Ecology and management of mahogany (Swietenia macrophylla King) in the Chimanes Forest, Bed, Bolivia

Mahogany ( Swietenia macrophylla King) regenerates in areas of erosion on high terraces and in forest killed by flooding and deposition of alluvial sediments in the Chimanes Forest, Bolivia. These hydrological disturbances are patchy, and only one of five stands of mahogany that we inventoried was regenerating. Mahogany survives these disturbances significantly better than the common tree species. The long time between disturbances appears to favour late maturation. Mahogany trees allocate little photosynthates to reproduction until they are very large emergents, at least 80 cm in diameter. The episodic nature of the regeneration sites means that mahogany stands are composed of one or a few cohorts, which are vulnerable to overharvesting, particularly with the current use of a minimum cutting diameter to regulate harvest. The delayed onset of fecundity means that the small trees that escape harvest are not very fecund, resulting in minimal seed input to logged forest. Only 7–9% of the gaps created by logging contain natural regeneration after 20 + yr. A successful management plan for mahogany would entail a monocyclic harvest, with a rotation age of 100 + years, the estimated time that it takes for trees to achieve commercial size in natural forest. Since the number of seed trees that will be left is small, they should be concentrated in sites that are likely to be conducive to natural regeneration, such as near rivers and flood damaged forest. Seed production will be maximized for a given basal area (opportunity cost to loggers) if trees c. 110 cm dbh are selected as seed trees. The mahogany stocks in the Chimanes Forest are nearly exhausted, but the findings of this study could be used to help rebuild the mahogany populations, or to design management plans for the commercial species that have similar ecologies to mahogany.     

The C57BL/6 nude, beige mouse: a model of combined T cell and NK effector cell immunodeficiency

This article describes the construction and establishment of a double congenic nude, beige C57BL/6 (B6 nu, bg) mouse strain. The mice do not show higher fragility than C57BL/6 nude mice and the double congenic strain can be maintained under conventional mouse housing conditions. Although the B6 nu, bg display a very low natural killer activity which cannot be enhanced by an interferon inducer (poly(I-C], they lack responsiveness to a T cell mitogen (concanavalin A); and they also show extremely low responsiveness to a B cell mitogen (0128: B12 Escherichia coli lipopolysaccharide) probably as a result of combined effects of the beige and nude genes in the C57BL/6 genetic context.     

Heterogeneous binding of high mobility group chromosomal proteins to nuclei

A dramatic difference is observed in the intracellular distribution of the high mobility group (HMG) proteins when chicken embryo fibroblasts are fractionated into nucleus and cytoplasm by either mass enucleation of cytochalasin-B-treated cells or by differential centrifugation of mechanically disrupted cells. Nuclei (karyoplasts) obtained by cytochalasin B treatment of cells contain more than 90 percent of the HMG 1, while enucleated cytoplasts contain the remainder. A similar distribution between karyoplasts and cytoplasts is observed for the H1 histones and the nucleosomal core histones as anticipated. The presence of these proteins, in low amounts, in the cytoplast preparation can be accounted for by the small percentage of unenucleated cells present. In contrast, the nuclei isolated from mechanically disrupted cells contain only 30-40 percent of the total HMGs 1 and 2, the remainder being recovered in the cytosol fraction. No histone is observed in the cytosol fraction. Unike the higher molecular weight HMGs, most of the HMGs 14 and 17 sediment with the nuclei after cell lysis by mechanical disruption. The distribution of HMGs is unaffected by incubating cells with cytochalasin B and mechanically fractionating rather than enucleating them. Therefore, the dramatic difference in HMG 1 distribution observed using the two fractionation techniques cannot be explained by a cytochalasin-B-induced redistribution. On reextraction and sedimentation of isolated nuclei obtained by mechanical cell disruption, only 8 percent of the HMG 1 is released to the supernate. Thus, the majority of the HMG 1 originally isolated with these nuclei, representing 35 percent of the total HMG 1, is stably bound, as is all the HMGs 14 and 17. The remaining 65 percent of the HMGs 1 and 2 is unstably bound and leaks to the cytosol fraction under the conditions of mechanical disruption. It is suggested that the unstably bound HMGs form a protein pool capable of equilibrating between cytoplasm and stably bound HMGs.     

Capping revisited: II. Low sensitivity to sulfydryl reagents

While thiols had been shown to either delay capping of lymphocyte membrane components or to leave it unaffected, sulfydryl reagents can block the process. However, capping is not completely blocked by doses of SH reagents below 10^?3M and thus, appears to be particularly resistant to SH poisoning. The extent of inhibition is concentration dependent, and in the lower dose range, some facilitation of capping can even be obtained. The effects of the drugs on the capping process actually depend on their reaction with SH radicals as shown by competition with free thiols. Some of the SH reagents used here are known inhibitors of lymphocyte triggering. There is no direct correlation, however, namely, there is in general rather little inhibition of capping at doses of SH reagents sufficient to completely abolish triggering.