Andropogon gerardii, big bluestem, has 60 and 90 chromosome cytotypes. Meiosis in the hexaploid was shown to be regular, although some secondary associations of bivalents form. Meiosis in the enneaploid (2n = 9x = 90) is irregular, leading to most gametes having unbalanced chromosome complements. Both cytotypes show considerable self‐incompatibility. Cytotypes crossed freely, forming a variety of fertile euploids and aneuploids. Indistinguishable exomorphology, intermixing in natural populations, and compatibility suggest that A. gerardii is best understood as a cytotypically complex single species.

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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.

In north‐eastern Argentina, Paraguay and south‐eastern Brazil, morphologically intermediate plants involving Andropogon lateralis, A. bicornis, A. glaziovii, A. arenarius and A. hypogynus were found. The possibility that they were natural hybrids was tested in two ways: (1) where they were sterile, their morphology was compared with that of the putative parents, and their meiosis and reproductive behaviour were studied; (2) where they were fertile, studies of artificial hybrids were also made. Most of the hybrids were sterile. The only fully fertile combination, generating recombination and hybrid swarms, was A. lateralis × A. hypogynus. In spite of apparently normal chromosome pairing, fertility was low in all other combinations on both the male and female sides. Sterility is probably a result of ‘cryptic’ or ‘gametic sterility’, which produces complete sterility of the gametes. Many of the hybrids survive and compete successfully with the parental species in natural populations, but their sterility maintains the genetic isolation of the majority of the taxa involved. Meiotic chromosome behaviour in all the hybrids indicates that the group of species shares slightly different forms of three basic genomes. Several specimens of natural hybrids were found in historical herbarium collections. In the past, they were given the status of type specimens of at least five taxonomic entities (A. lindmanii, A. coloratus, A. lateralis var. subtilior, A. multiflorus and A. lateralis var. bogotensis). The taxonomic consequences of these findings are discussed. © 2009 The Linnean Society of London, Botanical Journal of the Linnean Society, 2009, 159, 136–154.