Post-Zygotic Seed Abortion in Sexual Diploid × Apomictic Tetraploid Intraspecific Paspalum Crosses

Abstract

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.

Cytological and reproductive aspects in the Caespitosa group of Paspalum

“…These triploids may, in turn, be the origin of new tetraploids (3x+x=4x) (SIENA et al, 2008). Otherwise, the 2n gamete of an additional aposporous sac can be fertilized by a male haploid gamete from a neighbouring apomictic 4x plant (2n) to establish a new apomictic 4x genotype, as was achieved experimentally in P. rufum (NORRMANN et al, 1994). bivalents; (B) diakinesis in P. redondense with 10 bivalents; (C) metaphase I in P. pleostachyum with 10 bivalents; (D) anaphase I in triploid P. pleostachyum with 12+18 chromosome distribution; (E) metaphase I in tetraploid P. pleostachyum with 2 univalents (arrowheads), 10 bivalents, 2 trivalents (white arrow) and 3 quadrivalents (black arrow) associations; (F) anaphase I with laggards chromosomes in tetraploid P. pleostachyum.…”

“…Triploids seem to be rare individuals which arise in a diploid population, probably through unreduced gametes (2n+n) or in mixed 2x-4x populations, where there are two possible origins: by unreduced gametes as in diploid populations, or by 2x·4x crosses (NORRMANN et al, 1994). Unreduced egg-cells may be functional and, when fertilized by a reduced male gamete, they give rise to triploid plants (2x+x=3x).…”

Genetic and embryological evidences of apomixis at the diploid level in Paspalum rufum support recurrent auto-polyploidization in the species

“…Paspalum rufum Nees is a classical example of such a species. Diploid races (2n = 2x = 20) reproduce sexually and are mainly self-sterile, while tetraploids (2n = 4x = 40) are pseudogamous aposporous apomicts (Norrmann et al 1994). In spite of these differing modes of reproduction diploid and tetraploid forms are morphologically similar, with crosses between them giving rise to hybrids at a very low frequency (Norrmann et al 1994).…”

Unusual chromosome numbers in Paspalum L. (Poaceae: Paniceae) from Brazil

“…and Bonpl. ex Flüggé (Honfi et al, 1990), and the repeatedly confirmed counts of 2n = 12 and 2n = 24 for P. almum Chase (Quarín, 1974;Quarín and Hanna, 1980;Dandin and Chennaveeraiah, 1983;Pozzobon and Valls, 1987;Norrmann et al, 1994;Hunziker et al, 1998;Pozzobon et al, 2008). For that reason, the discovery of several accessions of Paspalum having unusual chromosome numbers in a relatively small collection from a more limited geographical area is highly surprising, and reinforces the need of intensifying collection and characterization of new materials.…”