Cytogenetic characteristics of a population of Chironomus riparius Meigen 1804 (Diptera, Chironomidae) from a polluted Po river station

A population of Chironomus riparius from a Po river station near Moncalieri (a trace-metal polluted station) was studied. In this population was established a great variability of band structure of polytene chromosomes as well as paracentric heterozygous inversions, deletions, deficiencies, partial breaks, diploid chromosome fragments, and changes in functional activity and appearance of heterochromatin. In arms A through F, some bands had an increased size compared to the standard chromosomic map. Some bands appeared in a heterozygous or normal homozygous state or were amplified. In all arms, many condensed stable bands appeared in the decondensed state when compared to the standard map. Asynaptic zones in arms E and G as well as heterozygous Balbiani rings and NORs were established. Very often the 4th chromosome was almost completely heteropycnotic and looded like a pompon chromosome. For the first time in this species, a high frequency of ectopic pairings of different arms was observed. Telomeric regions involved in ectopic pairings had a granular appearance, as did some centromeres. The hypothesis is advanced that such a high frequency of structural rearrangements could be correlated with genomic distribution of specific mobile elements.     

Genomewide spatial correspondence between nonsynonymous divergence and neutral polymorphism reveals extensive adaptation in Drosophila

The effect of recurrent selective sweeps is a spatially heterogeneous reduction in neutral polymorphism throughout the genome. The pattern of reduction depends on the selective advantage and recurrence rate of the sweeps. Because many adaptive substitutions responsible for these sweeps also contribute to nonsynonymous divergence, the spatial distribution of nonsynonymous divergence also reflects the distribution of adaptive substitutions. Thus, the spatial correspondence between neutral polymorphism and nonsynonymous divergence may be especially informative about the process of adaptation. Here we study this correspondence using genomewide polymorphism data from Drosophila simulans and the divergence between D. simulans and D. melanogaster. Focusing on highly recombining portions of the autosomes, at a spatial scale appropriate to the study of selective sweeps, we find that neutral polymorphism is both lower and, as measured by a new statistic Q(S), less homogeneous where nonsynonymous divergence is higher and that the spatial structure of this correlation is best explained by the action of strong recurrent selective sweeps. We introduce a method to infer, from the spatial correspondence between polymorphism and divergence, the rate and selective strength of adaptation. Our results independently confirm a high rate of adaptive substitution (approximately 1/3000 generations) and newly suggest that many adaptations are of surprisingly great selective effect (approximately 1%), reducing the effective population size by approximately 15% even in highly recombining regions of the genome.     

No accident: genetic codes freeze in error-correcting patterns of the standard genetic code

The standard genetic code poses a challenge in understanding the evolution of information processing at a fundamental level of biological organization. Genetic codes are generally coadapted with, or 'frozen' by, the protein-coding genes that they translate, and so cannot easily change by natural selection. Yet the standard code has a significantly non-random pattern that corrects common errors in the transmission of information in protein-coding genes. Because of the freezing effect and for other reasons, this pattern has been proposed not to be due to selection but rather to be incidental to other evolutionary forces or even entirely accidental. We present results from a deterministic population genetic model of code-message coevolution. We explicitly represent the freezing effect of genes on genetic codes and the perturbative effect of changes in genetic codes on genes. We incorporate characteristic patterns of mutation and translational error, namely, transition bias and positional asymmetry, respectively. Repeated selection over small successive changes produces genetic codes that are substantially, but not optimally, error correcting. In particular, our model reproduces the error-correcting patterns of the standard genetic code. Aspects of our model and results may be applicable to the general problem of adaptation to error in other natural information-processing systems.     

The Impact of Message Mutation on the Fitness of a Genetic Code

The Standard Genetic Code is organized such that similar codons encode similar amino acids. One explanation suggested that the Standard Code is the result of natural selection to reduce the fitness ``load' that derives from the mutation and mistranslation of protein-coding genes. We review the arguments against the mutational load-minimizing hypothesis and argue that they need to be reassessed. We review recent analyses of the organization of the Standard Code and conclude that under cautious interpretation they support the mutational load-minimizing hypothesis. We then present a deterministic asexual model with which we study the mode of selection for load minimization. In this model, individual fitness is determined by a protein phenotype resulting from the translation of a mutable set of protein-coding genes. We show that an equilibrium fitness may be associated with a population with the same genetic code and that genetic codes that assign similar codons to similar amino acids have a higher fitness. We also show that the number of mutant codons in each individual at equilibrium, which determines the strength of selection for load minimization, reflects a long-term evolutionary balance between mutations in messages and selection on proteins, rather than the number of mutations that occur in a single generation, as has been assumed by previous authors. We thereby establish that selection for mutational load minimization acts at the level of an individual in a single generation. We conclude with comments on the shortcomings and advantages of load minimization over other hypotheses for the origin of the Standard Code. Received: 4 April 2001 / Accepted: 22 October 2001     

On the dynamic persistence of cooperation: how lower individual fitness induces higher survivability

We study a model in which cooperation and defection coexist in a dynamical steady state. In our model, subpopulations of cooperators and defectors inhabit sites on a lattice. The interactions among the individuals at a site, in the form of a prisoner's dilemma (PD) game, determine their fitnesses. The chosen PD payoff allows cooperators, but not defectors, to maintain a homogeneous population. Individuals mutate between types and migrate to neighboring sites with low probabilities. We consider both density-dependent and density-independent versions of the model. The persistence of cooperation in this model can be explained in terms of the life cycle of a population at a site. This life cycle starts when one cooperator establishes a population. Then defectors invade and eventually take over, resulting finally in the death of the population. During this life cycle, single cooperators migrate to empty neighboring sites to found new cooperator populations. The system can reach a steady state where cooperation prevails if the global "birth" rate of populations is equal to their global "death" rate. The dynamic persistence of cooperation ranges over a large section of the model's parameter space. We compare these dynamics to those from other models for the persistence of altruism and to predator-prey models.     

Insertional polymorphism of a non-LTR mobile element (NLRCth1) in European populations of Chironomus riparius (Diptera, Chironomidae) as detected by transposon insertion display.

The midge Chironomus riparius is distributed all over the Palearctic region and is well characterized both at the morphological and cytogenetic levels. Here we describe a population study based on the insertional polymorphism of the retroposon NLRCth1, by means of a S-SAP (sequence-specific amplification polymorphism) derived technique (transposon insertion display; TID). While a previous study of allozyme polymorphism in Russian samples showed little variability, all the amplicons we identified are polymorphic. Genetic distances between 6 natural populations were calculated according to Nei and did not show a positive correlation with geographic distances. The genetic diversity detected among individuals of a given population was one order of magnitude higher than that among populations. However, the value of phi(ST) was significant (p < 0.001) and indicates that natural populations are more genetically differentiated than random samples of individuals.     

Footprints of ancient‐balanced polymorphisms in genetic variation data from closely related species

When long‐lasting, balancing selection can lead to “trans‐species” polymorphisms that are shared by two or more species identical by descent. In such cases, the gene genealogy at the selected site clusters by allele instead of by species, and nearby neutral sites also have unusual genealogies because of linkage. While this scenario is expected to leave discernible footprints in genetic variation data, the specific patterns remain poorly characterized. Motivated by recent findings in primates, we focus on the case of a biallelic polymorphism under ancient balancing selection and derive approximations for summaries of the polymorphism data from two species. Specifically, we characterize the length of the segment that carries most of the footprints, the expected number of shared neutral single nucleotide polymorphisms (SNPs), and the patterns of allelic associations among them. We confirm the accuracy of our approximations by coalescent simulations. We further show that for humans and chimpanzees—more generally, for pairs of species with low genetic diversity levels—these patterns are highly unlikely to be generated by neutral recurrent mutations. We discuss the implications for the design and interpretation of genome scans for ancient balanced polymorphisms in primates and other taxa.     

Study of the influence of sporulation conditions on heat resistance of Geobacillus stearothermophilus used in the development of biological indicators for steam sterilization

Biological indicators are important tools in infection control via sterilization process monitoring. The use of a standardized spore crop with a well-defined heat resistance will guarantee the quality of a biological indicator. Ambient factors during sporulation can affect spore characteristics and properties, including heat resistance. The aim of this study is to evaluate the main sporulation factors responsible for heat resistance in Geobacillus stearothermophilus, a useful biological indicator for steam sterilization. A sequence of a three-step optimization of variables (initial pH, nutrient concentration, tryptone, peptone, beef extract, yeast extract, manganese sulfate, magnesium sulfate, calcium chloride and potassium phosphate) was carried out to screen those that have a significant influence on heat resistance of produced spores. The variable exerting greatest influence on G. stearothermophilus heat resistance during sporulation was found to be the initial pH. Lower nutrient concentration and alkaline pH around 8.5 tended to enhance decimal reduction time at 121?°C (D_121°C). A central composite design enabled a fourfold enhancement in heat resistance, and the model obtained accurately describes positive pH and negative manganese sulfate concentration influence on spore heat resistance.     

Egg trading in the simultaneously hermaphroditic polychaete worm Ophryotrocha gracilis (Huth)

Ophryotrocha gracilis (Huth 1934) is a simultaneously hermaphroditicpolychaete worm with external fertilization and a brief protandrousphase. The mating system of this species seems to meet conditionsleading to the establishment of egg-trading behavior. Experimentsshowed that mating occurs in pairs composed of two simultaneoushermaphrodites; sex roles are sequentially alternated and self-fertilizationis avoided. Egg reciprocation is kept evolutionarily stableby laying eggs in multiple, small-sized egg clutches and reducingthe reproductive success of pairs in which one of the two partnersdoes not reciprocate egg exchange. The frequency of ovigeroushermaphrodites in mass cultures of O. gracilis is about 50".Such a high mate availability preadapts hermaphrodites of O.gracilis to change partners very frequently and to reduce investmentin parental care, contrary to what is observed in another egg-trading,simultaneously hermaphroditic species, O. diadema. Laboratorypopulations of O. diadema have a frequency of only 17% ovigeroushermaphrodites     

The expression of a bean PGIP in transgenic wheat confers increased resistance to the fungal pathogen Bipolaris sorokiniana

A possible strategy to control plant pathogens is the improvement of natural plant defense mechanisms against the tools that pathogens commonly use to penetrate and colonize the host tissue. One of these mechanisms is represented by the host plant's ability to inhibit the pathogen's capacity to degrade plant cell wall polysaccharides. Polygalacturonase-inhibiting proteins (PGIP) are plant defense cell wall glycoproteins that inhibit the activity of fungal endopolygalacturonases (endo-PGs). To assess the effectiveness of these proteins in protecting wheat from fungal pathogens, we produced a number of transgenic wheat lines expressing a bean PGIP (PvPGIP2) having a wide spectrum of specificities against fungal PGs. Three independent transgenic lines were characterized in detail, including determination of the levels of PvPGIP2 accumulation and its subcellular localization and inhibitory activity. Results show that the transgene-encoded protein is correctly secreted into the apoplast, maintains its characteristic recognition specificities, and endows the transgenic wheat with new PG recognition capabilities. As a consequence, transgenic wheat tissue showed increased resistance to digestion by the PG of Fusarium moniliforme. These new properties also were confirmed at the plant level during interactions with the fungal pathogen Bipolaris sorokiniana. All three lines showed significant reductions in symptom progression (46 to 50%) through the leaves following infection with this pathogen. Our results illustrate the feasibility of improving wheat's defenses against pathogens by expression of proteins with new capabilities to counteract those produced by the pathogens.