Selection caused by self-fertilization

The generally held view that increased self-fertilization should be advantageous in the absence of counteracting selective forces reducing viability or fertility is reexamined. It is pointed out that the models on which this view is based all imply a gain in male (pollen) fertility with increased selfing. Hence, the postulated advantage may equally well be due to increased fertility, a fact which reopens the discussion on the selective significance of differential selfing. A new model for differential self-fertilization is presented which avoids built-in fertility selection by explicitly considering pollen available for self- and for cross-pollination. Plant types are distinguished with respect to their amounts r_i of pollen available for self-pollination, and these types are assumed to be identical with respect to their pollen and ovule fertilities. Moreover, the efficiency of cross-pollination is allowed to depend, for example, on population density, thus giving rise to a parameter b called “crossing potential”, while the efficiency of self-pollination is described by a parameter a called “selfing potential”. These parameters may be conceived of as ecological parameters. Increasing r_i produces a simultaneous increase in each of the four measures of self-fertilization (introduced in Part I of the present series) irrespective of the values of the ecological parameters. It is then shown that increased selfing can be both advantageous and disadvantageous in terms of fitness, dependig on the ecological parameters as well as on the mode of self-fertilization (i.e. where selfing occurs before or after outcrossing). The main result is, roughly, that for both selfing modes high crossing and low selfing potential favour increased cross-fertilization, while the reverse favours increased self-fertilization. However, the regions for a and b in which this holds true differ substantially for the two selfing modes. In the complements of these regions strange conditions for the evolution of increased selfing or outcrossing, respectively, exist. The significance of these results for explaining experimental observations is discussed.     

Selection and fitness in bacteriocin-producing bacteria.

Bacteriocins are proteinaceous anticompetitor molecules produced by bacteria against closely related species. A number of theoretical models have been used to explain experimental data that indicate high polymorphisms among bacteriocins and a frequency-dependent nature of selection for bacteriocin-producing strains. The majority of these experimental data were, however, obtained from investigations into the colicin group of bacteriocins produced by Gram-negative bacteria. The conclusions drawn from these models have been extrapolated to other bacteriocins and allelopathic compounds in general. Examination of more recent experimental investigations into the bacteriocins of Gram-positive bacteria indicate a lower degree of polymorphism and a less frequency dependent mode of selection among these strains them among the colicin-producing strains. Here we examine these contradictions in the light of the assumptions and conclusions of the theoretical models and reported data. We show that fitness costs as indicated by decreased relative maximum growth rate associated with bacteriocin production may be much lower in many cases than is assumed in the present models. A lower fitness cost associated with bacteriocin production adequately explains the newer data from Gram-positive bacteria cited here, and indicates that extrapolation of existing models to all bacteriocins and other allelopathic compounds is not appropriate.     

Distribution of fitness and virulence effects caused by single-nucleotide substitutions in Tobacco Etch virus

Little is known about the fitness and virulence consequences of single-nucleotide substitutions in RNA viral genomes, and most information comes from the analysis of nonrandom sets of mutations with strong phenotypic effect or which have been assessed in vitro, with their relevance in vivo being unclear. Here we used site-directed mutagenesis to create a collection of 66 clones of Tobacco etch potyvirus, each carrying a different, randomly chosen, single-nucleotide substitution. Competition experiments between each mutant and the ancestral nonmutated clone were performed in planta to quantitatively assess the relative fitness of each mutant genotype. Among all mutations, 40.9% were lethal, and among the viable ones, 36.4% were significantly deleterious and 22.7% neutral. Not a single case of beneficial effects was observed within the level of resolution of our measures. On average, the fitness of a genotype carrying a deleterious but viable mutation was 49% smaller than that for its unmutated progenitor. Deleterious mutational effects conformed to a beta probability distribution. The virulence of a subset of viable mutants was assessed as the reduction in the number of viable seeds produced by infected plants. Mutational effects on virulence ranged between 17% reductions and 24.4% increases. Interestingly, the only mutations showing a significant effect on virulence were hypervirulent. Competitive fitness and virulence were uncorrelated traits.     

Predation risk effects on fitness related measures in a resident bird

Predation risk is thought to be highly variable in space and time. However, breeding avian predators may create locally fixed and spatially fairly predictable predation risk determined by the distance to their nest. From the prey perspective, this creates predation risk gradients that potentially have an effect on fitness and behavioural decisions of prey. We studied how breeding avian predators affect habitat selection (nest location) and the resulting fitness consequences in a northern population of resident willow tit ( Parus montanus ). Data included 429 willow tit nests over a four year period in a landscape containing a total of 33 avian predator nests. Willow tit nests were located randomly in the landscape and no predator avoidance in habitat selection or emptying of territories in proximity to predators was observed. Nestling size, however, was positively associated with distance from predator nests (n=252). Nestling mass and wing length were about 4.5% smaller close to predator nests compared to nestlings raised far from predator nests. Tarsus length also exhibited a positive relationship with increasing distance from predator nest but this was limited to habitats of young forests and pine bogs or dense mixed forests (4% increase). It is likely that habitat structural complexity influenced the perception of predation risk in different habitats. Our results indicate that willow tits do not provide reliable cues of predator free habitats for settling migrants. Nonetheless, breeding avian predators may create predictable predation risk in the landscape which is an important factor affecting reproductive success and potentially the demography of prey populations.     

The experimental assessment of fitness in Drosophila. I. Comparative measures of competitive reproductive success

Strains of D. melanogaster, representing a range of genetic diversity, were systematically subjected to each of several techniques that have been devised to assess total or net fitness. All of these techniques operationally define fitness in terms of reproductive success under competitive conditions. For the set of strains tested, an excellent correlation was obtained between the competitive indices produced by two single-generation intraspecific competitions, despite fundamental differences in the strains used as competitive standards. The results of two interspecific tests were also correlated, but the intraspecific vs. interspecific test result comparisons ranged from highly significant to nonsignificant correlations. For the fitness estimates, the overall variances produced by the intraspecific tests were greater than those obtained in the interspecific competitions. No significant correlations were found between the results of a long-term (multi-generation) competitive assessment of fitness and any of the single-generation tests. Therefore, although all of these tests refer to a net parameter called "fitness", they are clearly not all measuring the same thing. Differences in the competitive interactions operative in each technique are suggested to account for these results.     

Coevolution of self-fertilization and inbreeding depression. I. Mutation-selection balance at one and two loci

Simple theories for the evolution of breeding systems suggest that the fate of an allele that modifies the rate of self-fertilization hinges only on the degree to which selfing reduces opportunities for outcrossing ("pollen discounting") and the extent of inbreeding depression. These theories predict that outcrossing evolves whenever deleterious mutations have a more severe effect in combination than expected from their individual effects. We study the evolutionary dynamics of a modifier of the rate of self-fertilization in populations subject to complete pollen discounting and recurrent mutations which impair viability at a single locus in diploids and at two loci in haploids. Our analysis indicates that genetic associations arising immediately upon the introduction of a rare modifier allele generate substantial quantitative and qualitative departures from expectation. Higher rates of segregation under selfing in our one-locus diploid model generate positive associations between enhancers of selfing and wild-type viability alleles, which in turn favor the evolution of selfing under a wider range of conditions than expected. Greater opportunities for recombination under outcrossing in our two-locus haploid model generate positive associations between enhancers of outcrossing and wild-type viability alleles. These associations favor the evolution of outcrossing under a wider range of conditions, and introduce the possibility of stable mixed mating systems involving both selfing and outcrossing. Our explicit analysis of genetic associations between loci affecting viability and the rate of self-fertilization indicates that modifiers that enhance the production of offspring with very high (and very low) viability by promoting segregation or recombination develop positive associations with high viability. This advantage of producing extremes can compensate for an initial disadvantage in offspring number.     

Wolbachia属共生菌及其对节肢动物宿主适合度的影响

Wolbachia是广泛分布于节肢动物体生殖组织内呈母质遗传的一类共生细菌。近30多年来,大量的研究主要集中于Wolbachia对宿主生殖方式的调控方面;近年来的研究发现,Wolbachia对节肢动物宿主的适合度具有不同程度的影响。现对Wolbachia的宿主分布、存在部位及其对节肢动物宿主种群适合度的影响等方面进行了综述,探讨了Wolbachia在该领域的研究意义和潜在的应用价值。     

Selection in yeast I: Assessing genetic stability and relative fitness of commercial yeasts

Summary The potential for changes in allele frequencies in yeast populations by selection was examined. Cells from the wine yeastSaccharomyces cerevisiae (strain Montrachet) were grown over a large number of generations using two different culturing techniques, each with two variations: serial transfers on WLN agar plates with and without UV irradiation, and continuous culture in autoclaved and in filter-sterilized grape must. A low frequency of variant isozyme patterns was found in samples taken at the end of the experiment. Growth rates in must and on agar plates were also examined, and it was found that all samples were faster-growing than the original strain, to varying degrees. Applications for the selection system developed are discussed.     

Two patterns of daily hypoxic exposure and their effects on measures of chemosensitivity in humans.

The purpose of this study was to compare chemoresponses following two different intermittent hypoxia (IH) protocols in humans. Ten men underwent two 7-day courses of poikilocapnic IH. The long-duration IH (LDIH) protocol consisted of daily 60-min exposures to normobaric 12% O(2). The short-duration IH (SDIH) protocol comprised twelve 5-min bouts of 12% O(2), separated by 5-min bouts of room air, daily. Isocapnic hypoxic ventilatory response (HVR) was measured daily during the protocol and 1 and 7 days following. Hypercapnic ventilatory response (HCVR) and CO(2) threshold and sensitivity (by the modified Read rebreathing technique) were measured on days 1, 8, and 14. Following 7 days of IH, the mean HVR was significantly increased from 0.47 +/- 0.07 and 0.47 +/- 0.08 to 0.70 +/- 0.06 and 0.79 +/- 0.06 l.min(-1).%Sa(O(2))(-1) (LDIH and SDIH, respectively), where %Sa(O(2)) is percent arterial oxygen saturation. The increase in HVR reached a plateau after the third day. One week post-IH, HVR values were unchanged from baseline. HCVR increased from 3.0 +/- 0.4 to 4.0 +/- 0.5 l.min(-1).mmHg(-1). In both the hyperoxic and hypoxic modified Read rebreathing tests, the slope of the CO(2)/ventilation plot was unchanged by either intervention, but the CO(2)/ventilation curve shifted to the left following IH. There were no correlations between the changes in response to hypoxia and hypercapnia. There were no significant differences between the two IH protocols for any measures, indicating that comparable changes in chemoreflex control occur with either protocol. These results also suggest that the two methods of measuring CO(2) response are not completely concordant and that the changes in CO(2) control do not correlate with the increase in the HVR.     

Joint effects of self-fertilization and population structure on mutation load, inbreeding depression and heterosis

Both the spatial distribution of organisms and their mode of reproduction have important effects on the change in allele frequencies within populations. In this article, we study the combined effect of population structure and the rate of partial selfing of organisms on the efficiency of selection against recurrent deleterious mutations. Assuming an island model of population structure and weak selection, we express the mutation load, the within- and between-deme inbreeding depression, and heterosis as functions of the frequency of deleterious mutants in the metapopulation; we then use a diffusion model to calculate an expression for the equilibrium probability distribution of this frequency of deleterious mutants. This allows us to derive approximations for the average mutant frequency, mutation load, inbreeding depression, and heterosis, the simplest ones being Equations 35-39 in the text. We find that population structure can help to purge recessive deleterious mutations and reduce the load for some parameter values (in particular when the dominance coefficient of these mutations is <0.2-0.3), but that this effect is reversed when the selfing rate is above a given value. Conversely, within-deme inbreeding depression always decreases, while heterosis always increases, with the degree of population subdivision, for all selfing rates.     

The units of selection and measures of fitness

This paper presents a unified account of the properties of the measures, Malthusian parameter and entropy in predicting evolutionary change in populations of macromolecules, cells and individuals. The Malthusian parameter describes the intrinsic rate of increase of the population. The entropy describes the intrinsic variability in populations: it characterizes the variability in mutation and replication rates in populations of macromolecules; the rate of decay of synchrony in populations of cells; and the degree of iteroparity in populations of individuals. The Malthusian parameter determines ultimate population numbers: under constant environmental conditions, it is the rate of increase when equilibrium conditions are attained. Entropy determines population stability: the gain in the Malthusian parameter due to small fluctuations in the life-cycle variables is determined by entropy. These properties, which are valid for populations of macromolecules, cells and individuals, show that the Malthusian parameter and entropy act as complimentary fitness indices in understanding evolutionary change in populations.     

Selection of cryoprotectants based on their toxic effects on oyster gametes and embryos

Cryopreservation is a valuable tool for aquaculture by providing continuous seed production, regardless of the spawning seasons. This study aimed to select the least toxic among the cryoprotectants dimethyl sulfoxide (Me2SO), propylene glycol (PG), and methanol (MET) based on their toxicological effects on Crassostrea rhizophorae gametes and trochophores. They were exposed for 10, 20, and 30 min to a range of concentrations of those cryoprotectants. The endpoint was EC15-24 h (effective concentration which causes abnormalities in 15% of the population exposed to the cryoprotectants for 24 h), recently determined as the chronic value (the concentration at which chronic effects are first observed) for C. rhizophorae embryonic phases. There were no significant differences (p>0.05) among the exposure times in Me2SO toxic effects to either gametes or trochophores. For MET, the increase in exposure time resulted in higher toxicity for gametes, but not for trochophores, while for PG there was a significant (p>0.05) increase in toxicity with the increase of exposure for trochophores and spermatozoa, but not for oocytes. For gametes, MET was the most toxic among the cryoprotectants, while PG was the most toxic for trochophores.     

Pathogen and host genotype differently affect pathogen fitness through their effects on different life-history stages

ABSTRACT: BACKGROUND: Adaptation of pathogens to their hosts depends critically on factorsaffecting pathogen reproductive rate. While pathogen reproduction is the end result of an intricate interaction between host and pathogen, the relative contributions of host and pathogen genotype to variation in pathogen life history within the hostare not well understood. Untangling these contributions allows us to identify traits withsufficient genetic variation for selection to act and to identify mechanisms of coevolution between pathogens and their hosts. We investigated the effects of pathogen and host genotype on three life-history components of pathogen fitness; infection efficiency, latent period, and sporulation capacity, in the oat crown rust fungus, Puccinia coronata f.sp. avenae, as it infects oats (Avena sativa). RESULTS: We show that both pathogen and host genotype significantly affect total spore production butdo so through their effects on different life-history stages. Pathogen genotype has the strongest effect on the early stage of infection efficiency, while host genotype most strongly affects the later life-history stages of latent period and sporulation capacity.In addition, host genotype affected the relationship between pathogen density and the later life-history traits oflatent period and sporulation capacity. We did not find evidence of pathogen-by-host genotypic (GxG) interactions. CONCLUSION: Our results illustrate mechanisms by which variation in host populationswill affect the evolution of pathogen lifehistory. Results show that differentpathogen life-history stages have the potential to respond differently to selection by host or pathogen genotypeand suggest mechanisms of antagonistic coevolution. Pathogen populations may adapt tohost genotype through increased infection efficiency while their plant hosts may adapt by limiting the later stages ofpathogen growthand spore production within the host.