Biomedical subjects

R W Allard

Publications and source records attributed to R W Allard.

At least 37 records · Page 2Linked to original sources

PubMed1987

Correlations between development rates, enzyme activities, ribosomal DNA spacer-length phenotypes, and adaptation in Drosophila melanogaster.

Selection for "fast" preadult development rate among the progeny of flies collected in a natural population of Drosophila melanogaster produced a line that developed more rapidly than a line selected for "slow" preadult development rate. Assays for enzyme activity levels showed that the activities of alpha-glycerophosphate dehydrogenase, alcohol dehydrogenase, and malic enzyme were higher in the fast than in the slow line, but that the activity of superoxide dismutase was lower in the fast line. Differences in the frequencies of spacer-length phenotypes of X chromosome-linked rRNA genes (rDNA), which developed between the lines during the selection process, are larger than can be explained on the basis of genetic drift alone. Long rDNA spacers had high frequency in the fast line; short spacers, in the slow line. We conclude that enzyme levels affected adaptation under the selective regimes imposed and that the different X-linked rDNA spacer-length phenotypes are either adaptive in themselves or that they mark chromosomal segments carrying genes relevant to adaptation.

Adaptation, Physiological↗

PubMed1987

Chloroplast DNA polymorphisms in lodgepole and jack pines and their hybrids.

Samples taken from throughout the ranges of distribution of lodgepole pine (Pinus contorta Dougl. ex. Loud.) and jack pine (Pinus banksiana Lamb.) were assayed for Sal I and Sst I chloroplast DNA restriction fragment variation. Although the chloroplast genome is often regarded as highly conserved, at least 2 distinct Sal I and 13 distinct Sst I restriction fragment banding patterns occur in these closely related species. None of the chloroplast DNA restriction fragment banding patterns observed in allopatric lodgepole pine was observed in allopatric populations of jack pine, and vice versa, even though the two species share an extensive zone of sympatry, and gene flow between the species has been reported for nuclear genes. However, several atypical Sst I restriction fragment banding patterns occur only in or near the zone of sympatry. Chloroplasts have been reported to be inherited maternally in the great majority of species studied; however, restriction fragment analyses indicated that chloroplasts are inherited paternally in controlled matings between lodgepole pine (female) and jack pine (male).

Canada↗

PubMed1984

Ribosomal DNA spacer-length polymorphisms in barley: mendelian inheritance, chromosomal location, and population dynamics.

Spacer-length (sl) variation in ribosomal RNA gene clusters (rDNA) was surveyed in 502 individual barley plants, including samples from 50 accessions of cultivated barley, 25 accessions of its wild ancestor, and five generations of composite cross II (CCII), an experimental population of barley. In total, 17 rDNA sl phenotypes, made up of 15 different rDNA sl variants, were observed. The 15 rDNA sl variants comprise a complete ladder in which each variant differs in length from adjacent variants by approximately equal to 115 nucleotide pairs. Studies of four rDNA sl variants in an F2 population showed that these variants are located at two unlinked loci, Rrn1 and Rrn2, each with two codominant alleles. Using wheat-barley addition lines, we determined that Rrn1 and Rrn2 are located on chromosomes 6 and 7, respectively. The nonrandom distribution of sl variants between loci suggests that genetic exchange occurs much less frequently between than within the two loci, which demonstrates that Rrn1 and Rrn2 are useful as new genetic markers. Frequencies of rDNA sl phenotypes and variants were monitored over 54 generations in CCII. A phenotype that was originally infrequent in CCII ultimately became predominant, whereas the originally most frequent phenotype decreased drastically in frequency, and all other phenotypes originally present disappeared from the population. We conclude that the sl variants and/or associated loci are under selection in CCII.

Base Sequence↗

PubMed1984

Inheritance of nitrite reductase and regulation of nitrate reductase, nitrite reductase, and glutamine synthetase isozymes.

Banding patterns of nitrate reductase (NR), nitrite reductase (NiR), and glutamine synthetase (GS) from leaves of diploid barley (Hordeum vulgare), tetraploid wheat (Triticum durum), hexaploid wheat (Triticum aestivum), and tetraploid wild oats (Avena barbata) were compared following starch gel electrophoresis. Two NR isozymes, which appeared to be under different regulatory control, were observed in each of the three species. The activity of the more slowly migrating nitrate reductase isozyme (NR1) was induced by NO3- in green seedlings and cycloheximide inhibited induction. However, the activity of the faster NR isozyme (NR2) was unaffected by addition of KNO3, and it was not affected by treatments of cycloheximide or chloramphenicol. Only a single isozyme of nitrite reductase was detected in surveys of three tetraploid and 18 hexaploid wheat, and 48 barley accessions; however, three isozymes associated with different ecotypes were detected in the wild oats. Inheritance patterns showed that two of the wild oat isozymes were governed by a single Mendelian locus with two codominant alleles; however, no variation was detected for the third isozyme. Treatment of excised barely and wild oat seedlings with cycloheximide and chloramphenicol showed that induction of NiR activity was greatly inhibited by cycloheximide, but only slightly by chloramphenicol. Only a single GS isozyme was detected in extracts of green leaves of wheat, barley, and wild oat seedlings. No electrophoretic variation was observed within or among any of these three species. Thus, this enzyme appears to be the most structurally conserved of the three enzymes.

Avena↗

PubMed1981

A multilocus estimator of mating system parameters in plant populations.

A multilocus method of estimating mating system parameters in populations is presented that recovers information from classification over multiple loci that single-locus estimators do not detect. It is shown that the multilocus estimator provides a standard of reference (null hypothesis) that can be useful in analyzing the effects of factors such as population heterogeneity and post-mating zygotic selection on the transmission of genetic information at the population level.

Journal Article↗

PubMed1977

Effect of polyploidy on phosphoglucose isomerase diversity in Festuca microstachys.

Studies of the inheritance of electrophoretic banding patterns in Festuca microstachys support the hypothesis that three closely related loci, one located in each of the three ancestral genomes, code the multiple phosphoglucose isomerase (glucosephosphate isomerase; D-glucose-6-phosphate ketol-isomerase, EC 5.3.1.9) variants found in this hexaploid species. The close relationship among the three loci is indicated by the observation that hybrid enzymes of intercistronic origin form when the loci in different genomes carry alleles coding homodimers with unlike migration rates. Homozygous individuals fixed for different alleles in different genomes produce hybrid enzymes and, when self-fertilized, they breed true for isozyme patterns normally found only in the heterozygotes of diploid species. Biochemical diversity due to this "fixed heterozygosity" is high in F. microstachys; although this species is more than 99% self-fertilized the proportion of individuals with at least one heterodimer exceeded 61% in all of the 16 natural populations studied and it exceeded 92% in 11 of the populations. This great biochemical diversity may contribute to the ability of F. microstachys to survive in the wide range of habitats in which it is found over western North America.

Alleles↗

PubMed1975

Evolutionary Changes in the Mating System of an Experimental Population of Barley (Hordeum vulgare L.).

An analysis is presented of the reproductive cycle in an experimental population of barley. The experimental design included growing three different generations of the population in the same year and environment, thus permitting an assessment of the mating system at three stages in the evolutionary history of the population, unconfounded by environmental differences. Gene frequencies sometimes differed significantly in adults and in the effective pollen pool. It was also found that out-crossing rate more than doubled during the twenty generations spanned by the study. These results provide evidence for selection during reproductive phases of the life cycle. They also demonstrate that evolution in this predominantly self-pollinating population was in the direction of increased recombinational potential.

Journal Article↗

PubMed1975

The mating system and microevolution.

Studies of natural and experimental plant populations have revealed that genotypic frequency distributions are highly structured in predominantly selfing species. This high degree of genetic organization is manifested in intense correlations in allelic state over loci and also in striking micro-geographical heterogeneity. Both aspects of this structure are facilitated by self-fertilization. Development of non-random associations of alleles within populations is facilitated because inbreeding reduces heterozygosity and thus also the randomizing effect of recombination. Spatial differentiation is facilitated because self-fertilization retards gene flow from population to population. The effect of organizing the entire populational genotype into a sort of giant supergene is to increase the frequency in the population of genotypes which confers high fitness and hence to increase adaptation to the local environment. However the recombinational potential remains substantial. As a result considerable free genetic variability remains in the population and it is available for long-term response to natural selection. Thus the organization of genetic variability within populations provides for high immediate fitness and also for flexibility to meet longer term evolutionary needs. At the same time selfing is a barrier to migration and it promotes the development and maintenance of different multilocus organizations in adjacent populations occupying unlike habitats. In total, therfore, a pattern of genetic differentiation develops in space which is an almost exact overlay of the environmental heterogeneity. The plant genetic and plant breeding literature contains extensive evidence that the mating system in plants can be modified simply and drastically by selection and that different populations within the same species often practice very different amounts of inbreeding. Considering the ease with which the mating system can be altered, and the benefits of adjusting genetic variability through regulation of the mating system, it is not surprising that a high proportion of flowering plants self-fertilize to some extent and that at least one-third of species have adopted predominant selfing as a strategy in ecogenetic adaptation.

Adaptation, Biological↗

PubMed1972

Evidence for coadaptation in Avena barbata.

Two populations of Avena barbata have been sampled intensively and allelic frequencies have been determined on a fine grid for six enzyme loci. The distribution of alleles among microniches within the location occupied by one of these populations (CSA-1) was correlated with environment following patterns of macro- and microdifferentiation previously described in California. Multilocus analyses revealed striking gameticphase disequilibrium for both linked and unlinked loci, and also that two balanced five-locus gametic types, one associated with mesic conditions and the other with more xeric conditions, are in substantial excess at the CSA-1 location. Evidence is presented that these two gametic types are maintained by selection acting on two coadapted combinations of alleles, held together by restriction of recombination resulting from linkage and/or inbreeding. Gametic-phase disequilibrium featuring different allelic complexes were found in each of three microniches in the second population (Geyserville), which occupies a site that is environmentally atypical for A. barbata.

Adaptation, Physiological↗

PubMed1972

Patterns of Genetic Differentiation in the Slender Wild Oat Species Avena barbata.

Allozyme frequencies at five enzyme loci were determined for 14 California populations of Avena barbata, a species introduced to California from the Mediterranean Basin during the colonization of North America. Allelic frequencies at these loci were also determined in Mediterranean collections of this species. The pattern of divergence of the California populations from the ancestral gene pool was not random and was strongly correlated with environment; thus, the pattern is not in accord with the hypothesis that most electrophoretically detectable variants are adaptively neutral. Rates of gene substitution in California were also not in accord with the neutrality hypothesis. The observations are, however, compatible with predictions of Neo-Darwinian evolutionary theory. We interpret these observations to indicate that natural selection plays a major role in determining the unique patterns of distribution of genetic variability in the slender wild oat in California.

Journal Article↗

PubMed1972

Microgeographical Variation in Allozyme Frequencies in Avena barbata.

Microgeographical distribution of alleles of five enzyme loci and one morphological variant has been determined within populations of A. barbata occupying sites that are transitional between different vegetational zones. The results show that the spatial distribution of alleles is nonrandom and highly correlated with habitat in patterns that parallel the distribution of the same alleles in major climatic zones in California. It is concluded that the observed patterns of variation are best explained by Neo-Darwinian evolutionary models, in which selection plays a predominant role.

Journal Article↗

PubMed1972

Is the gene the unit of selection? Evidence from two experimental plant populations.

The dynamics of gametic frequency change have been analyzed in two experimental plant populations. Individual plants were scored for their genotype at four enzyme loci, and four-locus joint gametic frequencies were estimated from the genotypic data over generations. Striking correlations developed in allelic state over loci, including correlations between nonlinked loci, as these populations evolved. Furthermore, the same pair of four-locus complementary gametic types came into marked excess in the late generations of both populations. The results demonstrate that natural selection acted to structure the genetic resources of these populations into sets of highly interacting, coadapted gene complexes. They also provide evidence, at the level of the gene, that selection operating on correlated multilocus units is an important determinant of population structure.

Alleles↗

PubMed1966