摘要:

AbstractThe genusZeaconsists of the cultivar maize and six wild taxa. All taxa in the genus hybridize with one another, and the genus appears to be of relatively recent origin. The genusZeahas long been of interest to evolutionary and molecular geneticists, and a plethora of studies have established that maize and the teosintes are genetically diverse. This high diversity, coupled with relatively recent divergence among taxa, makesZeaan excellent model system for studying the distribution of nucleotide diversity both among recently diverged taxa and among a crop and its wild relatives. Nucleotide diversity has been studied in a number of maize loci. These studies have documented tremendous nucleotide variation in maize, and they suggest that allelic lineages in the maize gene pool can be quite long‐lived. For example, it has been estimated that allelic lineages have existed for several million years at both theAdh1 and theAdh2locus, suggesting that the common ancestor toZeataxa was highly polymorphic. These studies have also found ample evidence of recombination in maize loci, suggesting that recombination plays an important role in generating haplotype diversity. A recent study of the regulatory locusc1, a regulatory gene in the anthocyanin biosynthesis pathway, belies this general picture. Alleles from thec1locus have been sampled from maize and teosintes. The sampled alleles fall into two discrete groups, corresponding to two distinct functional classes. Both classes are found in teosinte, but only one of these classes is found in maize. Thec1locus is less diverse than any other maize locus yet examined. It is hypothesized that low nucleotide diversity at thec1locus is a consequence of selection, probably artificial by the early domesticators of maiz

ISSN:0913-557X    

DOI:10.1111/j.1442-1984.1996.tb00103.x

出版商:Blackwell Publishing Ltd    

年代:1996

数据来源: WILEY

摘要:

AbstractIn gametophytic self‐incompatibility, the S‐locus encodes an S‐protein whose expression results in successful pollination only when the pollen allele differs from both maternal alleles. Analysis of nucleotide and amino acid sequences of a number of S‐alleles has revealed extraordinary allelic sequence divergence and an excess of interspecific shared polymorphism. Population genetic theory and analysis of the distribution of the number of shared polymorphic sites verifies that the sequence diversity is consistent with the alleles being extraordinarily old. Theory also predicts that S‐alleles will exhibit less population structuring in a subdivided population than will a neutral locus, and data are being collected to test this. Self‐incompatibility must involve two features ‐ a pollen component that specifies the identity of the pollen and a pistil component that recognizes and elicits a response to self‐pollen. The S‐locus clearly determines the pistil component, but lack of expression of S in pollen leaves open the possibility that there may be another pollen factor. Experiments with transgenic plants have demonstrated that the S‐protein expression in the pistil is necessary and sufficient to determine the pistil phenotype. However, several lines of evidence suggest that the degree of elimination of self‐pollen in plants with gametophytic self‐incompatibility depends on additional genetic factors besides the S‐locus. Modifiers may affect self‐incompatibility by affecting either the pollen component, the pistil component or both. Population genetic models that test the consequences of modification of these two components are reviewed and extended. Conditions for invasion of reduced degree of self‐incompatibility depend in part on the l

ISSN:0913-557X    

DOI:10.1111/j.1442-1984.1996.tb00104.x

出版商:Blackwell Publishing Ltd    

年代:1996

数据来源: WILEY

摘要:

AbstractMolecular quantitative genetics seeks to map and characterize the effects of individual genes underlying traits (quantitative trait loci, or QTLs). Classically, it uses genetic differences between distinct strains or taxa and saturated marker maps to infer the location and effects of specific genes. Little attention has been devoted to characterizing QTL variability within populations, as higher heterozygosity and smaller allele effects require much larger experimental effort to detect specific QTLs. An alternative approach for within‐population studies is to utilize multiple crosses, each assayed for relatively few genetic markers. The genome regions surveyed for QTLs are regarded as random samples, with the objective of characterizing variability of QTLs. This approach is particularly appropriate for genetically heterogeneous traits such as inbreeding depression and life history (e.g., fitness) traits. In this paper, we explore this approach for QTLs affecting inbreeding depression, based upon a method using selfed progeny arrays, and illustrated with an experiment involving the common monkeyflower,Mimulus guttatu

ISSN:0913-557X    

DOI:10.1111/j.1442-1984.1996.tb00105.x

出版商:Blackwell Publishing Ltd    

年代:1996

数据来源: WILEY

摘要:

AbstractIsolated plant populations face a series of demographic and genetic challenges. They potentially may suffer from a loss of allelic and genotypic diversity or from inbreeding depression. The demographic stability associated with migration between components of a metapopulation is lost as fragmentation increasingly isolates populations. Such isolated populations are then increasingly subject to demographic stocasticity and to more frequent population bottlenecks. Some plant species clearly endure this challenge better than others. In studies of native plant species whose populations have long endured isolation, population numbers very often appear stable and populations often contain ample genetic variation. Of course, such observations are biased, since those species which did not accommodate isolation are presumed extinct and thus not studied.When one considers species which have recently undergone habitat fragmentation and population isolation, the ability to accommodate such change is clearly not present in many species. Such populations often show a dramatic decline in numbers as well as erosion of genetic variability. In these populations the role of somatic processes in the generation of genetic variation may assume an increased importance. Since only a few isolated studies have documented such somatic genomic changes in plants, the importance of such changes is subject to speculation.

ISSN:0913-557X    

DOI:10.1111/j.1442-1984.1996.tb00106.x

出版商:Blackwell Publishing Ltd    

年代:1996

数据来源: WILEY

摘要:

AbstractPopulations of the common morning glory in the southeastern US are characterized by a striking diversity of flower color polymorphisms. This diversity is probably a consequence of horticultural escapes from cultivation in the 18th and 19th centuries. More than 15 years of research in our laboratory has shown that some color phenotypes are selected by virtue of their differential attraction to insect pollinators. We have studied genetic diversity at isozyme and ribosomal DNA loci and we find reduced diversity in the southeastern US compared to Mexican populations. In an effort to link ecological genetics to molecular evolution, we have cloned and characterized the chalcone synthase (CHS) gene family in morning glory and we have studied the expression of CHS genes in flower development. We have also initiated an investigation of spatial patterns of diversity at CHS genes by sampling and sequencing genes from US and Mexican populations. These investigations reveal (1) that the four CHS genes (CHS A, B, C, and PS) characterized to date evolve rapidly in morning glory and that the gene family inIpomoeais of relatively recent origin (approximately 21 million years); (2) the duplicate genes inIpomoeagroup into two categories (CHS A, C versus CHS B, PS) that may indicate a functional divergence between chalcone synthase and stilbene synthase activities; (3) levels of molecular diversity for CHS A genes sampled from Mexico are much higher than observed in US collections suggesting a major population bottleneck associated with the introduction of morning glory into the southeastern US; and (4) the ratio of amino acid substitution to synonymous substitution between Ipomoea species is remarkably high (about 5.4 synonymous to amino acid substitutions) compared to CHS genes in other plant species. Taken together these data portray a rapidly evolving gene family where functional divergence may arise repeatedly in evolution, despite the central role of chalcone synthase in flavonoid metabolism.

ISSN:0913-557X    

DOI:10.1111/j.1442-1984.1996.tb00107.x

出版商:Blackwell Publishing Ltd    

年代:1996

数据来源: WILEY

摘要:

AbstractThe amount of DNA polymorphism can be estimated either from the average number of pairwise nucleotide differences per site (π) or from the proportion of segregating (polymorphic) site (s) among a sample of DNA sequences. Under a number of assumptions, including panmictic population and neutrality, it is known that the expectations of π and s are given by E(π) =θ and E(s) = a1(n)θ, where θ= 4Nv, a1(n) = 1 + 1/2 +…+ 1/(n‐1), N is the effective population size, v is the mutation rate per site per generation, and n is the sample size. Therefore, θ can be estimated by π or s/a1(n). These assumptions, however, are not always correct. In this paper, using a simple non‐random sampling model, I have examined the effect of non‐random sampling on the estimates of the amount of DNA polymorphism. The results indicate that the effect of non‐random sampling on the proportion of segregating site is substantially large whereas the effect of non‐random sampling on the average number of nucleotide differences per site is negligibly small unless non‐randomness is extremely large. Using a finite site model with and without rate variation, I have also examined the effect of rate variation among sites on the estimates of the amount of DNA polymorphism. The results indicate that if the neutral mutation rate varies among sites substantially, the estimates of θ based on the infinite site model are substantially underestimated. New methods for estimatin

ISSN:0913-557X    

DOI:10.1111/j.1442-1984.1996.tb00108.x

出版商:Blackwell Publishing Ltd    

年代:1996

数据来源: WILEY

摘要:

AbstractThe hypothesis that phenotypic plasticity is adaptive has rarely been tested explicitly. To support this hypothesis, it is necessary to demonstrate that the phenotype induced in each environment experienced by an organism has a high relative fitness in that environment. In plants, phytochrome‐mediated responses to the reduced ratio of red: far‐red light (R:FR) characteristic of dense vegetation have frequently been assumed to be adaptive. Such “shade avoidance” responses include stem elongation, suppression of branching, reallocation of biomass, and accelerated flowering. Phytochrome‐mediated responses to neighbors provide an ideal model system for studying the evolution of adaptive plasticity because the ecological context is clear and the physiological and genetic mechanisms are well studied at the molecular level. Moreover, plant phenotypes are easily manipulated by alteration of the R:FR signal. The shade avoidance hypothesis for adaptive plasticity is supported by studies of mutant and transgenic plants in which normal photomorphogenic responses are disabled, and by field experiments employing phenotypic manipulation in natural populations. Suppression of phytochrome‐mediated stem elongation at high density results in decreased fitness, as does constitutive expression of the elongated phenotype at low density. There is also evidence for ecotypic variation in shade avoidance responses. In a common garden, plants from woodland populations ofImpatiens capensisare less responsive to R:FR than plants from a nearby open site, suggesting local adaptive differentiation for plastic response to li

ISSN:0913-557X    

DOI:10.1111/j.1442-1984.1996.tb00109.x

出版商:Blackwell Publishing Ltd    

年代:1996

数据来源: WILEY

摘要:

AbstractHow micro‐ and macroevolutionary evolutionary processes produce phenotypic change is without question one of the most intriguing and perplexing issues facing evolutionary biologists. We believe that roadblocks to progress lie A) in the underestimation of the role of the environment, and in particular, that of the interaction of genotypes with environmental factors, and B) in the continuing lack of incorporation of development into the evolutionary synthesis. We propose the integration of genetic, environmental and developmental perspectives on the evolution of the phenotype in the form of the concept of thedevelopmental reaction norm (DRN)TheDRNrepresents the set of multivariate ontogenies that can be produced by a single genotype when it is exposed to environmental variation. It encompasses: 1) the processes that alter the phenotype throughout the ontogenetic trajectory, 2) the recognition that different aspects of the phenotype are (and must be) correlated and 3) the ability of a genotype to produce phenotypes in different environments. This perspective necessitates the explicit study of character expression during development, the evaluation of associations between pairs or groups of characters (e.g., multivariate allometries), and the exploration of reaction norms and phenotypic plasticity. We explicitly extend the concept of the DRN to encompass adjustments made in response to changes in the internal environment as well. Thus, ‘typical’ developmental sequences (e.g., cell fate determination) and plastic responses are simply manifestations of different scales of ‘environmental’ effects along a continuum. We present: (1) a brief conceptual review of three fundamental aspects of the generation and evolution of phenotypes: the changes in the trajectories describing growth and differentiation (ontogeny), the multivariate relationships among characters (allometry), and the effect of the environment (plasticity); (2) a discussion of how these components are merged in the concept of the developmental reaction norm; and (3) a reaction norm perspective of major determinants of phenotypes: epigenesis, selection and c

ISSN:0913-557X    

DOI:10.1111/j.1442-1984.1996.tb00110.x

出版商:Blackwell Publishing Ltd    

年代:1996

数据来源: WILEY

摘要:

AbstractThe growth habit of the rosette plantPlantago lanceolatais highly variable, and many vegetative and reproductive traits co‐vary. At one end of the range plants have relatively few but long and erect leaves, form few daughter rosettes, and produce a limited number of large spikes, with relatively heavy seeds. Plants at the other end of the range have the opposite characteristics. This suite of characters was shown to correlate with the height of the vegetation in mid‐summer. The causes for this association between different traits were investigated in different experiments, with the following results:Plants from two contrasting habitats both react strongly to light intensity and the red to far‐red (R/FR) ratio of the ambient light. Light intensity mainly affected plant size, whereas light quality affected the growth habit. Populations differ in their mean response rather than in the level of plasticity (i.e., slope of the reaction norms). Experiments show that genetic factors (population effects), R/FR ratio, and hormone treatments (GA or CCC) have similar effects on morphology, and are largely additive and interchangeable.Ten different populations were grown in a common garden, so that the genetic (clonal) correlation within populations, and their bivariate phenotypic means could be compared. Trait combinations which deviated in the same direction (both higher of both lower than the mean over all populations) on average had positive clonal correlations within populations, whereas combinations which deviated in opposite directions had negative correlations.Artificial selection on leaf length, performed under a high or a low R/FR ratio showed clear responses to selection, with heritabilities around 0.4. Correlated responses were found in many other traits, and genetic correlations were similar to the trait associations for the means of different natural populations. Correlated responses to selection depended on environmental circumstances. Under a high R/FR ratio (sun) evidence for a size/number trade‐off was found for leaf length and leaf number. Under a low R/FR ratio, however, a trade‐off between leaf length and leaf quality was found.In conclusion, the trait associations that are so characteristic for the growth habit inPlantago lanceolataare probably due to differences in hormone levels or activities. Genes and environmental factors affect growth habit in similar ways, by tapping into this regulatory mechanism. In the field, selection can lead to concerted changes in the mean of different traits, but changes in plasticity wil

ISSN:0913-557X    

DOI:10.1111/j.1442-1984.1996.tb00111.x

出版商:Blackwell Publishing Ltd    

年代:1996

数据来源: WILEY

摘要:

AbstractTo understand the causes of tradeoffs, particularly those involved in ageing, to test the reproductive effort model of life history evolution, and to measure the rate of evolution of induced life history responses, we have been performing an artificial selection experiment onDrosophila melanogastersince November 1993. It consists of four treatments, each with three replicates.The first treatment consists of constant high adult mortality; the second treatment of constant low adult mortality. In all four treatments, propyl acetate is present in the medium for 4 weeks, then absent for 4 weeks. In the third treatment, propyl acetate in the larval medium is associated with high adult mortality. In the fourth treatment, propyl acetate is associated with low adult mortality. To ensure that the effects of treatments are due to differences in mortality and not in density, adult and larval density are kept the same in all treatments.In the first two treatments, the evolution of reproductive effort and age and size at maturity has been rapid and has confirmed the predictions of the reproductive effort model. The relationships between age and size at eclosion and between size at eclosion and fecundity have also evolved. Because the only difference between the first two treatments was the level of adult mortality, this establishes differences in mortality rates as a sufficient cause for the evolution of age and size at eclosion and for the age‐distribution of fecundity rates.In the second two treatments, the evolution of induced responses has been slow, and responses are not yet consistent. The reasons for the slower evolution of induced responses suggest two new theoretical question

ISSN:0913-557X    

DOI:10.1111/j.1442-1984.1996.tb00112.x

出版商:Blackwell Publishing Ltd    

年代:1996

数据来源: WILEY