Developmental changes in the secondary xylem ofAcer rubrum induced by various auxins and 2,3,5-tri-iodobenzoic acid

Summary TIBA has been applied laterally to actively growing stems of uprightAcer rubrum seedlings. The frequency of initiation of tracheary elements is reduced and a complete ring of tension wood is developed in the stem locally below the TIBA application site. Rings of tension wood were never formed above the TIBA treatment site. In regard to anatomy, lignin distribution and peroxidase activity, the tension wood fibers formed as a result of TIBA treatment are identical to those which can be induced by bending.In the region of the stem above the site of TIBA application there is a particularly strong alteration in the development of tracheary elements.Application of IAA, NAA, or 2,4-D to the TIBA treatment site suppresses the capacity of TIBA to induce the development of tension wood and at the same time generally increases the frequency of initiation of tracheary elements.The effect of auxin alone on theAcer rubrum system has been studied. The secondary xylem formed during treatment with auxins (especially 2,4-D and NAA) at the stated concentrations is generally characterized by large groups of tracheary elements with a conspicuous angular outline in transverse section.The evidence suggests that auxins are involved in the regulatory systems which bring about the orderly development of the secondary xylem in arborescent angiosperms.This material was included in a doctoral thesis submitted by P. R.Morey to the graduate school of Yale University, New Haven.     

THE FINE STRUCTURE OF DIFFERENTIATING XYLEM ELEMENTS

Differentiating xylem elements of Avena coleoptiles have been examined by light and electron microscopy. Fixation in 2 per cent phosphate-buffered osmium tetroxide and in 6 per cent glutaraldehyde, followed by 2 per cent osmium tetroxide, revealed details of the cell wall and cytoplasmic fine structure. The localized secondary wall thickening identified the xylem elements and indicated their state of differentiation. These differentiating xylem elements have dense cytoplasmic contents in which the dictyosomes and elements of rough endoplasmic reticulum are especially numerous. Vesicles are associated with the dictyosomes and are found throughout the cytoplasm. In many cases, these vesicles have electron-opaque contents. "Microtubules" are abundant in the peripheral cytoplasm and are always associated with the secondary wall thickenings. These microtubules are oriented in a direction parallel to the microfibrillar direction of the thickenings. Other tubules are frequently found between the cell wall and the plasma membrane. Our results support the view that the morphological association of the "microtubules" with developing cell wall thickenings may have a functional significance, especially with respect to the orientation of the microfibrils. Dictyosomes and endoplasmic reticulum may have a function in some way connected with the synthetic mechanism of cell wall deposition.     

Effects of cytochalasin D on the distribution of actin and ACTH-induced steroidogenesis in cultured embryonic adrenal gland cells from the Pekin duck (Anas platyrhynchos)

Cultured steroidogenic cells derived from the adrenal glands of duck embryos were used to study changes in the distribution of actin associated with the corticotropic responsiveness. Actin-containing components were identified by rhodamine-phalloidin staining. The actin in most of the unstimulated cells occurred as stress fibers that either ran parallel throughout the cell or were present as domains of parallel fibers at angles to one another. When incubated in Krebs-Henseleit buffer containing 1–24 ACTH, the cells released approximately equal amounts of corticosterone and aldosterone. Incubation of the cells in buffer containing cytochalasin D caused the cells to lose their stress fibers, and the actin became distributed at the periphery in what appeared to be fragments of stress fibers and clumps of fibrous material in the central cytoplasm. Although cytochalasin D did not affect the basal output of corticosterone and aldosterone, the 1–24 ACTH-induced rates of both hormones were suppressed significantly. After the cells had been washed in unadulterated buffer, the normal distribution of actin stress fibers was restored and the cells responded normally when incubated in buffer containing 1–24 ACTH. These results suggest that the actin components of the cytoskeleton are important determinants of corticotropin-induced steroidogenic responsiveness.     

Phylogeny of Greya (Lepidoptera: Prodoxidae), based on nucleotide sequence variation in mitochondrial cytochrome oxidase I and II: congruence with morphological data

The phylogeny of Greya Busck (Lepidoptera: Prodoxidae) was inferred from nucleotide sequence variation across a 765-bp region in the cytochrome oxidase I and II genes of the mitochondrial genome. Most parsimonious relationships of 25 haplotypes from 16 Greya species and two outgroup genera (Tetragma and Prodoxus) showed substantial congruence with the species relationships indicated by morphological variation. Differences between mitochondrial and morphological trees were found primarily in the positions of two species, G. variabilis and G. pectinifera, and in the branching order of the three major species groups in the genus. Conflicts between the data sets were examined by comparing levels of homoplasy in characters supporting alternative hypotheses. The phylogeny of Greya species suggests that host-plant association at the family level and larval feeding mode are conservative characters. Transition/transversion ratios estimated by reconstruction of nucleotide substitutions on the phylogeny had a range of 2.0-9.3, when different subsets of the phylogeny were used. The decline of this ratio with the increase in maximum sequence divergence among taxa indicates that transitions are masked by transversions along deeper internodes or long branches of the phylogeny. Among transitions, substitutions of A-->G and T-->C outnumbered their reciprocal substitutions by 2-6 times, presumably because of the approximately 4:1 (77%) A+T-bias in nucleotide base composition. Of all transversions, 73%-80% were A<-->T substitutions, 85% of which occurred at third positions of codons; these estimates did not decrease with an increase in maximum sequence divergence of taxa included in the analysis. The high frequency of A<-->T substitutions is either a reflection or an explanation of the 92% A+T bias at third codon positions.