Does agricultural intensification cause tipping points in ecosystem services?

Landscape Ecology - Agricultural intensification is being widely pursued as a policy option to improve food security and human development. Yet, there is a need to understand the impact of...     

Evidence of a crossability barrier in diploid x hexaploid and diploid x octoploid crosses in the genus Fragaria

Summary A large number of diploid x hexaploid and diploid x octoploid crosses have been made by numerous workers with poor results. Crosses between synthetic tetraploids and species of higher levels of ploidy were successful. The success of these crosses and the high incidence of pentaploid progeny obtained by other workers from successful diploid x hexaploid crosses indicates that chromosomal or genetic imbalance in the endosperm or between zygote and endosperm is most likely the cause of failure to produce viable plants from 2x x 6x and 2x x 8x crosses.     

Alternative methods of interpreting measurements of resistance to the leafhopper Empoasca kraemeri Ross and Moore in the common bean,Phaseolus vulgaris L.

Summary Appropriate definitions of, and methods of measuring, resistance are sought using Phaseolus vulgaris, a major source of protein in Latin America, and its most important pest, Empoasca kraemeri. The concepts of general vigour and narrow-sense resistance are defined: these two components make up broad-sense resistance. The validity of these definitions, which depend on the inclusion of tolerance as a resistance mechanism, is discussed. In order to distinguish the components, 38 genotypes of P. vulgaris were grown with and without insecticidal proctection, in three replications in a split-plot design, with three planting dates. Seed yield per plant, and a visual score of damage symptoms or vigour, were measured. Three methods of estimating narrow sense resistance are considered: 1) the genotype × insecticide interaction effect in a split-plots analysis, 2) the yield of each genotype unprotected (Y_u) divided by its yield protected (Y_p). and 3) the deviation of each genotype from a regression of Y_u on Y_p. The third method overcomes some objections to the first two, and can be applied to damage and vigour scores as well as to yields. The 38 genotypes differed in narrow-sense resistance according to all three methods, and it appears that in practice Methods 2 and 3 were both valid. They also differed in general vigour. Breeders working with many pests and crops have found that low damage and high yield in the presence of the pest are effective selection criteria. The work reported here supports this view.     

Genetic markers and the selection of quantitative traits in forage grasses

Summary Marker assisted selection is based upon the principle that if a gene (or block of genes) is linked to an easily identifiable genetic marker it may be more efficient to select in a breeding programme for the marker than for the trait itself. The recent developments in molecular marker technology has allowed several approaches to be applied to the forage grasses. The most effective methods involve the production of detailed genetic maps which can be used for determination of the location of Quantitative Trait Loci (QTLs). Application of these methods toLolium has lead to the identification of 10 QTL's affecting mainly phenological characteristics. Up to 80% of the variation in inflorescence production in the establishment year may be accounted for by one region of the genome.     

The inheritance of resistance to Empoasca kraemeri Ross & Moore in the common bean,Phaseolus vulgaris L.

Summary Genotypes of Phaseolus vulgaris differ in their level of resistance to Empoasca kraemeri, but the speed with which these differences can be accumulated into a resistant variety will depend on whether they can be detected in single F₂ plants, and on whether the genetic control of resistance is additive. Thirty-eight homozygous genotypes were therefore grown in small plots with and without insecticidal protection, and also in simulated F₂ populations containing one plant of each genotype. In addition, the F₂ and F₃ generations of crosses were studied. Both generations of a 13 parent diallel were grown with and without insecticidal protection. Six crosses, involving some of the parents in the diallel and two additional genotypes, were studied in more detail. Protected and unprotected F₂ plants from these crosses were harvested individually, and the resulting F₃ families were grown with and without protection. In all the expriments, the level of damage to the unprotected plants and the vigour of the protected plants was assessed visually by means of a numerical score, and the seed yield was measured.The performance of a single homozygous plant was a reasonable predictor of the performance of the same genotype in a small plot, but in the six crosses studies in detail, the correlations between F₂ and F₃ performance were low. In the analysis of the diallel cross, the significant differences among crosses were due to specific combining ability and not general combining ability. These results indicate that the genetic control of resistance is largely non-additive, and that F₂ selection will probably be ineffective. However, F₃ selection should be effective, and transgressive segregants for resistance should be obtainable.In some of the six crosses, the F₃ families differed both in general vigour and in narrow-sense resistance to E. kraemeri, as defined by Galwey & Evans (1982). The visual assessment of damage symptoms in each plot was related to the subsequent seed yield of the plot, but was more heritable than yield. The relationship between these two variables was investigated by the genetic correlation coefficient and by an alternative method, and was found to be partly genetically determined, but partly due to the influence of environmental factors on both variables. This indicates that screening for E. kraemeri resistance should be done by means of a visual damage score in a carefully chosen, uniform environment.     

Cassini Imaging Science: initial results on Saturn's atmosphere

The Cassini Imaging Science Subsystem (ISS) began observing Saturn in early February 2004. From analysis of cloud motions through early October 2004, we report vertical wind shear in Saturn's equatorial jet and a maximum wind speed of approximately 375 meters per second, a value that differs from both Hubble Space Telescope and Voyager values. We also report a particularly active narrow southern mid-latitude region in which dark ovals are observed both to merge with each other and to arise from the eruptions of large, bright storms. Bright storm eruptions are correlated with Saturn's electrostatic discharges, which are thought to originate from lightning.     

Chromatographic silica gel: surface area determined by adsorption

The surface area of silicic acid, a form of silica gel, has been determined by adsorption of methanol from a benzene solvent. The method is straightforward, uses inexpensive apparatus, and should be applicable to other particulate adsorbents.     

SURVIVAL OF THE MAMMARY TUMOR MILK AGENTS OF MICE

The mammary tumor milk agent has been recovered from a transplanted mammary carcinoma that was carried for 10 passages in mice that did not themselves originally have the agent. It is possible that the agent, carried in the transplanted mammary tumor cells, may be responsible for the genetic mutations which have been detected in transplantable tumors. Preliminary results show that the agent can be recovered from the yolk sac of chick embryos 12 days after the injection into eggs of either tumor suspensions or cell-free filtrates of tissues containing the active agent.     

CLIMATE VARIABILITY: Enhanced: Do the Tropics Rule?

Climate is not the same from one decade to the next, but as Cane and Evans explain in their Perspective, the mechanisms and even the patterns of decadal climate change are not well understood. The proxy climate record reported by Linsley et al. is an example of the kind of highly resolved records required from across the world to understand climate on these time scales.