Thrasya petrosa (Trin.) Chase is the most widespread species of a grass genus indigenous to the New World. Genetic systems in diploid (2n = 2x = 20) and tetraploid (2n = 4x = 40) races of T. petrosa were investigated. The diploid race exhibited embryological development typical of sexual reproduction, but failed to produce seed because of self-incompatibility, whereas the tetraploid showed embryological pathways characteristic of facultative apomixis. Consequently, some ovules showed a normal meiotic embryo sac, others had one to several aposporous sacs, and, finally, some ovules had one or more aposporous sacs beside the meiotic one. A uniform progeny test assisted by molecular markers confirmed that the main reproductive mode for the tetraploid race was apomixis, despite some sexual reproductive structures observed by cytoembryological analyses. The chromosome pairing patterns at meiosis suggested that autoploidy was the genetic origin of the tetraploid races of T. petrosa. In addition, the close relationship between Thrasya Kunth and Paspalum L. previously supported by phylogenetic analyses is further sustained by the particular genetic system shared by the two genera. The system involves co-specific sexual self-incompatible diploids and apomictic, pseudogamous and self-compatible polyploids.