New evidence for the tribal placement of Sheareria within Astereae (Compositae)

Abstract

Sheareria S. Moore is a monotypic genus endemic to China, distributed from east to south and southwest China. This genus has been classified within the subtribe Milleriinae of the Heliantheae by the Chinese taxonomists, while Robinson and Nesom placed it in the Astereae. In the present study, we compared micromorphology, anatomy and chromosome numbers of this genus and representative species from the two tribes. The adaxial surfaces of the ligules of the only species in this genus, S. nana, have linear epidermal cell with transverse stripes and a longitudinal median promontory-like protrusion, which are identical or similar to those of the representative species of the Astereae. However, in the representative species of the Heliantheae, these cells are subround and papillose. Interfascicular parts of the cross-section of ovary walls in five species of the Heliantheae comprise 3–4 layers, including epidermis, hypodermis, small cell region and/or parenchyma tissue, and the epidermal cells are identical in size, shape and orderly arrangement to the hypodermal cells. In contrast, only two layers (epidermis and parenchyma tissue) were recovered in S. nana and five species of the tribe Astereae. The karyotype of S. nana is 2n=2x=18sm with x=9, a common basic number in the Astereae, but different from x=10 of the Heliantheae. All of these results together support the tribal delimitation of Sheareria within the Astereae, rather than in the Heliantheae.