Genetic Flexibility Research Articles

Morphological plasticity and genetic flexibility in a Cambrian trilobite

Morphometric analysis suggests that the uppermost Cambrian trilobite Dikelocephalus minnesotensis possessed a developmentally flexible genotype. Patterns of intraspecific variation in Dikelocephalus contrast with those seen in post-Cambrian trilobites. These results are consistent with other evidence suggesting that genetic systems were less strongly canalized during the Cambrian than in later times. Intraspecific variation in Dikelocephalus is partitioned into two components; one ontogenetically related, the other ontogenetically invariant. Both show a mosaic pattern of interpopulational variation. Slight heterochronic shifts are responsible for some differences between populations, but most characters appear to vary independently.

Pathogenic Neisseriae — A Model of Bacterial Virulence and Genetic Flexibility

The outcome of the early stages of a neisserial infection is determined by receptor-mediated events that culminate in the attachment and invasion of human mucosal tissues. The factors participating in this process, including pili, opacity proteins (Opa), and perhaps lipopolysaccharide (LPS), are subject to complex genetic controls that allow these factors to be produced in multiple forms. Antigenic variation allows the pathogenic Neisseriae to evade the human immune response, and facilitates their interaction with a variety of different cells and tissues of the human host. One of the major genetic mechanisms causing antigenic variation is transformation, which allows virulence genes to be exchanged and recombined between independent Neisseria strains within multiply infected individuals. A number of other factors, such as IgA protease, alpha-factor, and the meningococcal capsule are also implicated in pathogenesis and render the pathogenic Neisseriae an excellent model for the investigation of bacterial virulence.

Gene amplification and expression by RNA viruses and potential for further application to plant gene transfer

The great majority of plant viruses encapsidate messenger‐sense ssRNA and have no natural DNA phase in their life cycle. Despite their RNA nature, essentially any desired change can be introduced into such genomes by using recombinant DNA techniques with suitably constructed, expressible viral cDNA clones. For some viruses such as brome mosaic virus, these methods have been used to define the sequences controlling RNA‐directed genomic RNA replication and the expression of internal genes via subgenomic mRNAs. The results suggest a surprising degree of genetic flexibility, which appears to be reflected in the varied gene complements and genetic organizations of presumably related plant and animal RNA viruses sharing conserved replication genes. Foreign genes inserted in such RNA virus genomes can be amplified and expressed to a high level in transfected plant cells. In addition to the potential use of such viruses as episomal expression vectors, it should be possible to couple the viral pathways of RNA‐dependent RNA synthesis to amplify and to further regulate the expression of genes transformed into plant chromosomes.

Variation in gene flow levels among predominantly self‐pollinated plants

AbstractLevels of gene flow, Nm, were calculated for 34 predominantly self‐pollinated plants, using information on differentiation among populations, Gst. Gene flow levels varied from 0.01 to 6.55, and departed significantly from a uniform distribution. High, medium and low levels of gene flow were found among 15, 38, and 47 percent of the species, respectively. The average heterozygosity (H) and the Nm values showed a positively significant association. These results were compared to gene flow levels obtained with a limited number of predominantly outcrossed plants. Gene flow levels in several self‐pollinated species were comparable to those characteristic of some outcrossed species. Pollen flow, combined with long distance dispersal of propagules through various vectors may be responsible for the high levels of gene flow observed in self‐pollinated species. High gene flow may provide the genetic flexibility required for successful colonization, which is an essential feature of self‐pollinated plants.

Studies on the Life-Cycle, Infectivity and Clinical Effects of Capillaria-Hepatica (Bancroft) (Nematoda) in Mice, Mus-Musculus

The life cycle, infectivity and clinical effects of Capillaria hepatica, a nematode parasite of the liver of Mus musculus, were investigated as a prelude to assessment of the potential of the organism to suppress populations of this economic pest. Eggs of the parasite are deposited in the liver of the host and are not released during the life of that animal. C. hepatica was not transmitted by a trans-seminal, trans- placental or trans-mammary route. A high rate of transmission of C. hepatica (prevalence 71.7% at 145 days) occurred in an enclosed breeding colony of BALB/c mice after the founders of the colony were fed infected mouse liver. After ingestion of infected liver by mice, the majority of eggs were released unembryonated to the environment, via the faeces, within 40 h. Eggs have specific temperature, moisture and oxygen requirements for embryonation. We postulate that transmission of C. hepatica is dependent principally upon cannibalism and/or necrophagy for release of eggs, upon the mouse burrow and the host population density for embryonation and survival of eggs, and upon grooming behaviour within the burrow for ingestion of embryonated eggs. C. hepatica established, and produced clinical signs, in 11 strains of laboratory mice and in wild mice. Hepatomegaly and splenomegaly were characteristic features of infection, and death occurred in some animals. The slight variation in susceptibility to infection among genetically diverse mice suggests that mice may not have the genetic flexibility to rapidly develop resistance to C. hepatica. The parasite has been recorded only in M. rnusculus, Rattus norvegicus and R. rattus in urban areas in Australia. However, native murids Rattus fuscipes and Pseudomys australis, and a marsupial Trichosurus vulpecula, are susceptible to experimental infection. Suspected C. hepatica infection has been found only in native murids near Atherton, Qld. The absence of this conspicuous infection in free-ranging native mammals elsewhere in Australia suggests that they do not come into contact with embryonated eggs of C. hepatica, a situation compatible with the postulated transmission cycle. The life cycle of C. hepatica appears unique among helminth parasites of mammals, in that transmission and hence survival of the parasite is dependent upon death of the definitive host. This parasite may have the potential to suppress mouse population density.

CLONES IN FORESTRY AND IN NATURE

Summary Clonal propagation or apomixis is common in the plant kingdom. Among trees, agamospermy and vegetative propagation are relatively rare in nature. In this paper the roles of different modes of propagation in the survival strategy of three different plant groups are discussed. Conclusions for tree breeding strategies are drawn: clones have an important role to play. Greatest genetic gain and flexibility are attained when both sexual and clonal propagation occur. Genetic diversity should be maintained in any strategy.

SELECTION LIMITS: HAVE THEY BEEN REACHED WITH PIGS?

The likelihood of selection limits for performance traits of economic importance in pig production has been examined in the context of results from research with multiparous species and the context of the genetic improvement programs practiced in the industry. Research results with laboratory species (mice) have not identified insurmountable limits, either genetic or physiological, associated with intense unidirectional selection nor have such limits been identified in pig selection experiments spanning up to 12 generations. In terms of practical pig breeding this would represent approximately 25 yr. The evidence available on industry selection practices in Canada indicates that selection has been neither intense nor cohesive among breeders and that the criteria currently employed have been in place for less than one decade. For these reasons it appears that selection limits, if they exist, are unlikely to be evident for several more decades. This general outlook may not be applicable to all seed stock herds. Research evidence indicates that populations (e.g. fully closed herds) will show considerable individuality of response to selection which, coupled with variations among breeders in the intensity of selection and the performance criteria employed, will contribute to considerable divergence in the results obtained. While selection limits appear unlikely to compromise within-herd responses to sustained unidirectional selection, the universal adoption of specific criteria of performance merit, by fostering the progressive elimination of individual herds and breeds, diminishes the genetic flexibility of the population and restricts its potential to respond rapidly to changing production requirements. This aspect of genetic limits merits serious attention by the hog industry. Key words: Selection limits, pig breeding

Ecological aspects of the recombination problem

Some consequences of the effect of environmental factors on the recombination system are dealt with in this paper. There are two components involved - the system of individual adaptation (F-system) and the genetic system of population adaptation (R-system). Their interaction offers an optimum interrelation of immediate adaptivity and genetic flexibility within the population. Familiar data on the evolution of recombination control systems are considered in connection with the problems of induced broadening of genotypic variation spectrum with a view to selection.A notion of combined recombination rate and spectrum control is introduced here: disturbance control-due to direct influence of environmental factors on R-system (including meiotic processes), negative feedback control - due to dominance-recessiveness relationship between rec-loci alleles of the "fine" control system, and to the dependence of the R-system on the F-system norms of reaction to environment variation. The problem of dependence of the recombination spectrum on environmental factors has been considered and the hypothesis of a possible mechanism of such a dependence suggested.

Chromosome Studies on the Bambusoideae (Gramineae)

Meiotic chromosome numbers and observations were made in eleven species belonging to nine different genera of Bambusoideae. The numbers include counts in eight species that have never been investigated cytologically:Arundinaria aff.amabilis (n=24), Chusquea bambusoides (n=ca. 20),Cryptochloa dressleri (n=11), Olyra ciliatifolia (n=11), Sucrea monophylla (n=11), Raddia brasiliensis (n=11), Strephium distichophyllum (n=11), andPariana aurita (n=22). The generaStrephium andSucrea have not been studied previously with respect to their chromosomes. The significance of these results are discussed in relation to the over 140 published counts in the subfamily, which indicates a preponderance of polyploidy in the woody members, or bamboos, but not in their herbaceous allies. Aneuploidy is rare in the former but frequent in the latter. It is postulated that the herbaceous bambusoids, which flower every year and produce recombinants regularly and in short-time cycles, could have tolerated successive reductions of the linkage groups and their genetic flexibility. These changes would not have been favored in the bamboos, which have more genetic constancy due to the long lapse (often 20–120 years) between flowering generations.

An alternative philosophy for livestock breeding

The merits of animal breeding are related to the rate of improvement which can be offered. Objectives however may alter particularly as a result of social change, so that improved local livestock may be left adapted to outdated conditions, thus emphasising the need for flexibility. Historically such flexibility and indeed much improvement has depended on the introduction of superior strains, previously selected by others, which happened to meet local requirements. The development of strains with divergent biological characteristics as a supplement to existing policies would increase genetic flexibility, could facilitate a faster genetic response and aid the rapid improvement of indigenous breeds. Such a policy would ensure the availability of appropriate genetic variation in the future and thus provide a positive complement to the passive conservation of “rare” breeds.

An ancestral amplification of DNA in primates

Newly evolved, tandemly arrayed, highly repeated DNAs from three primates were compared using Markov-chain and random-simulation approaches. Markov-chain calculations suggested that the repeated DNA sequences derived from the amplification of a progenitor sequence some 55 million years ago. Divergence of the products of this ancestral amplification could have created a library of related DNA sequences from which newly evolved repeated DNA was drawn — by recent amplifications of library members. The ancestral DNA amplification may have provided the genetic flexibility for creating the primate order; more recent amplifications in old-world primates probably led to the present subfamilies. The random-simulation approach verified the idea that repeated DNA evolves nonrandomly. A variable region was identified within the sequence.

Density-dependent selection in a random environment: An evolutionary process that can maintain stable population dynamics

A theoretical analysis of natural selection is presented in which fitnesses depend on population density and randomly varying environmental processes. The theory is based on a general, heuristic analysis of a pair of coupled, nonlinear, stochastic difference equations that describe the joint dynamics of allele frequencies and population size. Four main conclusions emerge from the investigation of a particular class of models: (i) growth rates at low population densities tend to increase; (ii) individual selection, given sufficient genetic flexibility, will mold growth rates at higher densities so that in spite of i, stable deterministic population dynamics are maintained; (iii) "more fit" genotypes cannot be simply characterized-in particular, the mean population size need not be increased; and (iv) genetic polymorphisms can be maintained in both haploid and diploid organisms.

Analysis of growth curves and strategies for altering their shape.

Techniques for fitting longitudinal size-age data are reviewed with emphasis on the biological interpretation of parameters, goodness of fit, comparisons and computational difficulties. Particular attention is paid to the special cases and general form of Richards function, a nonlinear, four-parameter model; to the four-parameter, deterministic growth models derived from the "quadric law of damped exponential growth"; and to a simple two component model especially suited to fitting irregular size-age data. Genetic flexibility in the shape of the growth curve and various methods of altering this shape are discussed. Selection indices and expected changes in shape of the growth curve are described for various selection criteria, including growth and maturing rates, body weight and degree of maturity.

Distribution and dispersal in populations capable of resource depletion : A field study on Cinnabar moth.

Theoretical predictions from a simulation model of insect distributions and dispersal among isolated food plants have been tested with data gathered from 13 Cinnabar moth populations. Agreement with the predictions was good. Egg batch size was equal to or slightly larger than the number which could be supported by the average food plant. Egg batch distribution was more clumped when density was high and when egg batch size was small relative to food plant size. The tendency for larval dispersal was lower in populations in areas where plants were widely spaced. These findings indicate that the Cinnabar moth has the genetic or phenotypic flexibility to adjust egg batch size, egg distribution, and larval dispersal to characteristics of the habitat. A hypothesized model is proposed to describe the interactions between larval dispersal, food plant response to defoliation, and population fluctuations for the Cinnabar moth and its food plant, tansy ragwort.

Niche specialization and species diversity along a California transect.

The structure of the food-web at the flower-herbivore interface was examined along a transect of fourteen communities across central California. All results are commensurate with the hypothesis that in most environments there is selective pressure towards specialization. However, it is only in the most predictable or stable environments that the resultant diminishment of behavioral and genetic flexibility is in a sense "permitted" by subsequent natural selection. In the most extreme environments, behavioral specialization may be a necessary prerequiste permitting briefly thriving ephemeral populations which must recolonize frequently. The data results indicate: 1) Total species number increases with stability and predictability of the climate; 2) As the climate ameliorates, niche-specialization is a progressively more successful strategy; 3) The percentage of niche-specialized species of both plants and flower-feeding herbivores increases in the most severe environments at the expense of the more moderately specialized species; 4) Energetic flow chart redundancy increases in extreme environments; 5) Especially important to an understanding of pollination interactions is the fact that similar physiognomic communities at very different altitudes are in all cases much more similar than different community types within a research site.

Niche specialization and species diversity along a California transect.

The structure of the food-web at the flower-herbivore interface was examined along a transect of fourteen communities across central California. All results are commensurate with the hypothesis that in most environments there is selective pressure towards specialization. However, it is only in the most predictable or stable environments that the resultant diminishment of behavioral and genetic flexibility is in a sense "permitted" by subsequent natural selection. In the most extreme environments, behavioral specialization may be a necessary prerequiste permitting briefly thriving ephemeral populations which must recolonize frequently. The data results indicate: 1) Total species number increases with stability and predictability of the climate; 2) As the climate ameliorates, niche-specialization is a progressively more successful strategy; 3) The percentage of niche-specialized species of both plants and flower-feeding herbivores increases in the most severe environments at the expense of the more moderately specialized species; 4) Energetic flow chart redundancy increases in extreme environments; 5) Especially important to an understanding of pollination interactions is the fact that similar physiognomic communities at very different altitudes are in all cases much more similar than different community types within a research site.

MARGINAL POPULATIONS OF CEPAEA NEMORALIS (L.) ON THE BRENDON HILLS, ENGLAND. I. ECOLOGY AND ECOGENETICS.

A marginal population may be defined as one which exists on the edge of a species' range. Carson (1959) points out that since the distribution of any species is rarely continuous within its range marginal populations may be geographically internal as well as peripheral. In the absence of major barriers (such as the sea for terrestrial species) these populations exist presumably in the face of environmental stress or competitive interactions which prevent the species from extending its range and a number of generalizations have been made about them: 1) They may be restricted to a particular habitat to which the species is not otherwise confined (e.g., Ford, 1964). 2) They may show morphological adaptations different from those in central populations (Cook, 1961). 3) They are likely to be isolated anid less dense than central populations (Mayr, 1963). 4) They may show a reduction in the degree of polymorphism at the visible (e.g., Bovey, 1941) or chromosomal (e.g., Carson, 1955) level. None of these generalizations are universally true and it is at present impossible to predict what form the response of a species to sub-optimal conditions will take (Levins, 1968). For example it is known that marginal populations of some species of Drosophila (e.g., D. robusta, D. willistoni) are less polymorphic for chromosome inversions than central ones but also that this is not true for D. subobscura (Carson, 1959). For these species in which this reduction in inversion frequency occurs Carson has argued that increased recombination results from greater homozygosity for chromosome structure and that this is a reflection of the need to maintain genetic flexibility in terms of evolutionary adjustment to, unfavorable conditions. Marginal populations are likely to be of low density and consequently more inbred. Slizynski (1955) has shown that inbreeding lines of mice have greater recombination than outbreeding ones; it follows that a raised chiasma frequency in a small marginal population may be more a consequence of inbreeding than of a selective adjustment to a particular set of environmental conditions. Conclusions relating to the evolutionary significance of a raised chiasma frequency in marginal populations are valid only if it can be shown that this occurs independently of changes in population density. The landsnails Cepaea nemoralis (L.) and C. hortensis (Mull.) are widespread in Europe but neither species is continuously distributed. Both major geographical (Perrot and Perrot, 1938) and microgeographical (e.g., Cain and Currey, 1963) differences in distribution suggest that C. nemoralis is favored under warmer and drier conditions (e.g., Harvey, 1974); mixed populations of the 2 species are not common. Differences in climatic preferences between the species are suggested also by experimental data (Cameron, 1970a, b). Both species possess a similar genetic polymorphism for shell color and banding and circumstantial (e.g., Jones, 1973), direct (Richardson, 1974) and experimental (Lamotte, 1959) evidence suggests that climate (in particular temperature variation) is important in affecting morph frequencies.

Reflections on Heterodera rostochiensis1

AbstractIn 1955, the EPPO Heterodera rostochiensis Conference held in Wageningen (Anon., 1955) came to conclusions and recommendations which are analyzed together with other papers issued by EPPO between 1951–1959.In the course of the 18 years which elapsed since the Conference, major advances have been made in the following fields:– Origin, spread and distribution of the nematode, numerical dynamics and distribution of the nematode.– Numerical dynamics and genetic density and flexibility of nematode populations.– Breeding resistant potato varieties.– Development of new nematicides and integrated control.– Ecological limitations.Progress achieved along these lines is reviewed.

Foot infections in swimming baths.

A 10% random sample of all bathers at a public swimming bath were examined for tinea pedis and verruca.The overall incidence of tinea pedis was 8.5% and of verruca 4.8%. The incidence of tinea pedis in 205 male adults was 21.5%, in 288 boys 6.3%, in 60 adult females 3.3%, and in 220 girls 0.9%. The incidence of verruca in juveniles ranged from 4.2% in boys to 10.5% in girls.It was clear that both infections spread within the baths, and since a relatively small proportion of users admitted to taking precautions to avoid contracting or developing infections it seems advisable that more publicity about recommendations on foot care should be provided.

SCREENING OF WINTER SOFT WHEAT SAMPLES IN THE RYAZAN REGION CONDITIONS FOR DROUGHT RESISTANCE

Data on the determination of drought resistance in laboratory conditions are presented with a comparison of the results of field studies conducted in 2018-2020 in the fields of the Ryazan branch of the FSBI FNAC VIM. The diagnosis of plants against the background of sucrose revealed a depression in the length of sprouts and their number. The evaluation of the productivity of breeding numbers for drought resistance, compared with the data of the structural analysis of productivity, revealed that all the studied varieties and lines were medium-productive and only one line L 68-18 was highly productive. The maximum average yield (more than 6,0 t/ha) was obtained from promising lines: L 39-17, L 67-18, L 28-16, L 33-09, L 51-09. The assessment of yield, adaptability and a complex of economically valuable traits allowed us to select the most promising breeding lines. The line L 33-09 has been identified, which has the maximum indicators for resistance to modulated laboratory drought, high average yield, genetic flexibility, stability index and indicator of the level and stability of the variety. The selected varieties and lines are actively involved in hybridization to create a new breeding material adapted to the conditions of the region and further targeted selection based on drought resistance and potential productivity.