Systematics Of Chaetoseris And Stenoseris (Compositae-Lactuceae)

Chaetoseris and Stenoseris belong to Lactuceae, Compositae, and are distributed in Sino-Himalayan Region. These two genera and related ones form a complex named " Lactuca-Prenathes" in Lactuceae with unclear circumscription. The complex is of unstable and variable characters, which make it one of the most problematic groups in Compositae. The aim of the present study is to evaluate the separate genus status of Chaetoseris and Stenoseris to finally clarify the systematic relationship between these two genera and related ones, and to make a taxonomic revision based on evidence from macromorphology, anatomy, micro-characters, cytology and molecular study. The main results are summarized as follows:1. Macromorphological analysisOn the basis of examination of herbarium specimens and field observations, the variation pattern of all major macromorphological characters in Chaetoseris and Stenoseris were analyzed. Their taxonomic significance was re-evaluated. The results show that pappus, achene (beak, margin, shape) and involucral bracts (layer, arrangement) are important for the delimitation of these two genera and related ones; while the floret color, indumentum position on bracts, the ratio of ligule and tube, and the shape of achene are key characters to specific classification within these two genera. However, the shape of leaves and density of hair are greatly variable within a species and are of little significance in specific classification.2. Anatomy of leaf epidermisLeaf epidermis of 30 samples representing 15 species of Chaetoseris and Stenoseris were observed under a light microscope (LM). The shape of cell walls in adaxial and abaxial epidermis is irregular; the stomata are of the anomocytic type, composed of 2 guard cells and 3-5 irregular epidermal cells. Cuticular intrusions are known as "T" pieces or reduced to rod, sometimes no intrusion is presented between the ends of the two numbers of each pair of guard cells. The characters such as undulate degree, size of epidermal cells, stomatal index, the density of the degree ofcuticular intrusion are of some significance for specific delimitation within Chaetoseris and Stenoseris.3. Anatomy of achene wallThere are two types of achene transverse section in Chaetoseris, Stenoseris and related genera: "Winged type" and "Ribbed type". Winged is characterized by its protruding wings and costae in transverse section. The mesocarps are composed of parenchymatous cells and fiber cells, whose numbers are variable in different parts of the achenes. Distinct fibrous strands are confined to the costae or wings of achenes. This type is present in Lactuca, Cicerbita, Pterocypsela, Chaetoseris and Stenoseris. Ribbed Type is represented by Notoseris and Paraprenanthes, and is characterized by the ribbings in transverse section. Their mesocarps are composed only of fiber cells (including libriform fiber and fiber-sclereid cells). Characters such as the number and position of parenchymatous and fibrous cells in the mesocarp show much variation, and are of significance for the systematic analysis of these genera.4. SEM observation of surface sculpturing of acheneAchene surface sculpturing of 27 samples representing 20 species of Chaetoseris and Stenoseris and related genera were investigated under scanning electronic microscope (SEM). The results show that characters such as the shape of the epidermis, and the type of ornamentation of the outer cell wall are variable between genera, but similar within a genus. Chaetoseris with ellipsoid-shaped epidermal cells and with verrucose papillae in the outer cell wall and Stenoseris with long fusiform-shaped epidermal cells and verrucose papillae in outer cell wall are different from related genera. Characters such as the size of epidermis, shape of end wall and degree of wax development are of some significance for specific classification in Chaetoseris.5. CytologyTwo species of Lactuca, one of Chaetoseris and one of Pterocypsela were cytologically investigated, among which one species was reported for the first time. The species of Chaet...