Inhibition of RNA Synthesis and the Relationship between Nucleolar and Mitotic Cycles in Zea mays Root Meristems

The relationship between nucleolar and mitotic cycles has beendetermined after treatment of root apices of Zea mays with ethidiumbromide. In the meristematic regions of the stele the two cyclesare not much displaced in relation to each other except fora delay in the onset of the disorganization phase. A few nucleolipersist into metaphase and a few nuclei undergo an amitoticdivision. In the cap initials the drug greatly delays the onsetof disorganization of the nucleolus, which normally occurs beforeprophase in this region. It also delays the completion of reorganizationso that fully organized nucleoli are no longer available duringthe last half of telophase. In the quiescent centre the onsetof disorganization and the end of reorganization of the nucleoliare also delayed in relation to mitosis. There is no evidencefor a delay in the onset of reorganization in any region ofthe meristem. Some cells form multiple micronucleoli and this aberrant behaviouroccurs more often in the cap initials than elsewhere as doesamitotic division.     

Chemical structures in a lignin-carbohydrate complex isolated from the bovine rumen

The soluble, lignin-carbohydrate complex (LCC) from the rumen fluid of steers fed a diet of pure spear grass (Heteropogon contortus) has been purified by gel filtration. The purified LCC contained 7.4% of carbohydrate which, on hydrolysis, gave d-glucose, d-xylose, l-arabinose, l-rhamnose, and traces of d-galactose and d-mannose. The structure of the LCC was examined by methylation analysis, using g.l.c.-m.s. for the unequivocal classification of the sugar derivatives. d-Glucose, d-xylose, and l-rhamnose were shown to be glycosidically linked to lignin. Some of the d-glucosyl residues carry other (1→4)-linked d-glucose units, and some of the d-xylosyl residues bear other (1→4)-linked d-xylose units and (1→3)-linked l-arabinofuranosyl groups. The major carbohydrate component is a single d-glucopyranosyl group. The LCC was subjected to various chemical treatments in an investigation of the chemical nature of the bonding between lignin and the carbohydrates. d-Glucose could be enzymically hydrolyzed from the LCC, but only with a very high concentration of β-d-glucosidase. The presence of lignin in rumen LCC has been confirmed by nitrobenzene oxidation, vanillin and syringaldehyde being identified by g.l.c.-m.s. as oxidation products from both the original spear grass and the LCC.     

Persistent nuclear ribosomal DNA sequence polymorphism in the Amelanchier agamic complex (Rosaceae)

Individual plants of several Amelanchier taxa contain many polymorphic nucleotide sites in the internal transcribed spacers (ITS) of nuclear ribosomal DNA (nrDNA). This polymorphism is unusual because it is not recent in origin and thus has resisted homogenization by concerted evolution. Amelanchier ITS sequence polymorphism is hypothesized to be the result of gene flow between two major North American clades resolved by phylogenetic analysis of ITS sequences. Western North American species plus A. humilis and A. sanguinea of eastern North America form one clade (A), and the remaining eastern North American Amelanchier make up clade B. Five eastern North American taxa are polymorphic at many of the nucleotide sites where clades A and B have diverged and are thought to be of hybrid origin, with A. humilis or A. sanguinea as one parent and various members of clade B as the other parent. Morphological evidence suggests that A. humilis is one of the parents of one of the polymorphic taxa, a microspecies that we refer to informally as A. "erecta." Sequences of 21 cloned copies of the ITS1- 5.8S gene-ITS2 region from one A. "erecta" individual are identical to A. humilis sequence or to the clade B consensus sequence, or they are apparent recombinants of A. humilis and clade B ITS repeats. Amelanchier "erecta" and another polymorphic taxon are suspected to be relatively old because both grow several hundred kilometers beyond the range of one of their parents. ITS sequence polymorphisms have apparently persisted in these two taxa perhaps because of polyploidy and/or agamospermy (asexual seed production), which are prevalent in the genus.