoseris, Hololeion and Tolpis do not belong to this assemblage and their removal from the Hieraciinae should be reconsidered.