2n+n hybridization of apomictic Paspalum dilatatum with diploid Paspalum species

Common dallisgrass (Paspalum dilatatum) is an apomictic pentaploid (2n=5x=5 0) of hybrid origin with irregular meiosis and with the genome formula IIJJ X. The I and J genomes are homologous to those of diploid P. intermedium an d P. jurgensii, respectively, but the source of the X genome is unknown. Me mbers of the X genome may have genes of special biological significance, in cluding those controlling apomixis. Common dallisgrass was crossed with sev eral diploid Paspalum species in an attempt to identify the source of the X genome. Since common dallisgrass is apomictic, all hybrids produced will b e formed by fertilization of an unreduced egg (2n+n). Any hybrid showing 30 chromosome bivalents at meiosis would indicate that the male diploid paren t has a chromosome set that is homologous to the X genome of dallisgrass. O ver 36,000 spikelets of dallisgrass were emasculated and dusted with pollen of 15 different diploid species (diploid species bearing I or J genomes we re excluded). Only five (P. chaseanum, P. equitans, P. fasciculatum, P. not atum, and P. simplex) produced 2n+n hybrids with P, dilatatum. Meiotic chro mosome behavior was similar in all hexaploid hybrids showing ca. 20 bivalen ts and 20 univalents. Results indicated a very low rate of 2n+n hybridizati on; none of the five diploid species possessed the X genome. Because severa l diploid species failed to hybridize with 5x dallisgrass, other methods sh ould be attempted. Molecular markers specific for the X genome may help sol ve the question.