Evolutionary implications of meiotic Chromosome Behavior, Reproductive Biology, and Hybridization in 6X and 9X Cytotypes of Andropogon gerardii (Poaceae)

Abstract

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.

Impact of Intraspecific Polyploidy in Andropogon gerardii (Poaceae) Populations

“…Polyploidy within populations of a single species is relatively common (Lewis, 1980;Lumaret, 1988a;Keeler, 1997). Theoretical considerations suggest that mixed ploidy populations should be unstable, eventually fixing one cytotype (Fowler and Levin, 1984;Bever and Felber, 1993;Rodríguez, 1996a, b;Ramsey and Schemske, 1998).…”

Population biology of intraspecific polyploidy in grasses

“…Dactylis glomerata and Andropogon gerardii, both autopolyploid complexes, show interploidy level fertility (i.e. gene flow) (Zohary & Nur, 1959;Lumaret, 1988b;Norrmann, Quarin & Keeler, 1997). In Holcus, allopentaploids produce fertile diploid progeny (Richard et al, 1995).…”

“…In the case of Zohary & N ur's study, this logical argument was supported by the presence of B chromosomes in the progeny of the triploids, because in the Israeli populations studied, B chromosomes were found only in diploid plants. Similarly inter-cytotype (6x X 9x) hybrids of Andropogon gerardii form a potential bridge between the ploidy levels of that species because they have some pollen and seed fertility, despite being largely aneuploid (Norrmann et al, 1997).…”

“…For most intraspecific polyploid complexes studied, ploidy levels cannot be distinguished at the individual level with sufficient accuracy for taxonomic distinctions (Hedberg, 1967;Rothera & Davy 1986;Norrmann et al, 1997). Populations of different ploidy levels often differ, but there is such extensive overlap, especially by the higher ploidy levels, that individuals are difficult to categorize.…”

“…Much work needs to be done to understand the implication of these consequences of polyploidy to mixed populations and to adaptive evolution of grasses. Andropogon gerardii, for example, is an ecosystem dominant despite cytotypic variation that should lower fitness (Keeler, 1990;Norrmann et al, 1997) (Fig. 7.2).…”