The common races of bahiagrass, Paspalum notatum, are tetraploid (2n=4x=40) and reproduce by aposporous apomixis. Paspalum notatum var. saurae is the corresponding diploid (2n=2x=20) sexual race that outbreeds due to self-incompatibility. Chromosome doubling was induced by colchicine treatments in three individual plants from a natural diploid population. Embryological studies demonstrated that one of the induced autotetraploid plants reproduced sexually. The other two autotetraploids were facultative apomicts. These results indicate that an unexpressed gene(s) for apomixis exists at the diploid level. The expression of the trait is ploidydependent. The ploidy dependency may act either on the locus controlling apomixis through some transcription factors or via a secondary locus which requires a higher allele dosage to affect the expression of the main locus.

In many species of Paspalum, diploid (2x) cytotypes are usually outbreeders due to a self incompatibility system, while tetraploid (4x) conspecific counterparts are pseudogamous self-fertile apomicts. Intraspecific crossability between 2x female and 4x male cytotypes was investigated using classical methods of crossing in P. almum, P. brunneum, P. rufum, P. intermedium and P. quadrifarium. Expected triploid B,, hybrids were obtained in P. intermedium (crossability: 0.004%) and in P. brunneum (crossability: 0.015%). In P. rufum, only tetraploid B, , hybrids were achieved with a crossability of 0.025%. Looking for a better performance, in vitro ovary rescue was attempted 5 days after pollination in intraspecific 2x x 4x crosses of P. almum, P. quadrifarium, P. intermedium and P. rufum. The method was useful to recover some triploid B,, hybrids in P. almum (success rate: 0.49%) and in P. rufum (0.59%), but failed in P. intermedium and P. quadrifarium. Pollen tube growth was usually inhibited at the stigma or style levels after self-pollination in 2x plants, confirming that diploids are self-incompatible. Pollen of 4x cytotypes germinated and penetrated the pistils of diploid conspecific cytotypes. This indicated that no incompatibility system exists in these species to keep the 2x cytotypes from hybridising with 4x conspecific cytotypes as pollen donors. However, after pollination of 2x cytotypes with pollen of 4x counterparts, most ovules showed embryo and endosperm development, but a few days later, endosperms collapsed and embryos stopped their development. As a result of these processes, sexual self-incompatible 2x cytotypes of Paspalum species produced very few triploid hybrids when pollinated with pollen of their apomictic 4x counterparts. Thus, the low diploidtetraploid crossability was due to the existence of a post-zygotic abortive system and not caused by prefertilisation barriers.

No abstract

No abstract

No abstract