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Evolution of Genetic Systems
... Because of this, the implications of such data for general evolution are not universally accepted. However, Campbell (40) has made the pointed comment "if we ask 'how do natural bacterial populations evolve' we probably should not ignore the one case where the process has been observed." ...
... VOL. 40,1976 on December 19, 2020 by guest http://mmbr.asm.org/ Downloaded from 558 REANNEY Nonconjugative plasmid: This includes the quiescent "plasmids" listed by Novick (191) and the Col factors, E1-3, K, and X. ...
... The colloidal and particulate character of soil, and the variable nature of such factors as pH, temperature, and nutrient supply, mean that bacteria in soil will, under most conditions, be less likely to be in a VOL. 40,1976 on December 19, 2020 by guest http://mmbr.asm.org/ Downloaded from 560 REANNEY state of adequate physical proximity and metabolic fitness to permit an efficient process of gene transfer by connecting pili. ...
... According to some theoretical models of the population dynamics of transposable elements (Brookfield, 1982;Charlesworth, 1983;Charlesworth and Charlesworth, 1983;Langley, Brookfield andKaplan, 1983, Charlesworth andLangley, 1986), and recent experimental observations (Biémont and Aouar, 1986, in Drosophila;Alleman and Freeling, 1986, in maize), such an equilibrium may be maintained either by the regulation of transposition and excision in response to copy number by genome, or by a selective process on individuals as a complex function of the copy number of elements in the genome. However, we have only a few ideas on the level of heterozygosity of such mobile elements in populations. ...
... However, it depends on the number of insertion sites that is considered. In this calculation we considered each site of insertion as a classical Mendelian locus as done in some theoretical approaches of the dynamics of transposable elements (Charlesworth, 1983). The formula used was: ...
Using in situ hybridisation to the giant chromosomes of salivary glands, mdg-1 mobile element polymorphism was studied in individuals of Drosophila melanogaster from a mass-mated population and from inbred lines established from this population (analysed at the 15th, 27th and 35th generations). The number of mdg-1 copies and the number of heterozygous insertion sites were estimated in the expected hybrids, which resulted from theoretically crossing the inbred lines. The mean number of labelling sites in hybrids that originated from lines from the 35th generation was equal to 305 and was similar to the value observed in the initial population. An average proportion of heterozygous sites of 085 (with a range from 062 to 096), and an average number of copies of 35 (29 to 44) were found in these hybrids. These values may be considered as valid estimates of the values in the initial population. Other measurements of genetic heterogeneity, determined on the mass-mated population and the inbred lines, are proposed and discussed.
... In conclusion, to effectively exploit the additive genetic variation for (1) yield per plant, (2) tolerance to stresses, and (3) responsiveness to inputs, maize should be treated as an autogamous crop. This means that selection for inbred vigor (a term coined by Fasoulas 1974) should start from the F2 of outstanding hybrids grown in the absence of competition controlled selfing, and continue until the development of outstanding inbreds. Exploitation of heterosis will be practiced at the end, among the few inbreds excelling in yield per plant, tolerance to stresses, responsiveness to inputs, and quality. ...
... De acordo com o registo fóssil e de acordo com esta teoria, os eucariotas inferiores (protozoários, algas eucarió-ticas e fungos) podem agora ser incluídos em uma única árvore filogenética (Fig. 1). Ao contrário do que se pensava anteriormente sobre este assunto (Cronquist, 1960;Dougherty & Allen, 1960;Fritsch, 1935), muitos aspectos 4 «Mais recentemente, Wallin (1922), sustentou que os condriossomas podem ser considerados bactérias simbióticas cujas associações com outros componentes citoplasmáticos podem ter surgido nos estádios iniciais da evolução... para muitos, sem dúvida, tais especulações podem parecer demasiado fantásticas para serem actualmente consideradas pela comunidade biológica; no entanto, é possível que tais especulações possam algum dia vir a ser objecto de uma consideração mais séria» (Wilson, 1925, ver p.378). desta teoria são verificáveis pelas técnicas modernas da biologia molecular. ...
... In this section, we examine the genetic evidence supporting and detracting from the widespread expectation that extinction rates of selfing lineages exceed their speciation rates. Selfing may constrain adaptive evolution by reducing the levels of polymorphism and, particularly, additive genetic variation (Darlington, 1939;Mather, 1943;Stebbins, 1957). There is considerable support for the notion that selfing lineages harbor less nucleotide polymorphism, although it is unclear how genome-wide nucleotide polymorphism is linkedto potential adaptive phenotypic evolution (Pollak, 1987;Charlesworth & Charlesworth, 1995;Nordborg, 2000;Charlesworth & Wright, 2001;Gl emin et al., 2006). ...
A compound hypothesis positing that self‐fertilization is an evolutionary dead end conflates two distinct claims: the transition from outcrossing to selfing is unidirectional; and the diversification rate, or the balance of the speciation and extinction rate, is negative for selfing species. Both claims have enjoyed widespread informal support for decades, but have recently come under suspicion. Sources of data that apparently contradict strongly asymmetric mating system transitions often rely on statistical phylogenetic tests plagued by profound flaws. Although recently developed models mend preceding approaches, they have been employed sparingly, and many problems remain. Theoretical investigations, genetic data and applications of new phylogenetic methods provide indirect support for an association of selfing with negative diversification rates. We lack direct tests of reversals from selfing to outcrossing, and require data concerning the genetic basis and complexity of independently evolved outcrossing adaptations. The identification of the mechanisms that limit the longevity of selfing lineages has been difficult. Limitations may include brief and variable durations of selfing lineages, as well as ongoing difficulties in relating additive genetic and nucleotide variation. Furthermore, a common line of evidence for the stability of mixed mating – based simply on its frequent occurrence – is misleading. We make specific suggestions for research programs that aim to provide a richer understanding of mating system evolution and seriously challenge S tebbins’ venerable hypothesis.
Contents
Summary
386
I.
Introduction
387
II.
Microevolutionary perspectives on SEDE
388
III.
Macroevolutionary perspectives on SEDE
390
IV.
SEDE and the distribution of outcrossing rates in angiosperms
393
V.
Redirection of study to challenge Stebbins’ claims
394
Acknowledgements
395
References
395
... In addition, it may be expected that the two complexes associated in a naturally occurring complex hybrid are highly selected for mutual co-adaptation whereas complexes from different populations, which have not been mutually selected, are not coadapted and perform less well together. The selective advantage of complex hybridity has been explained, in the past, in terms of a " pursuit of hybridity " (Darlington 1939; James 1965; Carson 1967). Detailed study of the genetic properties and competitive abilities of natural populations of I. petraea distributed over the granite rocks of Western Australia has led to identification of the masking of deleterious mutations as the underlying basis of hybrid superiority and to a detailed appreciation of the pathways followed in this evolutionary pursuit. ...
The isolated granitic outcrops of Western Australia may harbour relics of the past such as Isoetes and Stylidium merrallii. They may also preserve a record of evolutionary change both within single populations and across population systems. Thus, the initial stages in the evolution of the complex hybrid genetic system are preserved amongst the extant lineages of Isotoma petraea on Pigeon Rock in the southwest of Western Australia. The elaboration of this bizarre genetic system is preserved across neighbouring outcrops. Study of I. petraea focuses attention on the evolutionary consequences of the controversial association of high levels of inbreeding with polymorphism for lethal genes within populations. In addition, it has generated novel and important insights into the genetic structure of Western Australian plant species and of evolutionary processes generally. Pigeon Rock and its unique living evolutionary museum is worthy of World Heritage listing, and should be preserved by removing the present stockyards and excluding the rock from the surrounding pastoral lease.
... In conclusion, to effectively exploit the additive genetic variation for (1) yield per plant, (2) tolerance to stresses, and (3) responsiveness to inputs, maize should be treated as an autogamous crop. This means that selection for inbred vigor (a term coined by Fasoulas 1974) should start from the F2 of outstanding hybrids grown in the absence of competition controlled selfing, and continue until the development of outstanding inbreds. Exploitation of heterosis will be practiced at the end, among the few inbreds excelling in yield per plant, tolerance to stresses, responsiveness to inputs, and quality. ...
Introduction Principles Underlying the Honeycomb Breeding The Masking Effects of Field Variation on Single-Plant Yields Honeycomb Selection Within Entries Honeycomb Selection Among Entries The Universality of Honeycomb Breeding Summary Literature Cited
Rubus subgen. Rubus (bramble) is one of the large plant genera in Europe, consisting of only a few sexual biological species and at least 700 apomictic lineages. In this thesis, it is argued that the stabilised apomict lineages should best be regarded species, even if their distribution area doesn’t meet the requirements of a regional species as defined in several publications by Weber. Included is a checklist of Dutch bramble species, comprising 191 species belonging to Rubus subgen. Rubus. In the Netherlands, 97 of the 191 species are classified as regional species, with a distribution area diameter under 500 km. On the basis of distribution data of bramble species in Ireland, the UK, Denmark, Germany, and the Netherlands, 12 phytogeographical bramble-regions are distinguished. Although ecological factors play a role in the realisation of these regions, it is argued that the found patterns are primarily the result of evolutionary processes. The density and species composition of Rubus scrubs in the Netherlands is studied using landscape transects. At landscape scale, the bramble species in the scrubs are not randomly distributed, causing a spatial clustering of floristically similar bramble scrubs. It was concluded that only a part of the diversity of Rubus scrubs was accounted for in the Dutch national vegetation classification, and a new scrub type (the Rubetum taxandriae) was described on the basis of these findings. Rubus scrubs are an important biotope for rare shrub species and endemic Rubus species. Additionally, scrubs rich in bramble species are important because they provide foraging and nesting habitats for numerous vertebrates and invertebrates. It is recommended to include the apomict Rubus species in biodiversity accounts, for instance in the national standard list of plant species, as well as the Red List.
A bacteriophage is termed defective if it is virtually unable to carry out a complete infectious cycle in known circumstances. As it is impossible to test infectivity on every host under all conditions, categorization is arbitrary and provisional. The significance of failure to infect is most readily interpretable for variants derived in a single step from an active phage. Operationally, ability to carry out a complete infectious cycle is frequently equated with plaque formation.
This paper explores the genetic basis of high and stable crop yield, and delineates the conditions that provide the link between genotypic and phenotypic superiority and, consequently, improve selection efficiency in plant breeding. Response to selection is viewed as the result of the conversion, through intralocus recombination, of two heterotic nonadditive allelic loci into a single super additive locus, and fixation of the latter. This concept connects the important phenomena of vigor, degeneration, and response to selection through a unifying genetic basis. Heterozygosity of nonadditive allelic loci leads to the nonheritable repulsion-phase vigor. Heterozygosity of a super additive locus leads to the partially heritable coupling-phase vigor. Homozygosity of the super additive locus leads to the fully heritable inbred vigor, which is responsible for genetic gain. A reliable explanation of heterosis is the one that considers repulsion- and coupling-phase vigor as being mutually essential and inter-convertible. Response to selection is the outcome of the evolution of hybrid to inbred vigor. The superiority of inbred vigor is due to the improved additive allelic complementation, that at the same time enhances epistatic interactions. The full exploitation of the above concept leads to maximization of homeostatic crop yield and can be successfully achieved using the principles of honeycomb breeding. These include clarification of the negative role of competition on: (1) crop yield, and (2) selection efficiency, and explain why crop yields are maximized when cultivars are monogenotypic. The unit of evaluation and selection becomes the individual plant grown at the critical distance, where the range of phenotypic expression is enlarged, and the negative effect of competition on selection efficiency is eliminated. Honeycomb breeding uncouples the reliable selection for yield and stability from the visual evaluation that predominates during the critical early generations of selection, and offers the transition from single-trait evaluation to whole-genome phenotypic evaluation. The concept of whole-genome phenotypic evaluation recognizes that genes controlling crop yield concern the genome as a whole and belong to three categories: (1) genes that control yield potential per plant and expand the lower limit of the optimal plant density range; (2) genes that confer tolerance to biotic and abiotic stresses and expand the upper limit of the optimal plant density range; (3) genes that control cultivar responsiveness to inputs. The outcome of selection, based on whole-genome phenotypic evaluation during all generations of a breeding program, is high yielding, stable, and density-independent cultivars.
Cyril Darlington (1903-1981) was the most famous cytologist in the world in the decades preceding the molecular revolution of the 1950s. He crossed disciplinary boundaries to create a synthesis of cytology, genetics and evolution by revealing the mechanics of chromosomal recombination and the importance of its evolution. Always controversial during his lifetime, obituaries ultimately referred to him as the 'Copernicus' or 'Newton' of cytology. This article reviews Darlington's scientific contributions, the reasons for their difficult reception at the time and their continuing relevance.
Understanding apomixis (asexual reproduction through seeds) is of great interest to both plant breeders and evolutionary biologists. The genus Boechera is an excellent system for studying apomixis because of its close relationship to Arabidopsis, the occurrence of apomixis at the diploid level, and its potentially simple inheritance by transmission of a heterochromatic (Het) chromosome. Diploid sexual Boechera stricta and diploid apomictic Boechera divaricarpa (carrying a Het chromosome) were crossed. Flow cytometry, karyotype analysis, genomic in situ hybridization, pollen staining and seed-production measurements were used to analyse the parents and resulting F1, F2 and selected F3 and test-cross (TC) generations. The F1 plant was a low-fertility triploid that produced a swarm of aneuploid and polyploid F2 progeny. Two of the F2 plants were fertile near-tetraploids, and analysis of their F3 and TC progeny revealed that they were sexual and genomically stabilized. The apomictic phenotype was not transmitted by genetic crossing as a single dominant locus on the Het chromosome, suggesting a complex genetic control of apomixis that has implications for future genetic and evolutionary analyses in this group.
This paper will review certain approaches to artifical intelligence research--mainly work done since 1960. An important area of research involves designing a machine that can adequately improve its own performance as well as solve other problems normally requiring human intelligence. Work in heuristic programming that seems most relevant to this goal will be discussed at length. Important subproblems are devising techniques for self-improvement, the general problem of deciding what task to best work on next in a network of tasks, and the general problem of how to mechanize learning or inductive inference. Some work in linguistics and pattern recognition is directly concerned with the induction problem. Another area of research that will be treated is simulation of organic evolution.
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