Analysis of Active Nucleolus Organizing Regions in Polyploid Prairie Cordgrass (Spartina pectinata Link) by Silver Staining

Prairie cordgrass has recently gained attention as an important biotic component of stressed ecosystems. This polyploid species is distributed broadly across the U.S. More cytogenetic data are needed to investigate how increased ploidy levels influence chromosome changes and gene expression in order to understand the formation of stable cytotypes. Silver staining is the cytogenetic method commonly used to study the number and distribution of active nucleolar organizing regions (NORs) on chromosomes at metaphase, and the number and size of nucleoli in nuclei at interphase. Intensity and distribution patterns of silver stained NORs (AgNORs) reveal differential ribosomal gene expressions in plants and animals. The purpose of this study is to estimate the number of AgNORs, their locations, and their activities in metaphase chromosomes and to determine heteromorphic variation in size and number of nucleoli within interphase nuclei for tetra-, hexa-, and octoploid prairie cordgrass populations. Increases in mean numbers of AgNORs in each metaphase and interphase cell were observed as ploidy level increases. Although distribution patterns of silver stained NORs at metaphase cells of tetraand octoploids reflect changes in ploidy, the neohexaploids did not follow the pattern, indicating that active NORs were not stable in early generation of formation of neo-hexaploidy. Collectively, these results suggest that distribution patterns of silver stained NORs in metaphase and number of silver stained nucleoli in interphase cells can be used as markers to detecting chromosome variations within and among ploidy levels and to determine when ploidy levels stabilize in prairie cordgrass.