Some population genetic models combining artificial and natural selection pressures in the presence of assortative mating

Summary We have attempted quantitatively through a series of assortative mating models to gain insight into the interaction between the usually antagonistic tendencies of artificial and natural selection pressures. We summarize some of the robust conclusions. In cases where natural selection is expressed only through the phenotype and acts in the opposite direction to the culling incline, then fixation of the dominant or recessive type can be achieved and which occurs depends critically on the initial composition of the population and the magnitude of the degree of culling compared to the selection coefficients.With traits determined at two loci in the case that the double heterozygote is the desired kind, the effect of selfing can only be overcome by very strong artificial selection pressures (high culling order). The degree of culling to achieve its objective can be relaxed with weakening of linkage. The relevant comparison is r ₂ + (1 – r)₂ < (1 – c)indicating the precise extent of culling needed, to prevent fixation. The relationships are more complex when natural selection forces are also involved (see Model IV).Supported in part by NIH Grant USPHS 10452-09.     

Phenotypic versus marker-assisted selection for stalk strength and second-generation European corn borer resistance in maize

Maize (Zea mays L.) stalk lodging is breakage of the stalk at or below the ear, which may result in loss of the ear at harvest. Stalk lodging is often intensified by the stalk tunneling action of the second-generation of the European corn borer (2-ECB) [Ostrinia nubilalis (Hübner)]. Rind penetrometer resistance (RPR) has been used to measure stalk strength and improve stalk lodging resistance, and quantitative trait loci (QTL) have been identified for both RPR and 2-ECB damage. Phenotypic recurrent selection (PS) increases the frequency of favorable alleles over cycles of selection. Several studies have indicated that marker-assisted selection (MAS) is also a potentially valuable selection tool. The objective of this study was to compare the efficiency of PS versus MAS for RPR and 2-ECB. Marker-assisted selection for high and low RPR was effective in the three populations studied. Phenotypic selection for both high and low RPR was more effective than MAS in two of the populations. However, in a third population, MAS for high RPR using QTL effects from the same population was more effective than PS, and using QTL effects from a separate population was just as effective as PS. Marker-assisted selection for resistance and susceptibility to 2-ECB using QTL effects from the same population was effective in increasing susceptibility, but not in increasing resistance. Marker-assisted selection using QTL effects from a separate population was effective in both directions of selection. Thus, MAS was effective in selecting for both resistance and susceptibility to 2-ECB. These results demonstrated that MAS can be an effective selection tool for both RPR and 2-ECB resistance. These results also validate the locations and effects of QTL for RPR and 2-ECB resistance identified in earlier studies.Communicated by D. Hoisington     

Gene controlled selection of mitochondria in Paramecium.

Summary The slow growing mutant cl₁ of Paramecium, previously described (Sainsard, Claisse and Balmefrezol, 1974) differs from wild-type by a single recessive nuclear mutation and by a particular mitochondrial phenotype (M^cl) that gene cl ₁ distinguishes from the wild-type mitochondrial phenotype (M⁺). A further analysis of these nucleo-mitochondrial interactions was carried out by confronting the genes cl ₁ and cl ₁ ⁺ with mixed populations of M⁺ and M^cl mitochondria obtained after cytoplasmic exchange at conjugation. The following results were obtained: 1. M⁺ and M^cl mitochondria introduced respectively into mutant and wild-type cells do not multiply easily; 2. when a mixed population (M⁺+M^cl) is established, both mitochondrial types are maintained during the growth of the F₁ heterozygous cl ₁/cl ₁ ⁺ clones; 3. when the nuclear segregation occurs in F₂, the formation of homozygotes cl ₁/cl ₁ or cl ₁ ⁺ /cl ₁ ⁺ is soon followed by the segregation of the two mitochondrial types, M^cl or M⁺, reconstituting the two parental nucleo-mitochondrial associations.This paper is dedicated to Professor T.M. Sonneborn on the occasion of his 70th birthday     

In situ growth of Gallionella biofilms and partitioning of lanthanides and actinides between biological material and ferric oxyhydroxides

Gallionella ferruginea is an iron‐oxidizing chemolithotrophic micro‐organism that lives in low‐oxygen conditions (0.1–1.5 mg L^?1 saturation). It produces a stalk structure from the concave side of the cell depending on population development, pH and redox conditions. After Gallionella oxidizes ferrous iron, bacteriogenic iron oxides (BIOS) precipitate on the stalk material and over time the stalks and/or the precipitated BIOS attenuate trace metals from surrounding groundwater. Gallionella ferruginea biofilms were cultured in situ in an artificial channel (2000 × 300 × 250 mm) using groundwater sourced from a borehole 297 m below sea level in the Äspö Hard Rock Laboratory in southern Sweden. The pH of the groundwater in the channels was always between 7.4 and 7.7 with oxygen saturation below 1.5 mg L^?1 and Eh between 100 and 200 mV. Oxygen eventually declined to <0.3 mg L^?1, terminating prolific biofilm growth. Biofilms formed within 2 weeks and were sampled every 2 weeks over 3 months. Cell number, stalk length and ferric iron concentration were measured for each sample and trace metal concentration was measured by inductively coupled plasma mass spectrometry. Results from well‐developed in situ biofilms suggest that Gallionella could concentrate metals at levels up to 1 × 10³‐fold higher than found within the host rock and more than 1 × 10⁶ times the levels found in the groundwater. These new experiments were used to support the results from the well‐developed biofilms and to relate biofilm development and population characteristics to metal attenuation. After 3 months, rare earth element (REE) plots indicated that BIOS can accumulate metals at levels up to 1 × 10⁴‐fold higher than found in the groundwater and fractionate heavy rare earth elements over light rare earth elements. Generally the presence of the organic phase promotes the adsorption of all lanthanides and actinides that are not adsorbed by the inorganic phase. The iron oxides are directly correlated with stalk length (R = 0.96), indicating that rapid REE and actinide adsorption requires both iron oxides and a nucleating biological structure for the iron oxides.     

Ontogenetic body-mass scaling of resting metabolic rate covaries with species-specific metabolic level and body size in spiders and snakes

According to common belief, metabolic rate usually scales with body mass to the 3/4-power, which is considered by some to be a universal law of nature. However, substantial variation in the metabolic scaling exponent (b) exists, much of which can be related to the overall metabolic level (L) of various taxonomic groups of organisms, as predicted by the recently proposed metabolic-level boundaries (MLB) hypothesis. Here the MLB hypothesis was tested using data for intraspecific (ontogenetic) body-mass scaling of resting metabolic rate in spiders and boid snakes. As predicted, in both animal groups b varies mostly between 2/3 and 1, and is significantly negatively related to L. L is, in turn, negatively related to species-specific body mass (M_m: estimated as the mass at the midpoint of a scaling relationship), and as a result, larger species tend to have steeper metabolic scaling slopes (b) than smaller species. After adjusting for the effects of M_m, b and L are still negatively related, though significantly only in the spiders, which exhibit a much wider range of L than the snakes. Therefore, in spiders and snakes the intraspecific scaling of metabolic rate with body mass itself scales with interspecific variation in both metabolic level and body mass.     

Comparison of potential fecundity models for walleye pollock Gadus chalcogrammus in the Pacific waters off Hokkaido,Japan

Potential fecundity models of walleye or Alaska pollock Gadus chalcogrammus in the Pacific waters off Hokkaido, Japan, were developed. They were compared using a generalized linear model with using either standard body length (L_S) or total body mass (M_T) as a main covariate along with Fulton's condition factor (K) and mean diameter of oocytes (D_O) as additional potential covariates to account for maternal conditions and maturity stage. The results of model selection showed that M_T was a better single predictor of potential fecundity (F_P) than L_S. The biological importance of K on F_P was obscure, because it was statistically significant when used in the predictor with L_S (i.e. length‐based model), but not significant when used with M_T (i.e. mass‐based model). Meanwhile, D_O was statistically significant in both length and mass‐based models, suggesting the importance of downregulation on the number of oocytes with advancing maturation. Among all candidate models, the model with M_T and D_O in the predictor had the lowest Akaike's information criterion value, suggesting its better predictive power. These newly developed models will improve future comparisons of the potential fecundity within and among stocks by excluding potential biases other than body size.     

Quantitative analysis of the effect of selection history on sugar yield adaptation of sugarcane clones

An objective of the CSR sugarcane breeding programme in Australia was to assess the scope for broadening the genetic base of the commercial sugarcane germ plasm through interspecific hybridization with Saccharum spontaneum clones. The contribution of both selection history and S. spontaneum to sugar yield and its components was investigated in the germ plasm pool assembled. The analysis was conducted on a data-set of 256 clones, consisting of parents and full-sib families generated from 32 biparental crosses, tested in six environments. The minimum number of generations back to S. spontaneum ancestor in the clone's pedigree was used as a germ plasm score. The geographical origin and selection history of each parent and their use in the biparental crosses were used to develop a selection history score for parents and offspring. The variation for seven attributes, cane yield, commercial cane sugar %, sugar yield, stalk number per stool, stalk weight, fibre % and ash % juice was partitioned according to the germ plasm and selection history scores. Significant (P<0.05) clone variation and clone x environment interaction for all attributes was present. The germ plasm scores accounted for a significant (P<0.05) component of the clone variation for all of the attributes except cane yield. There was an increase in sugar yield with an increase in the minimum number of generations back to a S. spontaneum clone. The selection history groups accounted for a high proportion of the variation among parental clones for all of the attributes except cane yield. This suggested that parents were the outcome of strong selection pressure for the commercial cane attributes. However, the selection history groups for the offspring produced by random mating of parents did not account for a high proportion of the variation for the attributes. Using the mixture method of classification we partitioned the 256 clones into five groups for patterns of performance for the seven attributes across the six environments. The five groups emphasized major differences in the patterns of performance for the seven attributes across environments. The distribution of germ plasm and selection history scores in each of the five groups indicated that their patterns of performance were associated with selection history and minimum generations to S. spontaneum. Therefore, both the analysis on selection history and germ plasm scores (extrinsic classification) and the analysis on the mixture method of classification (intrinsic classification) emphasized the influence of selection history on the sugar yield of sugarcane.     

Studies of esterase 6 in Drosophila melanogaster XII. Evidence for temperature selection of Est 6 and Adh alleles

Changes in allele frequencies at the esterase 6 (Est 6) and alcohol dehydrogenase (Adh) enzyme loci of Drosophila melanogaster and simulans are examined in natural populations and artificial populations maintained at two temperatures. Results from cage populations at 18 °C and 25 °C provide evidence for temperature selection at both loci. Seasonal population samples show no significant change in gene frequencies for either locus, a reasonable outcome given the small selection coefficients found in cage populations. The temperature effect for the Adh locus appears to be direct: natural selection of the fast allele in cool environs and of the slow allele in warm environs. The temperature effect for Est 6 is weaker and complicated by sex differences and deviations from Hardy-Weinberg expectation. This evidence and different Est 6 frequencies found for melanogaster and simulans, in conjunction with evidence of the male reproductive function of this enzyme, suggest that Est 6 polymorphisms are maintained in natural populations by a complex form of sexual selection.     

Callus induction and plant regeneration of U.S. rice genotypes as affected by medium constituents

Summary This study was conducted to establish and optimize a regeneration system for adapted U.S. rice genotypes including three commercial rice cultivars (LaGrue, Katy, and Alan) and two Arkansas breeding lines. Factors evaluated in the study were genotype, sugar type, and phytohormone concentration. The system consisted of two phases, callus induction and plant regeneration. In the callus induction phase, mature caryopses were cultured on MS medium containing either 1% sucrose combined with 3% sorbitol or 4% sucrose alone, and 0.5 to 4 mg·L⁻¹ (2.26 to 18.10 μM) 2,4-D with or without 0.5mg·L⁻¹) (2.32 μM) kinetin. In the plant regeneration phase, callus was transferred to 2,4-D-free MS medium containing 0 or 2 mg·L⁻¹ (9.29 μM) kinetin combined with 0 or 0.1 mg·L⁻¹ (0.54 μM) NAA. Callus induction commenced within a week, independent of the treatments. Callus growth and plant regeneration, however, were significantly influenced by interactions among experimental factors. Generally, the greatest callus growth and plant regeneration were obtained with 0.5 mg·L⁻¹ (2.26 μM) 2,4-D and decreased with increasing 2,4-D concentrations. Kinetin enhanced callus growth only when combined with 0.5 mg·L⁻¹ (2.26 μM) 2,4-D, and 4% sucrose. Inducing callus on kinetin-containing medium generally enhanced regeneration capacity in the presence of sucrose but not with a sucrose/sorbitol combination. Media containing sucrose alone generally supported more callus proliferation, but the sucrose/sorbitol combination improved regeneration of some cultivars. NAA and kinetin had little effect on regeneration.     

Gender based disruptive selection maintains body size polymorphism in Drosophila melanogaster

Darwinian fitness in holometabolous insects like the fruit fly Drosophila melanogaster is reported to be positively correlated with body size. If large individuals in a population have higher fitness, then one would expect directional selection to operate leading to uniformly large individuals. However, size polymorphism persists in nature and needs further probing. We assessed the effect of body size on some of the fitness and fitness-related traits in replicate populations of genotypically large, genotypically small and phenotypically small D. melanogaster flies. In this study, the time taken to attain reproductive maturity and copulation duration were independent of fly size. Fecundity and longevity of large females were significantly higher when they partnered genotypically small males than when they were with genotypically larger or phenotypically small males. The increased female longevity when in association with genotypically small males was not due to selective early death of males that would release the female partner from presumed cost of persistent courtship. On the contrary, the genotypically as well as phenotypically small males had significantly higher longevity than large males. The virility of the genotypically small males was not significantly different from that of genotypically large males. Our results clearly show that selection on body size operates in the opposite direction (disruptive selection) for the two genders, thus explaining the persistence of size polymorphisms in the holometabolous insect, Drosophila melanogaster.     

Bidirectional selection for body mass and correlated response of pyrethroid resistance and fitness in Sitophilus zeamais

Responses to artificial selection on body mass in the maize weevil Sitophilus zeamais (Coleoptera: Curculionidae) were investigated to determine whether changes in body mass are associated with insecticide susceptibility, rate of population growth, and metabolic rate. Two strains of the maize weevil differing in susceptibility to pyrethroid insecticides were subjected to bidirectional selection on body mass. The susceptible strain responded to selection resulting in individuals with lower or higher body mass, but the resistant strain responded significantly only to selection for lower body mass. The resistant strain selected for low body mass increased its level of deltamethrin resistance in 44 × . In contrast, selection for low body mass in the susceptible parental strain led to increased deltamethrin susceptibility (50 × ) and selection for high body mass increased deltamethrin resistance (4 × ). Thus, the correlated response of insecticide resistance to selection for body mass differed between strains, a likely consequence of their distinct genetic background. Regardless, body mass was positively correlated with fitness (reproductive output) (r = 0.79; P < 0.001), while such correlation with respiration rate was significant only at P = 0.07 (r = 0.44). Therefore, the association between body mass and deltamethrin resistance is population‐dependent in the maize weevil, and the confluence of deltamethrin resistance and high body mass in a given strain will likely favour its energy metabolism and lead to the mitigation of fitness costs usually associated with insecticide resistance. The genetic background and selection history of insecticide resistant populations should not be neglected since they may favour the confluence of insecticide resistance with mitigation mechanisms of its associated fitness costs limiting the tactics available to their management.     

Gender distribution,sexual size dimorphism and morphometric sexing in ballan wrasse Labrus bergylta

In wild ballan wrasse Labrus bergylta, mass–length relationships were not different between genders, and positive allometry was found in the mixed‐gender population. Male‐biased sexual size dimorphism was significant and the most effective morphometric method for sexing L. bergylta outside of the species spawning window used body mass (M_B in g), total body length (L_T in mm) and Fulton's condition factor (K) as discriminant variables to predict gender with 91% accuracy. The discriminant score (S_D) of a specimen can be calculated as S_D = 0·01 M_B ? 0·016 L_T ? 3·835 K + 6·252 to predict its gender as female or male if S_D is < 1·459 or S_D is > 1·504, respectively. There was a potential trend towards earlier sexual inversion compared to previous studies at comparable latitudes. Sex change is a phenotypically plastic trait under social control in haremic fishes and should be monitored in increasingly exploited L. bergylta.     

Diet and trophic niche of Antarctic silverfish Pleuragramma antarcticum in the Ross Sea,Antarctica

The diet of Antarctic silverfish Pleuragramma antarcticum was evaluated by examining stomach contents of specimens collected in the Ross Sea (71°–77° S; 165°–180° E) in January to March 2008. Pleuragramma antarcticum (50–236 mm standard length, L_S) and prey items were analysed for stable‐isotopic composition of carbon and nitrogen. According to index of relative importance (I_RI), which incorporates frequency of occurrence, mass and number of prey items, the most important prey items were copepods (81%I_RI over all specimens), predominantly Metridia gerlachei and Paraeuchaeta sp., with krill and fishes having low I_RI (2·2 and 5·6%I_RI overall). According to mass of prey (M) in stomachs, however, fishes (P. antarcticum and myctophids) and krill dominated overall diet (48 and 22%M, respectively), with copepods being a relatively minor constituent of overall diet by mass (9·9%M). Piscivory by P. antarcticum occurred mainly in the extreme south‐west of the region and near the continental slope. Krill identified to species level in P. antarcticum stomachs were predominantly Euphausia superba (14·1%M) with some Euphausia crystallophorias (4·8%M). Both DistLM modelling (PRIMER‐permanova+) on stomach contents (by I_RI) and stepwise generalized linear modelling on stable isotopes showed that L_S and location were significant predictors of P. antarcticum diet. Postlarval P. antarcticum (50–89 mm L_S) consumed exclusively copepods. Juvenile P. antarcticum (90–151 mm L_S) consumed predominantly krill and copepods by mass (46 and 30%M, respectively). Small adult P. antarcticum (152–178 mm L_S) consumed krill, fishes and copepods (37, 36 and 15%M, respectively). Large adult P. antarcticum (179–236 mm L_S) consumed predominantly fishes and krill (55 and 17%M, respectively), especially in the north (near the Ross Sea slope) and in the SW Ross Sea. Amphipods were occasionally important prey items for P. antarcticum (western Ross Sea, 39%M). General concordance between stomach contents and trophic level of P. antarcticum and prey based on δ¹⁵N was demonstrated. Pleuragramma antarcticum trophic level was estimated as 3·7 (postlarval fish) and 4·1 (fish aged 3+ years).     

Effect of sensitivity to abscisic acid on scaling relationships for biomass production rates and body size in Arabidopsis thaliana

The allometric relationships for plant daily biomass production rates, different measures of body size (dry weight and length) and photosynthetic biomass per plant are reported for two mutants of Arabidopsis thaliana (abi1-1, insensitive to ABA; era1-2, hypersensitive to ABA). Scaling relationships, such as daily rate of growth (G) vs body mass (M), plant body length or plant height (L) vs body mass (M), photosynthetic biomass (M _p ) vs non-photosynthetic biomass (M _n ), and daily rate of growth (G) vs. photosynthetic biomass (M _p ) were significantly different in abi1-1 and era1-2. It is implied that the sensitivity to abscisic acid may change the scaling relationships for plant biomass production rate and body size in Arabidopsis thaliana. Because these scaling relationships are closely related to sensitivity to abscisic acid, they are of importance for phytohormonal ecology.     

Net genetic and temporary epistatic or maternal environmental responses to selection for egg production in chickens

The difference between progeny from selected and unselected parents (T) was experimentally partitioned into net genetic change (G_A), temporary favorable epistatic combinations (G_EP) and egg-transmitted maternal environment (M) in two strains of Leghorns selected over 14 years for early pure-strain egg production. Differences among progeny from selected sires and dams, selected sires only, selected dams only, unselected sires and dams and the parental generation were equated to expected G_A, G_EP and M responses for each trait. Total response was 3.3% for early egg number, 3.7% for total egg number, 0.5% for egg weight, 3.8% for early egg mass and 4.2% for total egg mass. Among progeny that survived the test period and were judged to be normal, total response was 2.6% for total number of eggs, 3.0% for early egg mass and 3.1% for total egg mass. The percentage of T attributed to G_A was 9% for early egg number, 24% for total egg number, 43% for early egg mass and 47% for total egg mass; but 52% for total egg number, 98% for early egg mass and 71% for total egg mass of normal survivors. Temporary maternal selection responses (M) were (1) positive for number of eggs and egg masses, (2) greater for all progeny than for normal survivors, and (3) increased with progeny age. The results suggest that M was caused by reduced egg-transmitted disease. Epistatic selection response was positive for earlier sexual maturity and for number of eggs, but negative for egg weight and thus was small for egg masses. Temporary epistatic and maternal responses can explain overestimation of additive genetic response from offspring-parent regression or from replicated single-generation selection and apparent superiority of mass selection over family or combined selection.     

Statistical tests for detecting positive selection by utilizing high-frequency variants

By comparing the low-, intermediate-, and high-frequency parts of the frequency spectrum, we gain information on the evolutionary forces that influence the pattern of polymorphism in population samples. We emphasize the high-frequency variants on which positive selection and negative (background) selection exhibit different effects. We propose a new estimator of θ (the product of effective population size and neutral mutation rate), θ_L, which is sensitive to the changes in high-frequency variants. The new θ_L allows us to revise Fay and Wu's H-test by normalization. To complement the existing statistics (the H-test and Tajima's D-test), we propose a new test, E, which relies on the difference between θ_L and Watterson's θ_W. We show that this test is most powerful in detecting the recovery phase after the loss of genetic diversity, which includes the postselective sweep phase. The sensitivities of these tests to (or robustness against) background selection and demographic changes are also considered. Overall, D and H in combination can be most effective in detecting positive selection while being insensitive to other perturbations. We thus propose a joint test, referred to as the DH test. Simulations indicate that DH is indeed sensitive primarily to directional selection and no other driving forces.     

Product inhibition during the feeding phasein fed-batch ethanol fermentation of sugar-cane blackstrap molasses

Summary The following equations represent the influence of the ethanol concentration (E) on the specific growth rate of the yeast cells () and on the specific production rate of ethanol () during the reactor filling phase in fed-batch fermentation of sugar-cane blackstrap molasses: = ₀ - k · E and v = v ₀ · K/(K +E) Nomenclature E ethanol concentration in the aqueous phase of the fermenting medium (g.L^–1) - E_m value of E when = 0 or = 0 (g.L^–1) - F medium feeding rate (L.h^–1) - k empirical constant (L.g^–1.h^–1) - K empirical constant (g.L^–1) - M_as mass of TRS added to the, reactor (g) - M_cs mass of consumed TRS (g) - M_e mass of ethanol in the aqueous phase of the fermenting medium (g) - M_s mass of TRS in the aqueous phase of the fermenting medium (g) - M_x mass of yeast cells (dry matter) in the fermenting medium (g) - r correlation coefficient - S TRS concentration in the aqueous phase of the fermenting medium (g.L^–1) - S_m TRS concentration of the feeding medium (g.L^–1) - t time (h) - T temperature (° C) - TRS total reducing sugars calculated as glucose - V volume of the fermenting medium (L) - V₀ volume of the inoculum (L) - X yeast cells concentration (dry matter) in the fermenting medium (g.L^–1) - filling-up time (h) - specific growth rate of the yeast cells (h^–1) - ₀ value of when E=0 - specific production rate of ethanol (h^–1) - ₀ value of when E=0 - density of the yeast cells (g.L^–1) - dry matter content of the yeast cells     

Infrared characterization of conformational differences in the lamellar phases of 1,3-dipalmitoyl-sn-glycero-2-phosphocholine

Fourier transform infrared spectroscopy was used to characterize the lamellar phases of 1,3-dipalmitoyl-sn-glycero-2-phosphocholine (1,3-DPPC), a positional isomer of 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (1,2-DPPC). The molecule exists in three distinct phases over the temperature interval 0–70°C. In the low-temperature (L_c) phase, the spectra are indicative of acyl chains packed in an orthorhombic subcell, while the carbonyl groups and phosphate ester at the head group show evidence of only partial hydration. The transition from the low-temperature (L_c) phase to the intermediate-temperature (Lβ) phase at 25°C corresponds to a temperature-induced head-group hydration in which the hydration of the phosphate and carbonyl ester groups results in the reorganization of the hydrocarbon chain-packing subcell from orthorhombic to hexagonal. The transition from the intermediate (L_β) to the high-temperature (L_α) phase at 37°C is a gel-to-liquid-crystalline phase transition analogous to the 41.5°C transition of 1,2-DPPC. The spectra of the acyl-chain carbonyl groups show evidence of significant differences in molecular conformation at the carbonyl esters in the L_c phase. In the L_β and L_α phases, the carbonyl band contour becomes much more symmetric. However, two components are clearly present in the spectra indicating that the sn-1 and sn-3 carbonyls experience slightly different environments. The observed differences are likely due to a preferred conformation of the phosphocholine group relative to the glycerol backbone. Indications from the infrared spectra of differences in the structure of the C=O groups provide a possible explanation for the selection of the sn-1 chain of 1,3-DPPC by phospholipase A₂ on the basis of a preferred head group conformation.     

Dispersal-mediated selection on plant height in an autochorously dispersed herb

Dispersal ability is an important fitness component in most plant species. Therefore, some phenotypic traits can be selected due to their effect on dispersal. In this study I determine the potential for dispersal-mediated selection on plant height in an autochorous plant, Erysimum mediohispanicum (Brassicaceae). Selection was quantified by selection gradients, structural equation modeling and generalized additive models. I detected significant dispersal-mediated linear selection gradient on plant height, taller plants dispersing seeds farther. Nevertheless, the generalized additive models suggest that the selection on stalk height was non linear. Indeed, it detected a threshold in the effect of stalk height on dispersal ability; plants shorter than that threshold had an extremely short dispersal, whereas plants taller than that threshold dispersed the seeds very far. Furthermore, the structural equation modeling showed that stalk height indirectly affected dispersal distance through its significant effect on one reproduction-related fitness component, taller plants having greater fecundity. Selection on E. mediohispanicum stalk height occurs through two simultaneous paths, one via producing many seeds and the other through increasing probability of dispersing them far away.     

Population level processes in Rhizobium leguminosarum bv. trifolii: the role of founder effects

The importance of genotype-specific selection between host and symbiont, founder effect, and clonal reproduction in Rhizobia leguminosarum biovar trifolii populations is relatively unknown. A field experiment was conducted to sample 1268 isolates of R. l. bv. trifolii from four genotypically distinct Trifolium pratense plants for allozyme variation at nine loci. Genetic and genotypic variation, population genetic substructure, and linkage disequilibrium were estimated. Of the 1268 isolates 188 genotypically distinct strains (electrophoretic types or ETs) were identified with an average of 11.04 different ETs per plant. Total genetic diversity in the plot was 0.346 and most of the variation was found within plants (= 80%). Our data suggests that genotype-specific selection between the rhizobia and the four host-plant genotypes tested does not influence local population structure, but evidence of founder effect was present. Significant linkage disequilibrium was observed and is most likely due to the clonal reproduction of R. l. bv. trifolii.