ABSTRACT
Summary Common dallisgrass (Paspalum dilatatum Poir) is an important forage grass throughout many of the warmer regions of the world, including the southern U.S.A. Apomixis has been an important factor in the origin and preservation of this species. It is a 50-chromosome natural hybrid with three genomes that pair as 20 bivalents and 10 univalents at meiosis. Because it is a pentaploid obligate apomict, plant breeders have been unsuccessful in improving the species. The apomictic barrier has not been altered by radiation or interspecific hybridization. In an effort to circumvent this apomictic barrier, a phylogenetic investigation 171was initiated to identify its progenitors, to eventually resynthesize the species, and to allow breeding progress. A sexual tetraploid, yellow-anthered biotype is closely related to common dallisgrass, and its chromosomes are homologous with 40 chromosomes of common dallisgrass. The yellow-anthered biotype was used as a cytological substitute for common in the phylogenetic program and was crossed with other Paspalum species. Two diploid species, P. intermedium Munro. ex. Morong and P. jurgensii Hackel, have genomes homologous with a yellow-anthered dallisgrass genome. The genome formulas II, JJ, and II JJ have been assigned to P. intermedium, P. jurgensii, and yellow-anthered dallisgrass, respectively. Findings to date indicate that P. intermedium and P. jurgensii or closely related species are the ancestors of yellow-anthered dallisgrass. Information gained from hybrids involving other species indicates that the I and J genomes are widespread in the genus. One or both of the genomes or evolved forms have been identified in species of the Dilatata, Paniculata, Quadrifaria, and Virgata groups but are absent in the Disticha, Notata, and Setacea groups. Common dallisgrass has the genome formula II JJ X; X is an unknown genome. It was proposed that the grass originated from a cross between a sexual tetraploid with the genome constitution II JJ and an apomictic hexaploid II JJ XX. Efforts are underway to identify the hexaploid species.
