||Big bluestem (Andropogon gerardii Vitman) occurs as small, isolated populations in old field edges in the Piedmont of South Carolina. We collected seeds from several of these populations in Oconee County, South Carolina. These seedlots germinated very poorly in field
plantings and in standard germination tests. Upon physical xamination of seedlots, we discovered that most caryopses contained no filled seeds or severely shriveled seeds. Flowering appeared normal, with ample pollen and visible stigmas. The source populations were all small in extent and were all isolated by hundreds of meters to several kilometers. We undertook to establish the cause of the failure to produce normal seeds. Big bluestem is an obligate outcrossing species. We hypothesized that individual small populations comprised very similar genotypes or even clonal plants as a result of vegetative spread. Thus, small isolated populations might have little fertilization, due to lack of outcross pollen.
To test for the need for outcrossing for viable seed, we grew plants from each of four isolated populations (GV, CD, H28, and BC) in the following planting patterns: a) each plant isolated; b) each paired with a plant from one of the other populations; and c) plants from all four populations grouped in close (< 1 meter) proximity. We tested resulting caryopses for seed fill. Isolated individual plants produced < 1% filled seeds, significantly fewer (P = 0.05) than
grouped plants, with an average of 6% filled seeds. The paired plants averaged 3% filled seed, which was not different from the other two treatments. The germination percentage of filled seeds ranged from 18 to 45%. In comparison, a seedlot from a large (2 hectares) big bluestem population from Buck Creek, North Carolina, germinated at near 60%. The almost total lack of seed production in isolated individuals supports the obligate outcrossing nature of big bluestem.
We also tested for genetic diversity of shoots within populations and between populations via RAPD-PCR analysis of nuclear DNA. Newly expanding leaves were removed from widely spaced individual plants in the H28 and DC populations and extracted for nuclear DNA, which
was subjected to RAPD analysis. We analyzed the RAPD data for the two populations with Principal Component Analysis (PCA). Eigenvalues for the two most powerful principal components (60% of the total variance) were plotted in a two-dimensional graph. PC 1 explained
34% of the variance and generally separated the two populations genetically. On this axis, the two populations appeared as two distinctly different groupings. On the PC 2 axis, each of the two
populations appeared as a tight grouping, indicating little to no genetic variation in the individual populations. There was one significant outlier in the DC population data. These results indicated
that individual plants within populations are very closely related genetically, while separate populations can vary significantly in genetic makeup. These data indicate that unsuccessful seed production in isolated populations arose from lack of outcrossing among the individuals in these populations. Wide genetic differences between isolated populations suggests the opportunity to increase viable seed production by building composite populations with plants from a large number of isolated populations.