Evolution of rDNA in Nicotiana allopolyploids: a potential link between rDNA homogenization and epigenetics

Background: The evolution and biology of rDNA have interested biologistsfor many years, in part, because of two intriguing processes:(1) nucleolar dominance and (2) sequence homogenization. Wereview patterns of evolution in rDNA in the angiosperm genusNicotiana to determine consequences of allopolyploidy on theseprocesses. Scope: Allopolyploid species of Nicotiana are ideal for studying rDNAevolution because phylogenetic reconstruction of DNA sequenceshas revealed patterns of species divergence and their parents.From these studies we also know that polyploids formed overwidely different timeframes (thousands to millions of years),enabling comparative and temporal studies of rDNA structure,activity and chromosomal distribution. In addition studies onsynthetic polyploids enable the consequences of de novo polyploidyon rDNA activity to be determined. Conclusions: We propose that rDNA epigenetic expression patterns establishedeven in F₁ hybrids have a material influence on the likely patternsof divergence of rDNA. It is the active rDNA units that arevulnerable to homogenization, which probably acts to reducemutational load across the active array. Those rDNA units thatare epigenetically silenced may be less vulnerable to sequencehomogenization. Selection cannot act on these silenced genes,and they are likely to accumulate mutations and eventually beeliminated from the genome. It is likely that whole silencedarrays will be deleted in polyploids of 1 million years of ageand older.     

The tertiary endodermis in barley roots: Fine structure in relation to radial transport of ions and water

Summary The presence of numerous pits containing plasmodesmata in the inner tangential wall of the tertiary endodermis in barley roots is demonstrated by electron microscopy. The pit floor is covered by a thin layer of material which is continuous with and resembles the tertiary wall. The plasmodesmatal pore is constricted at its ends so that the plasmalemma lining the pore is appressed to the desmotubule. The frequency of plasmodesmata and their cross-sectional area is estimated, and phosphate and water fluxes through them are calculated on the assumption that they represent the only communication between the cortex and the vascular tissue. The pressure gradient across the ends of the plasmodesmata necessary to support the observed water flux is calculated for limiting cases of the pore radius and the viscosity of the fluid passing through the pore.