Cytology and reproductive behavior of hybrids between Paspalum urviUei and two hexaploid P. dilatatum biotypes

Sexual, tetraploid (2n = 4x = 40) vaseygrass (Paspalum urvillei Steud.) was crossed with two apomictic, hexaploid (2n = 6x = 60) dallisgrass (P. dilatatum Poir.) biotypes (Uruguayan and Uruguaiana) to determine their relationships and the reproductive behavior of the hybrids. One vaseygrass x Uruguayan and two vaseygrass x Uruguaiana hybrids were studied and all had 2n = 5x = 50 chromosomes. The mean pairing meiotic association for the vaseygrass x Uruguayan hybrid was 10.18 I and 19.91 11, and for the two vaseygrass x Uruguaiana hybrids was 10.28 I, 19.82 11, 0.006 111, and 0.01 IV. These findings indicate that the parents of both hybrids have two homologous genomes. Because vaseygrass and Uruguayan dallisgrass have the genome formulas IIJJ and IIJJXX, respectively, their hybrid has the formula IIJJX and the two homologous genomes are the I and J genomes. Likewise, chromosome pairing in the vaseygrass x Uruguaiana hybrids demonstrates that both species have two similar genomes, I and J. The Uruguaiana biotype usually has 8-10 univalents per cell and these result from the failure of some members of the third genome to pair. Thus the Uruguaiana biotype has different forms of the X genome, and its genome formula may be designated as IIJJXX,. Cytologically all three hybrids were facultative apomicts but their uniform F, progeny indicates that only the apomictic embryo sacs are functional. Seed fertility ranged from 10.8 to 16.2%.