| As previously circumscribed, Sphaerocardamum (Brassicaceae) included eight species endemic to limestone soils of Mexico's Chihuahuan desert and adjacent mountain regions. The circumscription of these species has been difficult because the genus was known from fewer than 43 different collections. In addition, further study of Sphaerocardamum was of interest because the genus displays a wide range of morphological characters that have been considered important in Brassicaceae taxonomy, even at higher taxonomic levels. This study focused on higher and lower level systematic questions related to Sphaerocardamum.; Higher level studies were used to address the relationships of the genus within the Halimolobine clade. As currently interpreted, the Halimolobine clade includes the majority of Halimolobos species, and all species of Mancoa, Pennellia, and Sphaerocardamum. The higher level project included: (1) a further test the monophyly of the Halimolobine clade; (2) a test the monophyly and circumscription of Halimolobos, Mancoa, Pennellia, and Sphaerocardamum ; and (3) the study of the evolution of morphological characters within the clade. Data were generated three independent DNA sequences and 17 morphological characters for 25 of the 42 Halimolobine species. Separate and total evidence analyses identified a monophyletic core group of Halimolobos, Mancoa, Pennellia, and Sphaerocardamum species that is related to members of Arabidopsis and relatives. Sphaerocardamum was supported as monophyletic, Pennellia was paraphyletic with respect to Arabis tricornuta, and Halimolobos and Mancoa were polyphyletic. Analysis of morphological variation in combination with molecular data revealed that individual morphological characters vary in their utility for classification of the group. However, the majority of the characters provided some grouping information.; Consideration of morphological variation from new collections for the genus identified potential species boundaries problems. The second portion of this project tested the species boundaries using the increased sampling of Sphaerocardamum collections and data from morphology, genome size, chromosome number, interfertility, and allelic diversity. Results from these data support the previous circumscriptions for S. compressum and S. nesliiforme. In contrast, the results support Sphaerocardamum divaricatum as conspecific with S. macropetalum , and S. fruticulosum, S. macrum, and S. ramosum as conspecific with S. stellatum. |
