As currently circumscribed, Sphaerocardamum (Brassicaceae) contains eight species that are endemic to limestone soils of Mexico’s Chihuahuan desert. Populations of Sphaerocardamum are small, scattered, and difficult to find, which has resulted in poor representation of the genus in herbaria. Previous authors have considered the current species limits relatively tentative and the present project was developed to address the species boundaries within the genus as part of a systematic study of Sphaerocardamum and its relatives. Field studies were conducted to obtain new material from known localities as well as from regions of the Chihuahaun desert from which Sphaerocardamum had not been previously collected. Data from crossing studies, DNA sequences (chloroplast trnL intron - trnL/F spacer, nrDNA ITS, and pistillata intron 1), flow cytometry, meiotic chromosome counts, and morphology were generated to address these problems. The results suggest that the genus should be reduced from eight species to a maximum of four and that some interspecific gene flow may have occurred. The results identify that these are diploid taxa with genome sizes ranging from 0.35-0.42 pg/2C. The genome sizes match species boundaries based on morphological and molecular data.

Key words: Cruciferae, Cytology, Cytometry, Genome, Molecular, Sphaerocardamum