Factors controlling fruit set in hermaphroditic plants: Studies with the Australian proteaceae

The number of fruits produced by many hermaphroditic plants is usually far fewer than the number of flowers available for fertilization. There are various possible explanations for the low fruit:flower ratio, some proximate and others ultimate. Recent studies, especially in northern hemisphere systems, have used field experiments to test some of them, but there are potential difficulties with the methodology of some experiments and with the testing of ultimate hypotheses. It is important to examine the possible explanations for low fruit: flower ratios with a range of different systems. This article reviews studies on Australian species of woody, perennial shrubs in the family Proteaceae; this evolutionarily distinct group of plants and pollinators has several unusual and interesting characteristics, and provides a valuable addition to the better-known northern hemisphere studies.     

Maintenance of the cellobiose utilization genes of Escherichia coli in a cryptic state

The genes for cellobiose utilization are normally cryptic in Escherichia coli. The cellobiose system was used as a model to understand the process by which silent genes are maintained in microbial populations. Previously reported was (1) the isolation of a mutant strain that expresses the cellobiose-utilization (Cel) genes and (2) that expression of those genes allows utilization of three beta- glucoside sugars: cellobiose, arbutin, and salicin. The Cel gene cluster has now been cloned from that mutant strain. In the course of locating the Cel genes within the cloned DNA segment, it was discovered that inactivation of the Cel-encoded hydrolase rendered the host strain sensitive to all three beta-glucosides as potent inhibitors. This sensitivity arises from the accumulation of the phosphorylated beta- glucosides. Because even the fully active genes conferred some degree of beta-glucoside sensitivity, the effects of cellobiose on a series of five Cel+ mutants of independent origin were investigated. Although each of those strains utilizes cellobiose as a sole carbon and energy source, cellobiose also acts as a potent inhibitor that reduces the growth rate on glycerol 2.5-16.5-fold. On the other hand, wild-type strains that cannot utilize cellobiose are not inhibited. The observation that the same compound can serve either as a nutrient or as an inhibitor suggests that, under most conditions in which cellobiose will be present together with other resources, there is a strong selective advantage to having the cryptic (Cel0) allele. In those environments in which cellobiose is the sole, or the best, resource, mutants that express the genes (Cel+) will have a strong selective advantage. It is suggested that temporal alternation between these two conditions is a major factor in the maintenance of these genes in E. coli populations. This alternation of environments and fitnesses was predicted by the model for cryptic-gene maintenance that was previously published.      

On alternatives to selection-induced mutation in the Bgl operon of Escherichia coli

Selection-induced mutations are nonrandom mutations that occur as specific and direct responses to environmental challenge. Examples of selection-induced mutations have been reported both in bacteria and in yeast. I previously showed (Hall 1988) that excisions of the mobile genetic element IS150 from within bglF are selection induced and argued that they occurred because they were potentially advantageous under the selective conditions employed. Mittler and Lenski (Mittler and Lenski 1992) have argued that such excisions are not selection induced but that they occur randomly in nondividing cells. Here I provide further evidence that IS150 excisions are induced by selection and that the excisions are immediately, rather than only potentially, advantageous to the cell. I also provide evidence that excisions, which Mittler and Lenski claim occur randomly in saturated broth cultures, actually occur after samples from those cultures are plated onto selective medium.      

The evolutionary ecology of corals

Corals display a wide range of complex life histories. The evolutionary consequences of factors such as clonality, indeterminate growth, asexual reproduction coupled with various (sexual) breeding systems, different levels of gene flow, and strongly overlapping generations have only just begun to be explored. We identify a series of problems and areas for new research that may be resolved b y the application of novel theoretical approaches (including nonequilibrium population genetic models and demographic models incorporating modular processes such as colony fission and polyp mortality), greater in situ experimentation, long-term monitoring of population dynamics and the use of new genetic techniques.     

Microsatellite diversity and genetic structure of fragmented populations of the rare,fire-dependent shrub Grevillea macleayana

Recent habitat loss and fragmentation superimposed upon ancient patterns of population subdivision are likely to have produced low levels of neutral genetic diversity and marked genetic structure in many plant species. The genetic effects of habitat fragmentation may be most pronounced in species that form small populations, are fully self-compatible and have limited seed dispersal. However, long-lived seed banks, mobile pollinators and long adult lifespans may prevent or delay the accumulation of genetic effects. We studied a rare Australian shrub species, Grevillea macleayana (Proteaceae), that occurs in many small populations, is self-compatible and has restricted seed dispersal. However, it has a relatively long adult lifespan (c. 30 years), a long-lived seed bank that germinates after fire and is pollinated by birds that are numerous and highly mobile. These latter characteristics raise the possibility that populations in the past may have been effectively large and genetically homogeneous. Using six microsatellites, we found that G. macleayana may have relatively low within-population diversity (3.2-4.2 alleles/locus; Hexp = 0.420-0.530), significant population differentiation and moderate genetic structure (FST = 0.218) showing isolation by distance, consistent with historically low gene flow. The frequency distribution of allele sizes suggest that this geographical differentiation is being driven by mutation. We found a lack mutation-drift equilibrium in some populations that is indicative of population bottlenecks. Combined with evidence for large spatiotemporal variation of selfing rates, this suggests that fluctuating population sizes characterize the demography in this species, promoting genetic drift. We argue that natural patterns of pollen and seed dispersal, coupled with the patchy, fire-shaped distribution, may have restricted long-distance gene flow in the past.     

Symplastic continuity between companion cells and the translocation stream: long-distance transport is controlled by retention and retrieval mechanisms in the phloem

Substantial symplastic continuity appears to exist between companion cells (CCs) and sieve elements of the phloem, which suggests that small solutes within the CC are subject to indiscriminate long-distance transport via the translocation stream. To test this hypothesis, the distributions of exotic and endogenous solutes synthesized in the CCs of minor veins were studied. Octopine, a charged molecule derived from arginine and pyruvate, was efficiently transported through the phloem but was also transferred in substantial amounts to the apoplast, and presumably other non-phloem compartments. The disaccharide galactinol also accumulated in non-phloem compartments, but long-distance transport was limited. Conversely, sucrose, raffinose, and especially stachyose demonstrated reduced accumulation and efficient transport out of the leaf. We conclude that small metabolites in the cytosol of CCs do enter the translocation stream indiscriminately but are also subject to distributive forces, such as nonselective and carrier-mediated membrane transport and symplastic dispersal, that may effectively clear a compound from the phloem or retain it for long-distance transport. A model is proposed in which the transport of oligosaccharides is an adaptive strategy to improve photoassimilate retention, and consequently translocation efficiency, in the phloem.     

High-level expression of the Endo-beta-N-acetylglucosaminidase F2 gene in E.coli: one step purification to homogeneity

The Endo F2gene was overexpressed in E.coli as a fusion protein joined to the maltose-binding protein. MBP-Endo F2was found in a highly enriched state as insoluble, inactive inclusion bodies. Extraction of the inclusion bodies with 20% acetic acid followed by exhaustive dialysis rendered the fusion protein active and soluble. MBP-Endo F2was digested with Factor Xaand purified on Q-Sepharose. The enzyme was homogeneous by SDS-PAGE, and appeared as a single symmetrical peak on HPLC. Analysis of the amino-terminus demonstrated conclusively that recombinant Endo F2was homogeneous and identical to the native enzyme.      

Exercise training reduces skeletal muscle membrane arachidonate in the obese (fa/fa) Zucker rat

Compared with the lean(Fa/) genotype, obese(fa/fa) Zucker rats have arelative deficiency of muscle phospholipid arachidonate, and skeletalmuscle arachidonate in humans is positively correlated with insulinsensitivity. To assess the hypothesis that the positive effects ofexercise training on insulin sensitivity are mediated by increasedmuscle arachidonate, we randomized 20 lean and 20 obese weanling maleZucker rats to sedentary or treadmill exercise groups. After 9 wk,fasting serum, three skeletal muscles (white gastrocnemius, soleus, andextensor digitorum longus), and heart were obtained. Fasting insulinwas halved by exercise training in the obese rat. In whitegastrocnemius and extensor digitorum longus (fast-twitch muscles), butnot in soleus (a slow-twitch muscle) or heart, phospholipidarachidonate was lower in obese than in lean rats(P < 0.001). In all muscles,exercise in the obese rats reduced arachidonate(P < 0.03, by ANOVA contrast). Weconclude that improved insulin sensitivity with exercise in the obesegenotype is not mediated by increased muscle arachidonate and thatreduced muscle arachidonate in obese Zucker rats is unique tofast-twitch muscles.