Chromosome pairing and its relationship to fertility in cytotypes of prairie cordgrass (Spartina pectinata Link)

Prairie cordgrass (Spartina pectinata Link) is a polyploid, perennial, C4 grass that is native to North America. The species is able to produce large amounts of biomass, and it is tolerant to a number of environmental stresses. Due to these characteristics, it is useful for purposes such as bioenergy production, wetland restoration, soil erosion control, and forage. The species is composed of three ploidy levels: tetraploids (2n=4x=40), hexaploids (2n=6x=60), and octoploids (2n=8x=80). It is hypothesized that the hexaploid cytotype originated from the tetraploid cytotype, and therefore select populations of the two cytotypes are adapted to the same area of the United States. This has provided the opportunity to study the effects of polyploidy on prairie cordgrass characteristics. Comparison of morphological traits between the two cytotypes has shown that the hexaploid may be more desirable for commercial production than the tetraploid cytotype. However, prairie cordgrass has a low seed set, which is problematic for commercial establishment and genetic improvement of the species. Polyploidy is known to affect fertility in other species, and therefore it may be the cause of reduced fertility in prairie cordgrass as well. In this study, chromosome pairing during meiosis I of microsporogenesis was analyzed in tetraploid, hexaploid, and octoploid prairie cordgrass cytotypes. Normal chromosome pairing and segregation was observed in tetraploids and octoploids whereas abnormality was observed in hexaploids. Chromosome pairing during meiosis I implied genome compositions of tetraploids, hexaploids, and octoploids were AABB, AAABBB, and AABBA’A’B’B’, respectively. Abnormality during meiosis I in the hexaploids may lead to inconsistent chromosome numbers in pollen grains, whereas normal meiosis I in the tetraploids is hypothesized to generate pollen with consistent chromosome numbers. To investigate this potential effect, pollen morphology and DNA content of pollen nuclei were analyzed in tetraploids and hexaploids. Analysis of iii pollen morphology showed that micropollen was not formed as a result of abnormal meiosis I, suggesting that abnormal meiosis I may not lead to vast differences in chromosome in pollen grains. Flow cytometric analysis of pollen nuclei DNA content revealed that the distribution of DNA content was significantly greater in hexaploids than in tetraploids, indicating that there may be a slight degree of heterogeneity in chromosome number between pollen nuclei. However, the degree of heterogeneity was lower than hypothesized. Seed set and seed germination was evaluated in hexaploids and tetraploids to study the effects of abnormal meiosis I and potential pollen heterogeneity. Seed set was slightly reduced in the hexaploids. A significant reduction in seed germination was observed in the hexaploids, though the magnitude of reduction was less than hypothesized. The results suggest that abnormal meiosis I in the hexaploid cytotype may not affect seed set to a great extent. Furthermore, improvement of seed set may be attained through selection, and the genome compositions of all prairie cordgrass cytotypes implied in this study may be useful to the selection process