The effect of seed freezing on morphological characteristics of four pink species

The effects of nondeep (?10°C) and deep (?196°C) seed freezing on the morphological characteristics of four pink species were studied. As a rule, various regimes of seed freezing weakly affected plant growth and development. Relatively stable traits (flower diameter, the number of stem nodes, and root length) did not change. Reproductive shoots became slightly shorter, and the pattern of their distribution changed. The number of variable traits (the number of vegetative shoots and the number of flowers) was reduced. However, in the following year, the number of flowers was restored. Characteristics valuable for cryopreservation (the number of fruits and seed germinability) were essentially unchanged. Some stimulatory effects of seed freezing were noted: enhanced seed germination under unfavorable conditions and an increased upper limit of some indices, including the number of reproductive organs.     

Reasons for the apparent variation in the probiotic response

The results obtained by probiotic supplementation are a function of the action of live microorganisms on the very complex population of microorganisms in the gut. The outcome of this extremely complicated interaction is bound to be subject to variation and can be influenced by several factors relating to the product, the administrator and the consumer. If we can define the reasons for these inconsistencies we will be better able to develop improved preparations which when given at the right time and in the right way will give the expected response. Presented at the Second Probiotic Conference, Košice, 15–19 September 2004, Slovakia     

The Loss of Genetic Diversity in Sichuan Taimen as Revealed by DNA Fingerprinting

Species endangerment often derives from the “endangerment” of genetic diversity, thus loss of genetic diversity is an important cause of species extinction. Since historical specimens were unavailable, previous studies mainly described the genetic diversity status in the current population rather than the loss of genetic variation over time. In this study, we collected samples during1998–1999 and obtained historical specimens from 1957 to 1958. Based on the two sets of fish, we determined the changes in genetic diversity of Sichuan taimen using DNA fingerprinting. The differences in genetic parameters between the present samples and historical taimens revealed their loss of genetic variation. As a result, the existing populations have lower viability, and proper management has to be implemented to preserve genetic diversity.     

Genetic population structure and call variation in a passerine bird, the satin bowerbird, Ptilonorhynchus violaceus

Geographic variation in vocalizations is widespread in passerine birds, but its origins and maintenance remain unclear. One hypothesis to explain this variation is that it is associated with geographic isolation among populations and therefore should follow a vicariant pattern similar to that typically found in neutral genetic markers. Alternatively, if environmental selection strongly influences vocalizations, then genetic divergence and vocal divergence may be disassociated. This study compared genetic divergence derived from 11 microsatellite markers with a metric of phenotypic divergence derived from male bower advertisement calls. Data were obtained from 16 populations throughout the entire distribution of the satin bowerbird, an Australian wet-forest-restricted passerine. There was no relationship between call divergence and genetic divergence, similar to most other studies on birds with learned vocalizations. Genetic divergence followed a vicariant model of evolution, with the differentiation of isolated populations and isolation-by-distance among continuous populations. Previous work on Ptilonorhynchus violaceus has shown that advertisement call structure is strongly influenced by the acoustic environment of different habitats. Divergence in vocalizations among genetically related populations in different habitats indicates that satin bowerbirds match their vocalizations to the environment in which they live, despite the homogenizing influence of gene flow. In combination with convergence of vocalizations among genetically divergent populations occurring in the same habitat, this shows the overriding importance that habitat-related selection can have on the establishment and maintenance of variation in vocalizations.     

Using the AMOVA framework to estimate a standardized genetic differentiation measure

Comparison of population structure between studies can be difficult, because the value of the often-used FST-statistic depends on the amount of genetic variation within populations. Recently, a standardized measure of genetic differentiation was developed based on GST, which addressed this problem, though no method was provided to estimate this standardized measure without bias. Here I present a method to estimate a standardized measure of population differentiation based on the analysis of molecular variance framework. One advantage of the method is that it can be readily expanded to include different hierarchical levels in the tested population structure.     

The evolution of genetic architecture under frequency-dependent disruptive selection

We propose a model to analyze a quantitative trait under frequency-dependent disruptive selection. Selection on the trait is a combination of stabilizing selection and intraspecific competition, where competition is maximal between individuals with equal phenotypes. In addition, there is a density-dependent component induced by population regulation. The trait is determined additively by a number of biallelic loci, which can have different effects on the trait value. In contrast to most previous models, we assume that the allelic effects at the loci can evolve due to epistatic interactions with the genetic background. Using a modifier approach, we derive analytical results under the assumption of weak selection and constant population size, and we investigate the full model by numerical simulations. We find that frequency-dependent disruptive selection favors the evolution of a highly asymmetric genetic architecture, where most of the genetic variation is concentrated on a small number of loci. We show that the evolution of genetic architecture can be understood in terms of the ecological niches created by competition. The phenotypic distribution of a population with an adapted genetic architecture closely matches this niche structure. Thus, evolution of the genetic architecture seems to be a plausible way for populations to adapt to regimes of frequency-dependent disruptive selection. As such, it should be seen as a potential evolutionary pathway to discrete polymorphisms and as a potential alternative to other evolutionary responses, such as the evolution of sexual dimorphism or assortative mating.     

Masking interference and the evolution of the acoustic communication system in the Amazonian dendrobatid frog Allobates femoralis

The efficacy of communication relies on detection of species-specific signals against the background noise. Features affecting signal detection are thus expected to evolve under selective pressures represented by masking noise. Spectral partitioning between the auditory signals of co-occurring species has been interpreted as the outcome of the selective effects of masking interference. However, masking interference depends not only on signal's frequency but on receiver's range of frequency sensitivity; moreover, selection on signal frequency can be confounded by selection on body size, because these traits are often correlated. To know whether geographic variation in communication traits agrees with predictions about masking interference effects, we tested the hypothesis that variation in the male-male communication system of the Amazonian frog, Allobates femoralis, is correlated with the occurrence of a single species calling within an overlapping frequency range, Epipedobates trivittatus. We studied frogs at eight sites, four where both species co-occur and four where A. femoralis occurs but E. trivittatus does not. To study the sender component of the communication system of A. femoralis and to describe the use of the spectral range, we analyzed the signal's spectral features of all coactive species at each site. To study the receiver component, we derived frequency-response curves from playback experiments conducted on territorial males of A. femoralis under natural conditions. Most geographic variation in studied traits was correlated with either call frequency or with response frequency range. The occurrence of E. trivittatus significantly predicted narrower and asymmetric frequency-response curves in A. femoralis, without concomitant differences in the call or in body size. The number of acoustically coactive species did not significantly predict variation in any of the studied traits. Our results strongly support that the receiver but not the sender component of the communication system changed due to masking interference by a single species.     

Adaptive change in the resource-exploitation traits of a generalist consumer: the evolution and coexistence of generalists and specialists

Mathematical models of consumer-resource systems are used to explore the evolution of traits related to resource acquisition in a generalist consumer species that is capable of exploiting two resources. The analysis focuses on whether evolution of traits determining the capture rates of two resources by a consumer species produce one generalist, two specialists, or all three types, when all types are characterized by a common fitness function. In systems with a stable equilibrium, evolution produces one generalist or two specialists, depending on the second derivative of the trade-off relationship. When there are sustained population fluctuations, the nature of the trade-off between the consumer's capture rates of the two resources still plays a key role in determining the evolutionary outcome. If the trade-off is described by a choice variable between zero and one that is raised to a power n, polymorphic states are possible when n > 1, which implies a positive second derivative of the curve. These states are either dimorphism, with two relatively specialized consumer types, or trimorphism, with a single generalist type and two specialists. Both endogenously driven consumer-resource cycles, and fluctuations driven by an environmental variable affecting resource growth are considered. Trimorphic evolutionary outcomes are relatively common in the case of endogenous cycles. In contrast to a previous study, these trimorphisms can often evolve even when new lineages are constrained to have phenotypes very similar to existing lineages. Exogenous cycles driven by environmental variation in resource growth rates appear to be much less likely to produce a mixture of generalists and specialists than are endogenous consumer-resource cycles.     

Evidence for overdominant selection maintaining X-linked fitness variation in Drosophila melanogaster

The role of balancing selection in maintaining genetic variation for fitness is largely unresolved. This reflects the inherent difficulty in distinguishing between models of recurrent mutation versus selection, which produce similar patterns of inbreeding depression, as well as the limitations of testing such hypotheses when fitness variation is averaged across the genome. Signatures of X-linked overdominant selection are less likely to be obscured by mutational variation because X-linked mutations are rapidly eliminated by purifying selection in males. Although models maintaining genetic variation for fitness are not necessarily mutually exclusive, a series of predictions for identifying X-linked overdominant selection can be used to separate its contribution from other underlying processes. We consider the role of overdominant selection in maintaining fitness variation in a sample of 12 X chromosomes from a population of Drosophila melanogaster. Substantial variation was observed for male reproductive success and female fecundity, with heterozygous-X genotypes exhibiting the greatest degree of variance, a finding that agrees well with predictions of the overdominance model. The importance of X-linked overdominant selection is discussed along with models of recurrent mutation and sexually antagonistic selection.