Homology of chromosomes of the X genomes in common and Uruguayan dallisgrass, Paspalum dilatatum

Two biotypes of dallisgrass, Paspalum dilatatum Poir., designated common and Uruguayan, have chromosome numbers and genome formulas of 2n = 5x = 50 (IIJJX) and 2n = 6x = 60 (IIJJXX), respectively. The relationship between the X genomes in these two biotypes is unknown, and each was arbitrarily assigned the letter X to designate an unknown genome. This study was undertaken to determine the relationship between the X genomes in these two biotypes. Because both biotypes are apomicts and cannot be crossed, a sexual intraspecific F 1 hybrid (2n = 45) between sexual yellow-anthered (2n = 4x = 40; IIJJ) and common dallisgrass biotypes was crossed with Uruguayan dallisgrass. This F 1 hybrid has complete sets of the I and J genomes but only 5 of the 10 chromosomes of the X genome from common dallisgrass. Two hybrids were recovered. One had 52 and the other had 53 chromosomes, which associated at metaphase I as 22 bivalents + 8 univalents and 23 bivalents + 7 univalents, respectively. Twenty bivalents represent pairing of members of the I and J genomes, and those in excess of 20 represent pairing between members of the two X genomes. The remaining members of the X genome from the Uruguayan biotype were present as univalents at metaphase I. This demonstrates that those chromosomes of the X genome from the common biotype that were present are homologous to members of the X genome of the Uruguayan biotype. Both hybrids are aposporous facultative apomicts with some sterility.