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.
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.
Paspalum rufum is a taxonomically well defined species which includes two cytotypes: sexual self‐incompatible diploids (2n = 2x = 20), and apomictic, pseudogamous, and self‐fertile tetraploids (2n = 4x = 40). The two cytotypes are morphologically very similar. The hypothesis of autoploidy in 4x apomictic strains was cytologically tested using 3x and 4x interploid hybrids as well as an induced autotetraploid plant. Controlled 2x × 4x crosses produced one 3x hybrid (BII, n + n) and two 4x hybrids (BIII, 2n + n). The autotetraploid plant was induced from the diploid strain by colchicine treatment. At meiosis the triploid BII hybrid showed a mean of 7.4 trivalents per pollen mother cell (PMC) and up to 9 trivalents were present in 28% of the PMCs. This indicates a strong homology between genomes of the 2x and the 4x parents. Moreover, the means of quadrivalent chromosome associations per PMC were similar in the natural tetraploid (2.6), the induced autotetraploid (3.1), and the BIII tetraploid hybrids (3.6) suggesting that natural tetraploid strains originated by autoploidy. These results are in agreement with previous findings and support the hypothesis that autoploidy has played an important role in the origin and evolution of apomixis in warm‐season grasses.
In many North American prairies, populations of Andropogon gerardii Vitman (Poaceae) are composed of hexaploid and enneaploid cytotypes (2 n = 60, 90), with intermediates occurring occasionally. Under controlled pollination, the two common cytotypes can be crossed, producing progeny with a range of chromosome numbers. In an investigation of fertility and compatibilities of intermediate cytotypes, individuals with chromosome numbers between 60 and 90 were crossed with each other, with the 2 n = 60 and 90 cytotypes, and with South American Andropogon species having 60 chromosomes. Regardless of cytotype, all A. gerardii plants had some fertility and virtually all crosses produced seeds. Cytotype is only partially predictive of fertility. Inter-specific hybrids between A. gerardii and South American hexaploid species were vigorous but sterile. Gene flow in natural A. gerardii populations of mixed cytotype probably involves plants of all cytotypes.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.