Ploidy Variation and Reproductive Pathways In Upland Switchgrass.

Many of the plant species that have been targeted for biofeedstock development have complex, polyploid genomes with limited prior research devoted to them. Switchgrass is a prime example of this. We employed a combination of approaches, including flow cytometry, classic cytology and molecular cytogenetics, to gain a better understanding of the extreme variation in chromosome numbers found in this species. Knowledge of chromosome-number variation in our key germplasm will be critical for the interpretation of genetic marker data, genetic mapping and breeding efforts that rely on marker-assisted selection.

Polyploidy and reproductive biology are linked by the underlying mechanism of unreduced gamete formation, that is, the production of eggs and/or sperm with the somatic chromosome number. This one alteration in the outcome of meiosis can have profound effects on the reproductive success of the individual plant and on the overall population structure, affecting gene flow and the distribution of genetic diversity. As a follow-up to our prior research on switchgrass ploidy and aneuploidy, we initiated a study of the reproductive pathways in tetraploids and octoploids, examining the ploidies and genetic relatedness of maternal parent plants (both 4X and 8X) and their offspring (seeds). A flow-cytometric seed screen (FCSS; Matzke et al. 2000.Plant Journal) was carried out to compare the ploidies of the embryo and endosperm cell populations in seeds with the ploidy of the parent. Sets of seed were germinated and grown up to confirm the FCSS analysis and will be genotyped to examine the apomictic versus sexual nature of their origin. A better understanding of the reproductive biology of this species will provide the foundation for more efficient breeding programs, as well as, improved analysis and interpretation of the sequence data being generated by ongoing genomics projects.