Morphological shifts in island-dwelling birds: the roles of generalist foraging and niche expansion

Abstract Passerine birds living on islands are usually larger than their mainland counterparts, in terms of both body size and bill size. One explanation for this island rule is that shifts in morphology are an adaptation to facilitate ecological niche expansion. In insular passerines, for instance, increased bill size may facilitate generalist foraging because it allows access to a broader range of feeding niches. Here we use morphologically and ecologically divergent races of white-eyes (Zosteropidae) to test three predictions of this explanation: (1) island populations show a wider feeding niche than mainland populations; (2) island-dwelling populations are made up of individual generalists; and (3) within insular populations there is a positive association between size and degree of foraging generalism. Our results provide only partial support for the traditional explanation. In agreement with the core prediction, island populations of white-eye do consistently display a wider feeding niche than comparative mainland populations. However, observations of individually marked birds reveal that island-dwelling individuals are actually more specialized than expected by chance. Additionally, neither large body size nor large bill size are associated with generalist foraging behavior per se. These latter results remained consistent whether we base our tests on natural foraging behavior or on observations at an experimental tree, and whether we use data from single or multiple cohorts. Taken together, our results suggest that generalist foraging and niche expansion are not the full explanation for morphological shifts in island-dwelling white-eyes. Hence, we review briefly five alternative explanations for morphological divergence in insular populations: environmental determination of morphology, reduced predation pressure, physiological optimization, limited dispersal, and intraspecific dominance.     

Immortalisation of human ovarian surface epithelium with telomerase and temperature-sensitive SV40 large T antigen

Epithelial ovarian cancer is the most common form of gynaecological malignancy. This lethal disease is thought to arise in ovarian surface epithelial (OSE) cells. The biology of these cells is not well understood, due to the limited amount of tissue that can be obtained from a single biopsy and their limited life span in culture. To overcome these problems, we have conditionally immortalised OSE cells with the catalytic subunit of telomerase (hTERT) and a temperature-sensitive form of SV40 Large T antigen (tsT). We have maintained these cells (designated OSE-C2) in culture for more than 100 population doublings after introduction of the immortalising genes. Early passage OSE-C2 cells have a near-tetraploid karyotype and exhibit a dual mesenchymal-epithelial phenotype, with consistent expression of vimentin and variable expression of cytokeratins and type III collagen, and absence of E cadherin expression. OSE-C2 cells proliferate steadily at the permissive temperature of 33 degrees C, but fail to increase in number at the nonpermissive temperature of 39 degrees C. Serum-deprived OSE-C2 cells are stimulated to grow at 33 degrees C by EGF, whereas they are growth inhibited at 33 degrees C by TGFbeta in the presence or the absence of serum. When temperature shifted to the nonpermissive temperature, OSE-C2 cells modulate to a more mesenchymal phenotype, and a proportion of the cells undergo senescence and/or apoptosis. Moreover, at the nonpermissive temperature, the levels of p53 and SV40 Large T antigen diminish, whilst the level of p21 increases, whereas the level of p16 and telomerase activity is unchanged. This experimental system shows that expression of telomerase alone only allows limited proliferative potential of OSE cells; expression of tsT is necessary to maintain these cells in culture for longer periods, perhaps by its ability to inactivate components of the p53/Rb pathway. OSE-C2 cells may be useful in studying the physiology and differentiation of human OSE cells and provide insight into the poorly understood earliest stages of epithelial ovarian cancer.     

Are non-functional, unfolded proteins ('junk proteins') common in the genome?

It has recently been shown that many proteins are unfolded in their functional state. In addition, a large number of stretches of protein sequences are predicted to be unfolded. It has been argued that the high frequency of occurrence of these predicted unfolded sequences indicates that the majority of these sequences must also be functional. These sequences tend to be of low complexity. It is well established that certain types of low-complexity sequences are genetically unstable, and are prone to expand in the genome. It is possible, therefore, that in addition to these well-characterised functional unfolded proteins, there are a large number of unfolded proteins that are non-functional. Analogous to 'junk DNA' these protein sequences may arise due to physical characteristics of DNA. Their high frequency may reflect, therefore, the high probability of expansion in the genome. Such 'junk proteins' would not be advantageous, and may be mildly deleterious to the cell.     

Control of Ras cycling by Ca2+

Ras GTPases are binary switches, cycling between an inactive GDP-bound form and an active GTP-bound form at the membrane. They transduce signals into the cytoplasm via effector pathways that regulate cell growth, differentiation and apoptosis. Ras activation is enhanced by guanine nucleotide exchange factors (GEFs); deactivation is accelerated by GTPase-activating proteins (GAPs). Recently, new roles for Ca(2+) and diacylglycerol (DAG) in the control of Ras cycling have emerged with the discovery of a series of novel GEFs and GAPs. These regulators of Ras cycling are likely to play a key role in the information processing of Ca(2+) and DAG signals.     

Genetic variation of resistance to mercury poisoning in steelhead (Oncorhynchus mykiss) alevins

Newly hatched steelhead alevins were obtained from the factorial breeding of 24 male and 10 female steelhead trout, Oncorhynchus mykiss. Each set of offspring were in a separate cell. They were tested for resistance to intoxication by methylmercuric chloride (CH3HgCl) in water at a nearly constant mercury concentration of 8 microg l(-1). High mortality (81% of the tested alevins) occurred within 2 weeks. Resistance to intoxication, as measured by the time to death, as well as by the survival rate, shared high paternal and maternal variation with negligible interaction. Heritability of time to death was 0.59 +/- 0.17; heritability of survival (all-or-none trait) was lower (0.26 +/- 0.09). Mercury in dead alevins increased with time to death, exhibiting a large environmental variation and (comparatively) negligible genetic influence. At the end of the bioassay, the mercury content in survivors varied widely (3-21 microg g(-1) wet weight). The content was greater than, but correlated with that of dead alevins from the same cells, and it showed little relation with survival rate. Thus, it seems that resistance to poisoning implies a tolerance to high levels of mercury rather than a limitation of its accumulation.     

Hydrocarbon distribution and colony odour homogenisation in <Emphasis Type="Italic">Pachycondyla apicalis</Emphasis>

Summary Within and between individuals hydrocarbon (HC)-circulation was studied in Pachycondyla apicalis workers, using radioactive labeling. Newly synthesized HCs occurred both in the PPG and on the epicuticle in appreciable amounts, lesser quantities were found in the crop. The front basitarsal brush contained a greater amount of radiolabeled HCs than could be predicted from its surface area, suggesting preferential secretion to these organs. We propose that the newly synthesized HCs are secreted primarily to the front basitarsal brushes and are thereafter either distributed throughout the body surface, or cleared via the PPG and the alimentary canal.Using labeled HCs as a model, we tracked the time-dependent dispersion of cuticular lipids among 11 workers, one of which was prelabeled for 24 hours. Distribution among the recipients became progressively uniform, reaching near homogenization between 5–10 days. The mean HCs transfer of P. apicalis to the PPG was substantially lower compared to that of Camponotus fellah or Aphaenogaster senilis. In contrast, transfer to the cuticle in this species was superior. We attribute the low transfer to the PPG to the inefficacy of passive body contact characteristic of P. apicalis, as opposed to trophallaxis and/or allogrooming that typify the other two species. The higher occurrence of radiolabeled HCs in P. apicalis cuticle can be attributed to their accumulation in the basitarsal brushes. The impact of cuticular lipid transfer and formation of uniform colony odour, as opposed to the maintenance of an idiosyncratic caste-specific composition, are discussed.Received 5 September 2002; revised 17 January 2003; accepted 10 February 2003.     

Agrobacterium tumefaciens VirB6 protein participates in formation of VirB7 and VirB9 complexes required for type IV secretion

This study characterized the contribution of Agrobacterium tumefaciens VirB6, a polytopic inner membrane protein, to the formation of outer membrane VirB7 lipoprotein and VirB9 protein multimers required for type IV secretion. VirB7 assembles as a disulfide cross-linked homodimer that associates with the T pilus and a VirB7-VirB9 heterodimer that stabilizes other VirB proteins during biogenesis of the secretion machine. Two presumptive VirB protein complexes, composed of VirB6, VirB7, and VirB9 and of VirB7, VirB9, and VirB10, were isolated by immunoprecipitation or glutathione S-transferase pulldown assays from detergent-solubilized membrane extracts of wild-type A348 and a strain producing only VirB6 through VirB10 among the VirB proteins. To examine the biological importance of VirB6 complex formation for type IV secretion, we monitored the effects of nonstoichiometric VirB6 production and the synthesis of VirB6 derivatives with 4-residue insertions (VirB6.i4) on VirB7 and VirB9 multimerization, T-pilus assembly, and substrate transfer. A virB6 gene deletion mutant accumulated VirB7 dimers at diminished steady-state levels, whereas complementation with a plasmid bearing wild-type virB6 partially restored accumulation of the dimers. VirB6 overproduction was correlated with formation of higher-order VirB9 complexes or aggregates and also blocked substrate transfer without a detectable disruption of T-pilus production; these phenotypes were displayed by cells grown at 28 degrees C, a temperature that favors VirB protein turnover, but not by cells grown at 20 degrees C. Strains producing several VirB6.i4 mutant proteins assembled novel VirB7 and VirB9 complexes detectable by nonreducing sodium dodecyl sulfate-polyacrylamide gel electrophoresis, and two strains producing the D60.i4 and L191.i4 mutant proteins translocated IncQ plasmid and VirE2 effector protein substrates in the absence of a detectable T pilus. Our findings support a model that VirB6 mediates formation of VirB7 and VirB9 complexes required for biogenesis of the T pilus and the secretion channel.     

Beta-helix model for the filamentous haemagglutinin adhesin of Bordetella pertussis and related bacterial secretory proteins

Bordetella pertussis establishes infection by attaching to epithelial cells of the respiratory tract. One of its adhesins is filamentous haemagglutinin (FHA), a 500-A-long secreted protein that is rich in beta-structure and contains two regions, R1 and R2, of tandem 19-residue repeats. Two models have been proposed in which the central shaft is (i) a hairpin made up of a pairing of two long antiparallel beta-sheets; or (ii) a beta-helix in which the polypeptide chain is coiled to form three long parallel beta-sheets. We have analysed a truncated variant of FHA by electron microscopy (negative staining, shadowing and scanning transmission electron microscopy of unstained specimens): these observations support the latter model. Further support comes from detailed sequence analysis and molecular modelling studies. We applied a profile search method to the sequences adjacent to and between R1 and R2 and found additional "covert" copies of the same motifs that may be recognized in overt form in the R1 and R2 sequence repeats. Their total number is sufficient to support the tenet of the beta-helix model that the shaft domain--a 350 A rod--should consist of a continuous run of these motifs, apart from loop inserts. The N-terminus, which does not contain such repeats, was found to be weakly homologous to cyclodextrin transferase, a protein of known immunoglobulin-like structure. Drawing on crystal structures of known beta-helical proteins, we developed structural models of the coil motifs putatively formed by the R1 and R2 repeats. Finally, we applied the same profile search method to the sequence database and found several other proteins--all large secreted proteins of bacterial provenance--that have similar repeats and probably also similar structures.