Comparative evolution of cereals.
TL;DR: The gene pool classification suggested by Harlan and de Wet (1971) is used in order to treat the several cereals on a uniform basis and is not intended to be a formal taxonomic system but rather a simple device to bring a genetic focus to bear on the taxonomies already available.
Abstract: First, we shall introduce the materials with which we are dealing, and this requires some understanding of formal names and botanical classification. The taxonomy of cultivated plants has long been in a state of confusion. The same array of variation is treated in radically different ways by different taxonomists (see Jirasek, 1966; Jeffrey, 1968). Classifications are cluttered with Latin names that have little or no biological meaning, and some individual taxa are given ranks ranging from variety to genus depending on who is doing the classifying. Inept classifications have probably caused more difficulty in understanding the origin and evolution of cultivated plants than any other factor. We shall use the gene pool classification suggested by Harlan and de Wet (1971) in order to treat the several cereals on a uniform basis. In this system the total array of variation within maximum genetic reach is partitioned into primary, secondary, and tertiary gene pools. The primary gene pool includes all those races that can be crossed with the crop, yielding reasonably fertile hybrids in which the chromosomes pair well and in whose offspring genetic segregation is reasonably normal. The primary gene pool corresponds to the widely accepted concept of the biological species. The secondary gene pool includes all those species that can be crossed with the crop but with restricted gene flow. Genes can be transferred from the secondary to the primary gene pool, but one must struggle with those barriers that separate biological species such as sterility, poor chromosome pairing, lethal or weak hybrids, or poorly adapted hybrid derivatives and so on. The tertiary gene pool includes all those species that can be crossed with the crop, but the hybrids lead essentially nowhere. The hybrids are lethal, completely sterile, or anomalous. If any gene transfer is possible at all, it must be through radical manipulation of some sort such as embryo culture, tissue culture, use of complex hybrid bridges and so on (see Harlan and de Wet, 1971). Polyploid series in cultivated plants pose some special problems. As a general rule, we have suggested (Harlan and de Wet, 1971) that each level be treated as a separate gene pool. The barriers between ploidy levels are not necessarily strong, however, and morphological differences are sometimes minimal and difficult to describe. Each series is different and appropriate treatments must be worked out crop by crop. Separate gene pools for ploidy levels in potato or sugarcane may not be appropriate at all. In the cereals considered here, the only problem of separation by ploidy level occurs in oats where A vena strigosa and A. barbata races are difficult to distinguish morphologically. Our gene pool classification is not intended to be a formal taxonomic system but rather a simple device to bring a genetic focus to bear on the taxonomies already available. The conventional epithets can be used without undue confusion pro-
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01 Jan 2011
810 citations
Cites background from "Comparative evolution of cereals."
...One explanation might be that, in these species, seed size increased as a result of indirect selection for greater seedling vigor and germination uniformity under field production (Harlan et al. 1973) or as an overall allometric effect....
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Book•
31 Jan 1992
TL;DR: The chloroplast DNA restriction site variation and the evolution of the annual habit in North American Coreopsis (Asteraceae) and the suitability of molecular and morphological evidence in reconstructing plant phylogeny are examined.
Abstract: Preface. Part I: Molecules and genomes in plant systematics. Chloroplast DNA and the study of plant phylogeny: present status and future prospects - M T Clegg and G Zurawski Use of chloroplast DNA rearrangements in reconstructing plant phylogeny - S R Downie and J D Palmer Mitochondrial DNA in plant systematics: applications and limitations - J D Palmer Ribosomal RNA as a phylogenetic tool in plant systematics - R K Hamby and E A Zimmer Evolution of the NOR and 5S DNA loci in the Triticeae - R Appels and B Baum Part II: Molecular approaches to plant evolution Intraspecific chloroplast DNA variation: systematic and phylogenetic implications - D E Soltis, P S Soltis and B G Milligan Molecular data and polyploid evolution in plants - P S Soltis, J J Doyle and D E Soltis Molecular systematics and crop evolution - J Deobley Part III: Model studies of phylogenetic relationships Contributions of molecular data to polyploid evolution in plants - P S Soltis, J J Doyle and D E Soltis Molecular systematics and crop evolution - J Deobley Contributions of molecular data to papilionoid legume systematics - J J Doyle, M Levin and A Bruneau Chloroplast DNA variation in the asteraceae: phylogenetic and evolutionary implications - R K Jansen, H J Michaels, R S Wallace, K-J Kim, S C Keeley, L E Watson and J D Palmer Chloroplast DNA restriction site variation and the evolution of the annual habit in North American Coreopsis (Asteraceae) - D J Crawford, J D Palmer and M Kobayashi Molecular systematics of onagraceae: examples from Clarkia and Fuschia - K J Systema and J E Smith Floral morphology and chromosome number in the subtribe oncidiinae (Orchidaceae): evolutionary insights from a phylogenetic analysis of the chloroplast DNA restriction site variation - M W Chase and J D Palmer Part IV: Theoretical perspectives The suitability of molecular and morphological evidence in reconstructing plant phylogeny -M J Donaghue and M J Sanderson Character-site weighting for restriction site data in phylogenetic reconstruction, with an example from chloroplast DNA - V A Albert, B D Mishler and M W Chase Polymorphism, hybridization and variable evolutionary rate in molecular phylogenies - K Ritland and J E Eckenwalder Index.
743 citations
TL;DR: It is demonstrated that a synthesis from the twin vantage points of genetics and archaeology can expand the understanding of the nature of evolutionary selection that accompanies domestication.
Abstract: Plant domestication is an outstanding example of plant–animal co-evolution and is a far richer model for studying evolution than is generally appreciated. There have been numerous studies to identify genes associated with domestication, and archaeological work has provided a clear understanding of the dynamics of human cultivation practices during the Neolithic period. Together, these have provided a better understanding of the selective pressures that accompany crop domestication, and they demonstrate that a synthesis from the twin vantage points of genetics and archaeology can expand our understanding of the nature of evolutionary selection that accompanies domestication.
739 citations
TL;DR: Data suggest that in domesticated grasses, changes in grain size and shape evolved prior to non-shattering ears or panicles, suggesting a need to reconsider the role of sickle harvesting in domestication.
597 citations
Cites background from "Comparative evolution of cereals."
...The ‘domestication syndrome’ provides such a framework by highlighting a set of characters that differ between domesticated crops and their wild ancestors (Harlan et al., 1973; Hawkes, 1983; Zohary and Hopf, 2000; Gepts, 2004)....
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...The beginnings of food production represents a strategic shift in human behaviour, towards the manipulation of the soil environment (clearance, tillage) and through an influence on the composition of plant populations in that soil (via preferential seeding and tending of one or a few species)....
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...This is likely to be selected for by open environments in which larger seedlings have advantages, surviving deeper burial within disturbed soils and thus this trait should be selected for by tillage and cultivation generally (Harlan et al., 1973)....
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...Key words: Domestication, cultivation, cereals, pulses, archaeobotany, Triticum, Hordeum, Oryza, Vigna, Pennisetum....
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01 Jan 2009
TL;DR: In this paper, the authors explored the relationship between seed size increase and seed dispersal in a variety of cereals, including wheat, barley, rice, corn, beans, and pearl millet.
Abstract: † Background Archaeobotany, the study of plant remains from sites of ancient human activity, provides data for studying the initial evolution of domesticated plants. An important background to this is defining the domestication syndrome, those traits by which domesticated plants differ from wild relatives. These traits include features that have been selected under the conditions of cultivation. From archaeological remains the easiest traits to study are seed size and in cereal crops the loss of natural seed dispersal. † Scope The rate at which these features evolved and the ordering in which they evolved can now be documented for a few crops of Asia and Africa. This paper explores this in einkorn wheat (Triticum monococcum) and barley (Hordeum vulgare) from the Near East, rice (Oryza sativa )f rom China, mung (Vigna radiata )a nd urd (Vigna mungo) beans from India, and pearl millet (Pennisetum glaucum) from west Africa. Brief reference is made to similar data on lentils (Lens culinaris), peas (Pisum sativum), soybean (Glycine max) and adzuki bean (Vigna angularis). Available quantitative data from archaeological finds are compiled to explore changes with domestication. The disjunction in cereals between seed size increase and dispersal is explored, and rates at which these features evolved are estimated from archaeobotanical data. Contrasts between crops, especially between cereals and pulses, are examined. † Conclusions These data suggest that in domesticated grasses, changes in grain size and shape evolved prior to non-shattering ears or panicles. Initial grain size increases may have evolved during the first centuries of cultivation, within perhaps 500‐1000 years. Non-shattering infructescences were much slower, becoming fixed about 1000‐2000 years later. This suggests a need to reconsider the role of sickle harvesting in domestication. Pulses, by contrast, do not show evidence for seed size increase in relation to the earliest cultivation, and seed size increase may be delayed by 2000‐4000 years. This implies that conditions that were sufficient to select for larger seed size in Poaceae were not sufficient in Fabaceae. It is proposed that animal-drawn ploughs (or ards) provided the selection pressure for larger seeds in legumes. This implies different thresholds of selective pressure, for example in relation to differing seed ontogenetics and underlying genetic architecture in these families. Pearl millet (Pennisetum glaucum) may show some similarities to the pulses in terms of a lag-time before truly largergrained forms evolved.
571 citations
References
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TL;DR: The results of these investigations were sufficient to show that even for relatively simple cases (two loci, simple symmetrical selective values) linkage might have profound effects on the course of natural selection and, pari passu, natural selection may have major effect on the distribution of coupling and repulsion linkage in a population.
Abstract: HILE the theory of the genetic changes in a population due to selection is quite well understood for single loci, our theory for multiple-gene characters is in a rudimentary stage. Most of the formulations for multiple-gene characters are simply extensions of single-locus models, extensions which ignore the problem of linkage. There are, however, a few papers in which the role of linkage has been investigated for more or less special cases of selection (KIMURA 1956; LEWONTIN and KOJIMA 1960; BODMER and PARSONS 1962). The results of these investigations were sufficient to show that even for relatively simple cases (two loci, simple symmetrical selective values) linkage might have profound effects on the course of natural selection and, pari passu, natural selection may have major effects on the distribution of coupling and repulsion linkage in a population. The results of the investigations of LEWONTIN and KOJIMA (1960) of the twolocus model can be summarized as follows: (1) If the fitnesses are additive between loci (no epistasis), linkage does not effect the final equilibrium state of the population. (2) If linkage is tighter than the value demanded by the magnitude of the epistasis (the greater the epistasis the greater the value) there may be permanent linkage disequilibrium and alteration of equilibrium gene frequencies. (3) The rate of genetic change with time is affected by the tightness of the linkage. (4) In some cases stable gene frequency equilibria are possible only if linkage is tight enough. Although these conclusions were based only on two-locus model and for selective values of a fairly restricted sort, they point clearly to the importance of taking linkage into account in understanding the changes of gene frequencies in populations. In fact, some experimental results (an example of which will be given below) can be understood only if the interaction of selection and linkage is taken into account, The equations describing the interaction between selection and linkage (see below) do not usually have general literal solutions. It is for this reason that the authors cited above have restricted themselves to relatively simple cases. In view of the interesting findings of those previous papers, however, it is worthwhile to explore the subject more intensively. To do so requires the numerical rather than general literal solutions to the equations, but such numerical solutions apply, obviously, only to the particular parameter values chosen. To make such a nu-
1,913 citations
1,248 citations
TL;DR: An attempt is made to provide a framework in which both taxonomy and infraspecific classification can operate with a minimum of confusion.
Abstract: The methods of formal taxonomy have not been very satisfactory for the classification of cultivated plants. As a result, the people who deal with cultivated plants the most have developed their own informal and intuitive classifications based on experience as to what constitutes useful groupings. An attempt is made to provide a framework in which both systems can operate with a minimum of confusion. The structure of the total available gene pool is characterized by assigning taxa to primary, secondary and tertiary gene pools. At the infraspecific level, cultivars are grouped into races and subraces in an informal way without rigid rules for the use of terms.
946 citations
933 citations