Predictably simple: assemblages of caterpillars (Lepidoptera) feeding on rainforest trees in Papua New Guinea

Predictability in the composition of tropical assemblages of insect herbivores was studied using a sample of 35,952 caterpillars (Lepidoptera) from 534 species, feeding on 69 woody species from 45 genera and 23 families in a lowland rainforest in Papua New Guinea. Caterpillar assemblages were strongly dominated by a single species (median 48% of individuals and 49% of biomass). They were spatially and temporally constant (median normalized expected species shared (NESS) similarity between assemblages from the same host was greater than or equal to 0.85 for three sites 8-17 km apart as well as for three four-month periods of the year). Further, the median presence of species was 11 months per year. Assemblages on hosts from different families and genera were virtually disjunct (NESS similarity less than 0.05) as the caterpillars were mostly specialized to a single plant family (77% of species) and, within families, to a single genus (66% of species), while capable of feeding on multiple congeneric hosts (89% of species). The dominance of caterpillar assemblages by a small number of specialized species, which also exhibited low spatial and temporal variability, permitted robust and reliable estimates of assemblage composition and between-assemblage similarity from small samples, typically less than 300 individuals per host plant. By contrast, even considerably larger samples were insufficient for estimates of species richness. A sample of 300 individuals was typically obtained from 1,050 m(2) of foliage sampled during 596 tree inspections (i.e. a particular tree sampled at a particular time) in the course of 19 sampling days (median values from 69 assemblages). These results demonstrate that, contrary to some previous suggestions, insect herbivore assemblages in tropical rainforests have a predictable structure and, as such, are amenable to study.     

Mammalian cell cycle checkpoints: signalling pathways and their organization in space and time

The major mission of the cell division cycle is a faithful and complete duplication of the genome followed by an equal partitioning of chromosomes to subsequent cell generations. In this review, we discuss the advances in our understanding of how mammalian cells control the fidelity of these fundamental processes when exposed to diverse genotoxic insults. We focus on the most recent insights into the molecular pathways that link the sites of DNA lesions with the cell cycle machinery in specific phases of the cell cycle. We also highlight the potential of a new technology allowing direct visualization of molecular interactions and redistribution of checkpoint proteins in live cell nuclei, and document the emerging significance of live-cell imaging for elucidation of the spatio-temporal organization of the DNA damage response network.     

Dimerization of Toc-GTPases at the chloroplast protein import machinery

Import of chloroplast precursor proteins is controlled by the coordinate action of two homologous GTPases, Toc159 and Toc33, located at the cytosol-outer membrane interface. Recent studies in Arabidopsis showed that the cytosolic form of the precursor binding protein Toc159 is targeted to its receptor at the import machinery, Toc33, via heterodimerization of their GTP-binding domains. Toc33 may also form GDP-bound homodimers, as suggested by the crystal structure of its pea ortholog. Moreover, the structural data suggested that arginine 130 (Arg130) of Arabidopsis Toc33 may function as a GTPase-activating "arginine-finger" at the other monomer in the Toc33 dimer. Here, we demonstrate that Arg130 of Toc33 does not function as an Arginine-finger. A mutant, Toc33-R130A, binds and hydrolyzes GTP like the wild type. However, we demonstrate that Arg130 is involved in both homodimerization of Toc33 and in heterodimerization with the GTP-binding domain of Toc159. The dependence of Toc33 homodimerization on Arg130 is mutual, requiring the presence of Arg130 at both monomers. As the GTPase is not activated by dimerization, it may be activated independently at either monomer, possibly even before dimerization. Independent regulation of GTPase activity may serve to coordinate the interactions of the GTPases during the import of proteins into the chloroplast.     

Solution structures of stem-loop RNAs that bind to the two N-terminal RNA-binding domains of nucleolin

Nucleolin, a multi-domain protein involved in ribosome biogenesis, has been shown to bind the consensus sequence (U/G)CCCG(A/G) in the context of a hairpin loop structure (nucleolin recognition element; NRE). Previous studies have shown that the first two RNA-binding domains in nucleolin (RBD12) are responsible for the interaction with the in vitro selected NRE (sNRE). We have previously reported the structures of nucleolin RBD12, sNRE and nucleolin RBD12–sNRE complex. A comparison of free and bound sNRE shows that the NRE loop becomes structured upon binding. From this observation, we hypothesized that the disordered hairpin loop of sNRE facilitates conformational rearrangements when the protein binds. Here, we show that nucleolin RBD12 is also sufficient for sequence- specific binding of two NRE sequences found in pre-rRNA, b1NRE and b2NRE. Structural investigations of the free NREs using NMR spectroscopy show that the b1NRE loop is conformationally heterogeneous, while the b2NRE loop is structured. The b2NRE forms a hairpin capped by a YNMG-like tetraloop. Comparison of the chemical shifts of sNRE and b2NRE in complex with nucleolin RBD12 suggests that the NRE consensus nucleotides adopt a similar conformation. These results show that a disordered NRE consensus sequence is not a prerequisite for nucleolin RBD12 binding.     

Distinct versus redundant properties among members of the INK4 family of cyclin-dependent kinase inhibitors

p16(INK4a), p15(INK4b), p18(INK4c) and p19(INK4d) comprise a family of cyclin-dependent kinase inhibitors and tumor suppressors. We report that the INK4 proteins share the ability to arrest cells in G1, and interact with CDK4 or CDK6 with similar avidity. In contrast, only p18 and particularly p19 are phosphorylated in vivo, and each of the human INK4 proteins shows unique expression patterns dependent on cell and tissue type, and differentiation stage. Thus, the INK4 proteins harbor redundant as well as non-overlapping properties, suggesting distinct regulatory modes, and diverse roles for the individual INK4 family members in cell cycle control, cellular differentiation, and multistep oncogenesis.     

Individual and geographical variation in display behaviour of male harbour seals in Scotland

Studying variations in behaviour at the individual or population level enables insight into the reproductive strategies within a species. We examined individual and geographical variation in the vocal and dive behaviour of male harbour seals, Phoca vitulina, which is associated with aquatic mating. This display behaviour was recorded in the Moray Firth, Scotland, from July 1994 to 1997, and in Orkney, Scotland, during July 1998. One vocalization type was apparent in the Moray Firth and two in Orkney. Time parameters (total and pulse duration) varied between males in the population in the Moray Firth. We used both frequency and time parameters in a discriminant analysis, which showed that 73.2% of individual male vocalizations could be correctly classified; 94.6% of male vocalizations from the Moray Firth and Orkney could be correctly classified according to their geographical areas. Therefore, vocal variation was greater between geographical areas than between individuals. No individual variation was apparent between dive and surface interval durations. However, individuals varied significantly in the percentage of short surface intervals. Male harbour seals showed substantial variability in the parameters affecting their vocal and dive behaviour during the mating season. We suggest that these variations may be indicative of adaptations to varying environmental challenges influencing the reproductive strategies of discrete populations. Copyright 2000 The Association for the Study of Animal Behaviour.