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PubMed1997

Evaluation of chromatin condensation in human spermatozoa: a flow cytometric assay using acridine orange staining.

The quality of sperm chromatin is an important factor in fertilization and is especially critical where one spermatozoon is artificially selected for fertilizing an egg (as in intracytoplasmic sperm injection). In this study, flow cytometry after staining of human spermatozoa with Acridine Orange was used to study chromatin structure. A method is described for estimating the percentage of cells in a human sperm sample that have completed epididymal maturation in regard to chromatin condensation. Of the 121 samples of the semen that were examined, nine contained a higher percentage of hypocondensed spermatozoa and six samples contained elevated amounts of hypercondensed spermatozoa. In addition to aberrancies in chromatin condensation other defects showed up as satellite populations of spermatozoa with higher than normal ratios of red/green fluorescence after Acridine Orange staining. Such defects were found in 15 semen samples. The use of swim-up and Percoll gradient centrifugation methods was shown to improve the percentage of spermatozoa with normal chromatin structure in some samples with poor initial quality.

Acridine Orange↗
PubMed1983

Epidemiology of canine heartworm infection.

Factors affecting transmission of D. immitis between dogs are summarized in table 4. Although this list may not be exhaustive, it illustrates the complexity of heartworm epidemiology. There are interesting and provocative similarities in the epidemiology of canine and human filariasis. Ottesen (148) has described the spectrum of disease observed in regions endemic for lymphatic filariasis. It includes patients with 1) microfilariae, but no symptoms; 2) disease, but no microfilariae; 3) tropical pulmonary eosinophilia of filarial etiology; and 4) no disease and no evidence of infection. A dog population in areas endemic for D. immitis may show a similar range of clinical manifestations. For example, dogs with microfilariae may not demonstrate clinical disease; dogs with occult infection may show signs ranging from none to severe, including signs similar to the human tropical pulmonary eosinophilia syndrome. Although some dogs live in endemic areas and, apparently, are not infected, it is difficult to determine whether those dogs are resistant to infection or if they have not had adequate exposure. One study has explored the relationship between human W. bancrofti infection and genetic factors (149). Although familial clustering of cases was apparent, the authors were unable to distinguish with certainty whether this association was due to environmental or genetic influences. Carefully designed seroepidemiologic studies of canine heartworm infection may shed some light on genetic or other factors which confer resistance or susceptibility to filarial infections. Natural strain variation has been recognized in several filariae of medical importance including O. volvulus (150), B. malayi (151), and L. loa (2). Strain variation may be manifested as differences in behavior patterns such as microfilarial periodicity, vector and host affinities, and pathogenicity. A laboratory study (88) demonstrated how such strains could arise. In this study, a strain of Brugia patei was adapted to a new, normally refractory host, by artificial selection. The possible existence of different physiologic races of D. immitis should be investigated. Experimental infections with D. immitis have been achieved under laboratory conditions in numerous mosquito species (77).(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗
PubMedDate not supplied

Rapid unidirectional change of hybrid dysgenesis potential in Drosophila.

Two main types of Drosophila melanogaster strains have been previously described in relation to hybrid gonadal sterility: P (paternally derived) and M (maternally derived). When M strain females are mated with P strain males, cytoplasm-chromosome interactions result in variable frequencies of sterility in their hybrid progeny. A third neutral strain type, Q, generally has no significant potential for sterility in any hybrid strain combination. Mixed populations of these three types of strains were set up in various combinations and initial frequencies. They were subsequently maintained without artificial selection and monitored for their gonadal sterility and potential. All 24 mixed cultures initiated with P and M parents independently evolved rapidly towards a state of at least moderate P activity, which was maintained in succeeding generations. The M cytotype was not maintained after 10-20 generations except at very low frequencies. Changes in sterility potential were less clear cut in those mixed populations that originally included neutral strain flies, but there was a tendency for the frequencies of sterility of the majority of PQ and QM mixed cultures to change in the same direction as for the P and M combinations. The results are discussed in terms of conventional mechanisms of segregation and selection and in terms of an alternative hypothesis of chromosome contamination in which sterility factors may be transposed from P to M chromosomes, resulting in extremely rapid unidirectional population changes. Possible implications of these results for general models of the evolution of hybrid dysgenesis are explored.

Animals↗
PubMed2003

Phase shifts and Per gene expression in mouse suprachiasmatic nucleus.

In mammals, circadian rhythms controlled by the suprachiasmatic nuclei are entrained by photic stimuli. To investigate the molecular mechanism of photic entrainment, we examined light-induced behavioral phase delays and associated changes in mPer1 and mPer2 gene expression in the suprachiasmatic nuclei of two mouse lines artificially selected for nest-building behavior. Big nest-builders show larger phase delays than small nest-builders. Light-induced mPer1 and mPer2 expression was examined in individual mice previously tested for phase shifting at circadian time 16. Light-induced mPer2 expression was significantly higher in big compared to small nest-builders. No difference was found between lines in light-induced mPer1 expression. The results suggest a more important role for mPer2 than for mPer1 gene expression in behavioral phase delays.

Animals↗
PubMed2006

As you reap, so shall you sow: coupling of harvesting and inoculating stabilizes the mutualism between termites and fungi.

At present there is no consensus theory explaining the evolutionary stability of mutualistic interactions. However, the question is whether there are general 'rules', or whether each particular mutualism needs a unique explanation. Here, I address the ultimate evolutionary stability of the 'agricultural' mutualism between fungus-growing termites and Termitomyces fungi, and provide a proximate mechanism for how stability is achieved. The key to the proposed mechanism is the within-nest propagation mode of fungal symbionts by termites. The termites suppress horizontal fungal transmission by consuming modified unripe mushrooms (nodules) for food. However, these nodules provide asexual gut-resistant spores that form the inoculum of new substrate. This within-nest propagation has two important consequences: (i) the mutualistic fungi undergo severe, recurrent bottlenecks, so that the fungus is likely to be in monoculture and (ii) the termites 'artificially' select for high nodule production, because their fungal food source also provides the inoculum for the next harvest. I also provide a brief comparison of the termite-fungus mutualism with the analogous agricultural mutualism between attine ants and fungi. This comparison shows that--although common factors for the ultimate evolutionary stability of mutualisms can be identified--the proximate mechanisms can be fundamentally different between different mutualisms.

Agaricales↗
PubMed1993

Accounting for bias in estimates of the rate of polygenic mutation.

Experimental data on the rate of response to artificial selection in initially inbred lines or the rate of divergence among inbred sublines can be used to estimate the rate of increase in variance of quantitative traits from new mutations. So far estimates have been based on the infinitesimal model of many genes with small additive effects which imply a rate of increase in heritability for Drosophila melanogaster bristle number traits of about 0.1% per generation. Such estimates are biased because mutants tend to have large effects, to have non-additive gene action, and to be deleterious. Here, recent information on the distribution of effects of new mutations on Drosophila melanogaster bristle number and viability is used to infer the direction and magnitude of this bias. The infinitesimal model tends to underestimate the mutational variance, typically by a factor of about 3, but this factor depends on the experimental design. Averages of revised estimates, accounting for this bias, of the per generation increment in heritability from mutation are 0.36% and 0.21% for abdominal and sternopleural bristle number, respectively, in experiments involving M strains, and 1.4% and 0.7% for abdominals and sternopleurals, respectively, in P strains.

Animals↗
PubMed2001

Basis of the trade-off between parasitoid resistance and larval competitive ability in Drosophila melanogaster.

Drosophila melanogaster can be artificially selected for increased resistance against parasitoid wasps that attack the larvae. Lines selected for greater resistance are poorer larval competitors under conditions of resource scarcity. Here we investigated the mechanistic basis of this apparent trade-off. We found that resistant lines have approximately twice the density of haemocytes (blood cells) than that of controls. Haemocytes are involved in encapsulation, the chief cellular immune defence against parasitoids. We have previously shown that resistant lines feed more slowly than controls and hypothesize that limiting resources are being switched from trophic to defensive functions.

Animals↗
PubMed2003

Parasite infection, host resistance and mate choice: battle of the genders in a simultaneous hermaphrodite.

Parasites have been proposed to be fundamental in the evolution of mate choice because differential mating on the basis of heritable disease resistance is expected to lead to progeny with a better genome-environment match than random mating. However, direct empirical data in support of this hypothesis are often lacking, and the relative influences of current and potential infection status (i.e. resistance genotype), and of mate choice versus mate conflict, remain largely unknown. We demonstrate experimentally, using simultaneous hermaphroditic snails (Biomphalaria glabrata) artificially selected for resistance or susceptibility to Schistosoma mansoni infection, that mate choice is influenced by a combination of current and potential parasitic infection status. As predicted by game-theory models, we also found a picture of conflict and cooperation: resistant and susceptible genotypes copulated in either gender and reciprocated (i.e. switched gender) equally when faced with an uninfected partner, but, by contrast, resistant snails actively refused to copulate as females with an infected partner. Such recognition and discrimination has implications for the maintenance of sex and the evolution of recognition systems.

Animals↗
PubMed2004

Is death-feigning adaptive? Heritable variation in fitness difference of death-feigning behaviour.

The adaptation of death-feigning (thanatosis), a subject that has been overlooked in evolutionary biology, was inferred in a model prey-and-predator system. We studied phenotypic variation among individuals, fitness differences, and the inheritance of death-feigning behaviour in the red flour beetle, Tribolium castaneum (Herbst) (Coleoptera: Tenebrionidae). Two-way artificial selections for the duration of death-feigning, over 10 generations, showed a clear direct response in the trait and a correlated response in the frequency of death-feigning, thus indicating variation and inheritance of death-feigning behaviour. A comparison of the two selected strains with divergent frequencies of death-feigning showed a significant difference in the fitness for survival when a model predator, a female Adanson jumper spider, Hasarius adansoni Audouin (Araneomophae: Salticidae), was presented to the beetles. The frequency of predation was lower among beetles from strains selected for long-duration than among those for short-duration death-feigning. The results indicate the possibility of the evolution of death-feigning under natural selection.

Adaptation, Biological↗
PubMed2003

Photomorphogenic responses in maize seedling development.

As an emerging maize (Zea mays) seedling senses light, there is a decrease in the rate of mesocotyl elongation, an induction of root growth, and an expansion of leaves. In leaf tissues, mesophyll and bundle sheath cell fate is determined, and the proplastids of each differentiate into the dimorphic chloroplasts typical of each cell type. Although it has been inferred from recent studies in several model plant species that multiple photoreceptor systems mediate this process, surprisingly little is known of light signal transduction in maize. Here, we examine two photomorphogenic responses in maize: inhibition of mesocotyl elongation and C4 photosynthetic differentiation. Through an extensive survey of white, red, far-red, and blue light responses among a diverse collection of germplasm, including a phytochrome-deficient mutant elm1, we show that light response is a highly variable trait in maize. Although all inbreds examined appear to have a functional phytochrome signal transduction pathway, several lines showed reduced sensitivity to blue light. A significant correlation was observed between light response and subpopulation, suggesting that light responsiveness may be a target of artificial selection. An examination of C4 gene expression patterns under various light regimes in the standard W22 inbred and elm1 indicate that cell-specific patterns of C4 gene expression are maintained in fully differentiated tissues independent of light quality. To our knowledge, these findings represent the first comprehensive survey of light response in maize and are discussed in relation to maize breeding strategies.

Base Sequence↗
PubMed1990

Leaf cuticles behave as asymmetric membranes : evidence from the measurement of diffusion potentials.

Cuticles were isolated enzymatically from the leaves of two maple species (Acer saccharum Marsh and A. platanoides L.) and from orange (Citrus aurantium L.). The cuticles were placed in a plastic cuvette and different concentrations of KCl were perfused over the physiological inner and outer surfaces while the electrical potential (E(10)) that developed across the cuticles and was caused by ion diffusion was measured. E(10) was always positive, indicating that the permeability of K(+) was always greater than that of Cl(-). Measured E(10) in cuticles did not fit the Goldman equation, whereas, E(10) measured during KCl diffusion across selected artificial membranes fit the equation. The magnitude of E(10) in cuticles and artificial membranes also was dependent on ionic strength, decreasing as ionic strength increased. These observations are explained by combining classical transport equations with equations that describe the equilibrium ion distribution between ionic double layers in the cuticle or membranes and the bathing solution.

Journal Article↗
PubMed2006

Neural-electronic inhibition simulated with a neuron model implemented in SPICE.

There have been numerous studies presented in the literature related to the simulation of the interaction between biological neurons and electronic devices. A complicating factor associated with these simulations is the algebraic complexity involved in implementation. This complication has impeded simulation of more involved neural-electronic circuitry and consequently has limited potential advancements in the integration of biological neurons with synthetic electronics. In this paper, we describe a modification to a previously proposed SPICE based Hodgkin-Huxley neuron model that demonstrates more physiologically relevant electrical behavior. We utilize this SPICE based neuron model in conjunction with an external circuit that allows for artificial selective inhibition of neural spiking. The neural firing control scheme proposed herein would allow for action potential frequency modulation of neural activity that, if developed further, could potentially be applied to suppress undesirable neural activity that manifests symptomatically as the tremors or seizures associated with specific pathologies of the nervous system.

Action Potentials↗
PubMed2000

Heritable variation in a family-diagnostic trait.

Derived characters that have not changed during the diversification of a clade provide traits that are diagnostic at higher taxonomic levels. The tetradynamous stamen condition (four long and two short stamens) of the Brassicaceae is an example of a diagnostic trait that has not changed during the diversification of this large flowering plant family. We investigated one hypothesis that might explain the long-term stasis of this trait-that tetradynamous stamens have persisted because of an absence of genetic variation underlying the trait. Through a sib-analysis with Raphanus raphanistrum and an artificial selection experiment with Brassica rapa, we demonstrate that significant genetic variation is present for the tetradynamous condition in both species and that the trait is therefore not constrained from evolutionary change by a lack of heritable genetic variation.

Brassica↗
PubMed2001

Costs of counterdefenses to host resistance in a parasitoid of Drosophila.

The ability of a parasitoid to evolve enhanced counterdefenses against host resistance and its possible costs were studied in a Drosophila-parasitoid system. We reared Asobara tabida (Braconidae, Hymenoptera) exclusively on D. melanogaster to impose artificial selection for improved counterdefenses against cellular encapsulation, the main host defense against parasitism. Controls were reared on D. subobscura, the main host of the population of wasps from which the laboratory culture was derived and a species that never encapsulates parasitoids. We observed improved survival and avoidance of encapsulation in all five selection lines compared to their paired control lines, although there was unexpected variation among pairs. Improved survival was associated with parasitoid eggs becoming embedded in host tissue, where they were protected from circulating haemocytes. There were no differences among lines in average adult size, fat content, egg load, or performance on D. subobscura. However, the duration of the egg stage in selection lines was longer than that of control lines, probably because of reduced nutrient and/or oxygen supply when eggs are embedded in host tissue. We suggest that this delay in hatching reduces the probability of parasitoid survival if another parasitoid egg is laid in the same host (superparasitism or multiparasitism) and hence is a cost of enhanced counterdefenses against host resistance.

Animals↗
PubMed2003

Fitness costs of chemical defense in Plantago lanceolata L.: effects of nutrient and competition stress.

Fitness costs of defense are often invoked to explain the maintenance of genetic variation in levels of chemical defense compounds in natural plant populations. We investigated fitness costs of iridoid glycosides (IGs), terpenoid compounds that strongly deter generalist insect herbivores, in ribwort plantain (Plantago lanceolata L.) using lines that had been artificially selected for high and low leaf IG concentrations for four generations. Twelve maternal half-sib families from each selection line were grown in four environments, consisting of two nutrient and two competition treatments. We tested whether: (1) in the absence of herbivores and pathogens, plants from lines selected for high IG levels have a lower fitness than plants selected for low IG levels; and (2) costs of chemical defense increase with environmental stress. Vegetative biomass did not differ between selection lines, but plants selected for high IG levels produced fewer inflorescences and had a significantly lower reproductive dry weight than plants selected for low IG levels, indicating a fitness cost of IG production. Line-by-nutrient and line-by-competition interactions were not significant for any of the fitness-related traits. Hence, there was no evidence that fitness costs increased with environmental stress. Two factors may have contributed to the absence of higher costs under environmental stress. First, IGs are carbon-based chemicals. Under nutrient limitation, the relative carbon excess may result in the production of IGs without imposing a further constraint on growth and reproduction. Second, correlated responses to selection on IG levels indicate the existence of a positive genetic association between IG level and cotyledon size. At low nutrient level, a path analysis based on family means revealed that in the presence of competitors, the negative direct effect of a high IG level on aboveground plant dry weight was partly offset by a positive direct effect of the associated larger cotyledon size. This indicates that fitness costs of defense may be modulated by environment-specific fitness effects of genetically associated traits.

Adaptation, Physiological↗
PubMed2004

Fitness of indirectly transmitted pathogens: restraint and constraint.

Many pathogens of medical and veterinary importance have obligatory multihost life cycles. Yet, theoretical models aiming to predict patterns of pathogen reproductive success and the limited empirical data available with which to evaluate them, focus on directly transmitted microparasites. Patterns of host exploitation and the relative fitness of individual pathogen genotypes throughout the different host stages of multihost life cycles have thus remained ignored. We examined correlated responses to artificial selection of Schistosoma mansoni lines selected for high or low infection intensity in the intermediate host. Pathogen fitness in the intermediate host was strongly inversely correlated with pathogen fitness in the definitive host. Moreover, high pathogen infection intensity was associated with decreased, rather than increased, virulence to its intermediate host. These results raise important implications regarding the impact of genetic constraints on the maintenance of genetic and phenotypic polymorphisms in natural populations, the evolution and coevolution of parasite virulence and host specialization, as well as the success of host-directed control programs.

Animals↗
PubMed2006

Behavioural trade-offs between growth and mortality explain evolution of submaximal growth rates.

1. The importance of body size and growth rate in ecological interactions is widely recognized, and both are frequently used as surrogates for fitness. However, if there are significant costs associated with rapid growth rates then its fitness benefits may be questioned. 2. In replicated whole-lake experiments, we show that a domestic strain of rainbow trout (artificially selected for maximum intrinsic growth rate) use productive but risky habitats more than wild trout. Consequently, domestic trout grow faster in all situations, experience greater survival in the absence of predators, but have lower survival in the presence of predators. Therefore, rapid growth rates are selected against due to increased foraging effort (or conversely, lower antipredator behaviour) that increases vulnerability to predators. In other words, there is a behaviourally mediated trade-off between growth and mortality rates. 3. Whereas rapid growth is beneficial in many ecological interactions, our results show the mortality costs of achieving it are large in the presence of predators, which can help explain the absence of an average phenotype with maximized growth rates in nature.

Animals↗
PubMed2004

Quantitative trait loci affecting knockdown resistance to high temperature in Drosophila melanogaster.

Knockdown resistance to high temperature is an ecologically important trait in small insects. A composite interval mapping was performed on the two major autosomes of Drosophila melanogaster to search for quantitative trait loci (QTL) affecting knockdown resistance to high temperature (KRHT). Two dramatically divergent lines from geographically different thermal environments were artificially selected on KRHT. These lines were crossed to produce two backcross (BC) populations. Each BC was analysed for 200 males with 18 marker loci on chromosomes 2 and 3. Three X-linked markers were used to test for X-linked QTL in an exploratory way. The largest estimate of autosome additive effects was found in the pericentromeric region of chromosome 2, accounting for 19.26% (BC to the low line) and 29.15% (BC to the high line) of the phenotypic variance in BC populations, but it could represent multiple closely linked QTL. Complete dominance was apparent for three QTL on chromosome 3, where heat-shock genes are concentrated. Exploratory analysis of chromosome X indicated a substantial contribution of this chromosome to KRHT. The results show that a large-effect QTL with dominant gene action maps on the right arm of chromosome 3. Further, the results confirm that QTL for heat resistance are not limited to chromosome 3.

Animals↗