Genome Relationships between Tetraploid and Hexaploid Biotypes of Dallisgrass, Paspalum dilatatum

A sexual, yellow-anthered, tetraploid (2n = 4X = 40) biotype of dallisgrass, Paspalum dilatatum Poir., was crossed with an apomictic, purple-anthered, hexaploid (2n = 6X = 60) dallisgrass biotype from Uruguay to determine their cytological relationship and to establish the genomic composition of the hexaploid Uruguayan biotype. Forty-one intraspecific F1 hybrids were produced and 20 were studied cytologically. Nineteen plants had 50 chromosomes and one was aneuploid with 47 chromosomes. During metaphase I, the mean chromosome associations for 50-chromosome hybrids were 10.31I, 19.83II, 0.003III, and 0.003IV. The mean meiotic associations of the aneuploid hybrid were 7.12I and 19.94II. Most bivalents occurred as rings. These findings indicate that the biotypes have two homologous genomes. Based on the genome formula (IIJJ) of the yellow-anthered biotype, the 60-chromosome Uruguayan biotype has both the I and J genomes plus a third genome and its formula may be expressed as IIJJXX, where X is the third genome. The formula of the F1 hybrids is IIJJX. The chromosome number (2n = 5X = 50) and meiotic pairing behavior (20II + 10I) of these hybrids are similar to those of apomictic common dallisgrass. These results suggest that they may have the same genomic formula, but similarity of the X genomes in the Uruguayan and common biotypes has not been proven. If they are similar, apomictic common dallisgrass probably originated from a natural cross between the sexual yellow-anthered biotype or a closely related sexual tetraploid relative and the apomictic, hexaploid Uruguayan biotype.