Leaf phenology mediates provenance differences in herbivore populations on valley oaks in a common garden

1. Plants from different populations often display a variation in herbivore resistance. However, it is rarely understood what plant traits mediate such differences. 2. It was tested how leaf phenology affects herbivore populations in a 15‐year‐old common garden of valley oaks (Quercus lobata Née) with different populations and maternal parents from throughout the Q. lobata range. 3. The abundance of leaf miners (Stigmella sp. Shrank) and leaf phenology of oaks in the common garden was measured. 4. Leaf miner abundance varied among provenance locations (population), but not among maternal parents within populations. Leaf phenology varied by provenance location and maternal parent, and trees that leafed out earlier accrued higher leaf‐miner abundance. Path analysis indicated that leaf phenology was the likely driver of provenance and parental differences in resistance to leaf miners. 5. Understanding population differences is particularly important when considering transport of genotypes for ornamental or restoration purposes. The present study suggests that similarity in leaf phenology may be one factor that could be used to find genotypes with a similar herbivore resistance to local genotypes.     

Molecular Method for Determining Sex of Walruses

Abstract: We evaluated the ability of a set of published trans-species molecular sexing primers and a set of walrus-specific primers, which we developed, to accurately identify sex of 235 Pacific walruses (Odobenus rosmarus divergens). The trans-species primers were developed for mammals and targeted the X- and Y-gametologs of the zinc finger protein genes (ZFX, ZFY). We extended this method by using these primers to obtain sequence from Pacific and Atlantic walrus (O. r. rosmarus) ZFX and ZFY genes to develop new walrus-specific primers, which yield polymerase chain reaction products of distinct lengths (327 and 288 base pairs from the X- and Y-chromosome, respectively), allowing them to be used for sex determination. Both methods yielded a determination of sex in all but 1–2% of samples with an accuracy of 99.6–100%. Our walrus-specific primers offer the advantage of small fragment size and facile application to automated electrophoresis and visualization.     

Feeding rates and carbohydrate metabolism by Bemisia tabaci (Homoptera: Aleyrodidae) on different quality phloem saps

Abstract.The effects of water stress on phloem sap quality of the melon, Cucumis melo, and how this, in turn, has an impact on the sweet potato whitefly, Bemisia tabaci were studied. Melon plants were grown under watering regimes that produced plants with or without water stress. Plants showed strong developmental responses to the treatments; water-stressed plants were shorter, with fewer, smaller leaves than those without stress. There was, however, no effect of plant water stress on the development period of whiteflies feeding on these plants, or on the weights of male or female adults. Honeydew production was used as an indirect measure to test whether the absence of insect developmental or behavioural effects was due to differential phloem sap ingestion. Feeding rates on the stressed plants were almost half those on unstressed plants, and there was also variation in the daily pattern of honeydew production. Phloem sap and honeydew were analysed to determine why the feeding behaviours differed. Amino acid composition of the phloem sap was similar in both groups of plants, but carbohydrate concentrations were greater in water-stressed plants, indicating that lower feeding rates may have been due either to the physical or nutritional quality of the phloem sap. The honeydew of insects that were feeding on water-stressed plants contained a greater concentration of carbohydrate than those on unstressed plants, and was composed of a significantly greater proportion of glucose and the disaccharide, trehalulose. This isomerization of more complex sugars from those in the diet suggests that B. tabaci uses a mechanism of osmoregulation to actively maintain its internal water status. It is concluded that transient conditions of water stress in this host plant do not affect the development of B. tabaci, due to physiological and behavioural changes in response to diets with different nutritional and physical properties. The implications of this finding for the feeding biology of B. tabaci on desert-grown crops are discussed.     

Mitochondrial DNA and two perspectives on evolutionary genetics

This essay reviews comparative studies of animal mitochondrial DNA (mtDNA), with emphasis on findings made and ideas developed at Berkeley. It argues that such studies are bringing together two previous paths of progress in evolutionary biology. One path is that of those who worked far above the species level and were concerned with genealogical trees, time scales and the accumulation of new mutations on surviving molecular lineages. The other path is that of those who worked at and below the species level and were concerned mainly with population structure, migration and the frequencies of alleles that existed in an ancestral population. This fusion of paths is made possible by the high rate at which mutations accumulate on mtDNA lineages and by this molecule's uniparental and apparently haploid mode of inheritance. These properties make mtDNA a superb tool for building trees and time scales relating molecular lineages at and below the species level. In addition, owing to its mode of inheritance, mtDNA is more sensitive to bottlenecks in population size and to population subdivision than are nuclear genes. Joint comparative studies of both mtDNA and nuclear DNA variability give us valuable insights into how effective population size has varied through time. Such studies also give insight into the conditions under which mtDNA from one species can colonize another species.     

Evidence for Stigmatic Self-incompatibility, Pollination Induced Ovule Enlargement and Transmitting Tissue Exudates in the Paleoherb, Saururus cernuus L. (Saururaceae)

Saururus cernuus, a species belonging to the primitive herbaceousangiosperm family Saururaceae, exhibits high rates of self-sterility.We investigated the structural and functional aspects of pollen-carpelinteractions following cross and self pollination to assessthe tissue specific site and timing of self-sterility and factorsimportant for successful cross pollen tube growth. Self-sterilitywas due to inhibition of self pollen germination at a dry stigma.Self pollination was associated with anomalous foot formation,reduced cell wall expansion and secretory activity of stigmaticpapillae, and callose production in stigmatic papillae. Followinggermination, cross compatible pollen tubes entered a solid coreof transmitting tissue and grew to the base of a short style.Entry of cross pollen tubes into the ovary was coincident withovule enlargement which placed the micropyle in the proximityof cross pollen tube tips. Ovule enlargement also occurred followingself pollination. Cross pollen tubes either entered an exudate-filledmicropyle directly from the style, or growth in the ovary waslocalized to the epidermis of the locule and outer integumentprior to entry into the micropyle. Prior to pollination, thetransmitting tract was void of secretions except for exudatein the micropyle. Growth of pollen tubes on the locule and integumentwas associated with exudate apparently arising from transmittingcells adjacent to growing pollen tubes. The present study providesthe first evidence in a primitive herbaceous species of stigmaticself-incompatibility (SI) in association with a dry stigma,pollination-induced signalling events affecting developmentof carpellary tissues, and micropylar exudates. Copyright 1999Annals of Botany Company SI evolution, dry stigma, exudates, pollen-carpel signalling, Saururaceae.     

C3 plants enhance rates of photosynthesis by reassimilating photorespired and respired CO2

Photosynthetic carbon gain in plants using the C₃ photosynthetic pathway is substantially inhibited by photorespiration in warm environments, particularly in atmospheres with low CO₂ concentrations. Unlike C₄ plants, C₃ plants are thought to lack any mechanism to compensate for the loss of photosynthetic productivity caused by photorespiration. Here, for the first time, we demonstrate that the C₃ plants rice and wheat employ a specific mechanism to trap and reassimilate photorespired CO₂. A continuous layer of chloroplasts covering the portion of the mesophyll cell periphery that is exposed to the intercellular air space creates a diffusion barrier for CO₂ exiting the cell. This facilitates the capture and reassimilation of photorespired CO₂ in the chloroplast stroma. In both species, 24–38% of photorespired and respired CO₂ were reassimilated within the cell, thereby boosting photosynthesis by 8–11% at ambient atmospheric CO₂ concentration and 17–33% at a CO₂ concentration of 200 µmol mol^?1. Widespread use of this mechanism in tropical and subtropical C₃ plants could explain why the diversity of the world's C₃ flora, and dominance of terrestrial net primary productivity, was maintained during the Pleistocene, when atmospheric CO₂ concentrations fell below 200 µmol mol^?1.     

Dynamic photo-inhibition and carbon gain in a C4 and a C3 grass native to high latitudes

C₄ plants are rare in the cool climates characteristic of high latitudes and altitudes, perhaps because of an enhanced susceptibility to photo‐inhibition at low temperatures relative to C₃ species. In the present study we tested the hypothesis that low‐temperature photo‐inhibition is more detrimental to carbon gain in the C₄ grass Muhlenbergia glomerata than the C₃ species Calamogrostis Canadensis. These grasses occur together in boreal fens in northern Canada. Plants were grown under cool (14/10 °C day/night) and warm (26/22 °C) temperatures before measurement of the light responses of photosynthesis and chlorophyll fluorescence at different temperatures. Cool growth temperatures led to reduced rates of photosynthesis in M. glomerata at all measurement temperatures, but had a smaller effect on the C₃ species. In both species the amount of xanthophyll cycle pigments increased when plants were grown at 14/10 °C, and in M. glomerata the xanthophyll epoxidation state was greatly reduced. The detrimental effect of low growth temperature on photosynthesis in M. glomerata was almost completely reversed by a 24‐h exposure to the warm‐temperature regime. These data indicate that reversible dynamic photo‐inhibition is a strategy by which C₄ species may tolerate cool climates and overcome the Rubisco limitation that is prevalent at low temperatures in C₄ plants.     

Hydathodal leaf teeth of Chloranthus japonicus (Chloranthaceae) prevent guttation-induced flooding of the mesophyll

Why the leaves of cold temperate deciduous and moisture-loving angiosperms are so often toothed has long puzzled biologists because the functional consequences of teeth remain poorly understood. Here we provide functional and structural evidence that marginal leaf teeth of Chloranthus japonicus, an understory herb, enable the release of guttation sap during root pressure. When guttation from teeth hydathodes was experimentally blocked, we found that the leaf intercellular airspaces became flooded. Measurements of chlorophyll a fluorescence revealed that internal flooding resulted in an inhibition of photosynthesis, most likely through the formation of a film of water within the leaf that reduced CO₂ diffusion. Comparing a developmental series of leaves with and without teeth experimentally covered with wax, we found that teeth did not affect overall leaf stomatal conductance and CO₂ uptake. However, maximal and effective light-saturation PSII quantum yields of teeth were found to be lower or equal to the surrounding lamina throughout leaf ontogeny. Collectively, our results suggest hydathodes and their development on teeth apices enable the avoidance of mesophyll flooding by root pressure. We discuss how these new findings bear on the potential physiological interpretations of models that apply leaf marginal traits to infer ancient climates.