POST-ZYGOTIC SEED ABORTION IN SEXUAL DIPLOID X APOMICTIC TETRAPLOID INTRASPECIFIC PASPALUM CROSSES

In many species of Paspalum, diploid (2x) cytotypes are usually outbre eders due to a self incompatibility system, while tetraploid (4x) cons pecific counterparts are pseudogamous self-fertile apomicts. Intraspec ific crossability between 2x female and 4x male cytotypes was investig ated using classical methods of crossing in P. almum, P. brunneum, P. rufum, P. intermedium and P. quadrifarium. Expected triploid B-II hybr ids were obtained in P. intermedium (crossability: 0.004%) and in P. b runneum (crossability: 0.015%). In P. rufum, only tetraploid B-III hyb rids were achieved with a crossability of 0.025% Looking for a better performance, in vitro ovary rescue was attempted 5 days after pollinat ion in intraspecific 2x x 4x crosses of P. almum, P. quadrifarium, P. intermedium and P. rufum. The method was useful to recover some triplo id B-II hybrids in P. almum (success rate: 0.49%) and in P. rufum (0.5 9%), but failed in P. inter-medium and P. quadrifarium. Pollen tube gr owth was usually inhibited at the stigma or style levels after self-po llination in 2x plants, confirming that diploids are self-incompatible . Pollen of 4x cytotypes germinated and penetrated the pistils of dipl oid conspecific cytotypes. This indicated that no incompatibility syst em exists in these species to keep the 2x cytotypes from hybridising w ith 4x conspecific cytotypes as pollen donors. However, after pollinat ion of 2x cytotypes with pollen of 4x counterparts, most ovules showed embryo and endosperm development, but a few days later, endosperms co llapsed and embryos stopped their development. As a result of these pr ocesses, sexual self-incompatible 2x cytotypes of Paspalum species pro duced very few triploid hybrids when pollinated with pollen of their a pomictic 4x counterparts. Thus, the low diploid-tetraploid crossabilit y was due to the existence of a post-zygotic abortive system and not c aused by pre-fertilisation barriers.