This review considers the possibility that apomixis is a highly conserved but evolutionarily suppressed alternative to sex, that apomixis and sex are anciently polyphenic, and that the switch between ...them is controlled by upstream epigenetic processes that are environmentally triggered
Abstract
In angiosperms, there are two pathways of reproduction through seeds: sexual, or amphimictic, and asexual, or apomictic. The essential feature of apomixis is that an embryo in an ovule is formed autonomously. It may form from a cell of the nucellus or integuments in an otherwise sexual ovule, a process referred to as adventitious embryony. Alternatively, the embryo may form by parthenogenesis from an unreduced egg that forms in an unreduced embryo sac. The latter may form from an ameiotic megasporocyte, in which case it is referred to as diplospory, or from a cell of the nucellus or integument, in which case it is referred to as apospory. Progeny of apomictic plants are generally identical to the mother plant. Apomixis has been seen over the years as either a gain- or loss-of-function over sexuality, implying that the latter is the default condition. Here, we consider an additional point of view, that apomixis may be anciently polyphenic with sex and that both reproductive phenisms involve anciently canalized components of complex molecular processes. This polyphenism viewpoint suggests that apomixis fails to occur in obligately sexual eukaryotes because genetic or epigenetic modifications have silenced the primitive sex apomixis switch and/or disrupted molecular capacities for apomixis. In eukaryotes where sex and apomixis are clearly polyphenic, apomixis exponentially drives clonal fecundity during reproductively favorable conditions, while stress induces sex for stress-tolerant spore or egg formation. The latter often guarantees species survival during environmentally harsh seasons.
Apomixis in angiosperms is asexual reproduction from seed. Its importance to angiospermous evolution and biodiversity has been difficult to assess mainly because of insufficient taxonomic ...documentation. Thus, we assembled literature reporting apomixis occurrences among angiosperms and transferred the information to an internet database (
http://www.apomixis.uni-goettingen.de
). We then searched for correlations between apomixis occurrences and well-established measures of taxonomic diversity and biogeography. Apomixis was found to be taxonomically widespread with no clear tendency to specific groups and to occur with sexuality at all taxonomic levels. Adventitious embryony was the most frequent form (148 genera) followed by apospory (110) and diplospory (68). All three forms are phylogenetically scattered, but this scattering is strongly associated with measures of biodiversity. Across apomictic-containing orders and families, numbers of apomict-containing genera were positively correlated with total numbers of genera. In general, apomict-containing orders, families, and subfamilies of Asteraceae, Poaceae, and Orchidaceae were larger, i.e., they possessed more families or genera, than non-apomict-containing orders, families or subfamilies. Furthermore, many apomict-containing genera were found to be highly cosmopolitan. In this respect, 62% occupy multiple geographic zones. Numbers of genera containing sporophytic or gametophytic apomicts decreased from the tropics to the arctic, a trend that parallels general biodiversity. While angiosperms appear to be predisposed to shift from sex to apomixis, there is also evidence of reversions to sexuality. Such reversions may result from genetic or epigenetic destabilization events accompanying hybridization, polyploidy, or other cytogenetic alterations. Because of increased within-plant genetic and genomic heterogeneity, range expansions and diversifications at the species and genus levels may occur more rapidly upon reversion to sexuality. The significantly-enriched representations of apomicts among highly diverse and geographically-extensive taxa, from genera to orders, support this conclusion.
Increasingly across the globe, heritage agencies have taken on board the critique of heritage management enshrined in Smith’s (2006) argument about the dominance of an ‘authorised heritage discourse’ ...and the way this has been developed by other writers in the field. To this end, they are increasingly engaged in extending their work towards working with communities. This is highly commendable and contributes towards the sustainability of heritage as a category.What gets lost in the debates about community engagement and involvement, however, is any consideration of the nature or role of the heritage agency as a type of institution; and yet an understanding of the role of these institutions in the process is essential if we are truly to break away from limited, top-down, highly managerial conceptions of what heritage is for. It is not just a matter of organisations doing what they do in a more inclusive manner; it is more than them moving from positions of authority to acting as facilitators. There needs to be a clear understanding by all involved in the heritage process – including those who work for institutions – of how institutions function in relation to the object of their attention and others who have an interest in that object.
Apomixis (asexual seed formation) in angiosperms occurs either sporophytically, through adventitious embryony, or gametophytically, where an unreduced female gametophyte (embryo sac) forms and ...produces an unreduced egg that develops into an embryo parthenogenetically. Multiple types of gametophytic apomixis occur, and these are differentiated based on where and when the unreduced gametophyte forms, a process referred to as apomeiosis. Apomeiotic gametophytes form directly from ameiotic megasporocytes, as in Antennaria-type diplospory, from unreduced spores derived from 1st division meiotic restitutions, as in Taraxacum-type diplospory, or from cells of the ovule wall, as in Hieracium-type apospory. Multiple types of apomeiosis occasionally occur in the same plant, which suggests that the different types occur in response to temporal and/or spatial shifts in termination of sexual processes and onset timing of apomeiosis processes. To better understand the origins and evolutionary implications of apomixis in
(Brassicaceae), we determined apomeiosis type for 64 accessions representing 44 taxonomic units. Plants expressing apospory and diplospory were equally common, and these generally produced reduced and unreduced pollen, respectively. Apospory and diplospory occurred simultaneously in individual plants of seven taxa. In
, apomixis perpetuates otherwise sterile or semisterile interspecific hybrids (allodiploids) through multiple generations. Accordingly, ample time, in these multigenerational clones, is available for rare meioses to produce haploid, intergenomically recombined male and female gametes. The fusion of such gametes could then produce segmentally autoploidized progeny. If sex re-emerges among such progeny, then new and genomically unique sexual species could evolve. Herein, we present evidence that such apomixis-facilitated speciation is occurring in
, and we hypothesize that it might also be occurring in facultatively apomictic allodiploids of other angiospermous taxa.
In angiosperms, meiotic failure coupled with the formation of genetically unreduced gametophytes in ovules (apomeiosis) constitute major components of gametophytic apomixis. These aberrant ...developmental events are generally thought to be caused by mutation. However, efforts to locate the responsible mutations have failed. Herein, we tested a fundamentally different hypothesis: apomeiosis is a polyphenism of meiosis, with meiosis and apomeiosis being maintained by different states of metabolic homeostasis. Microarray analyses of ovules and pistils were used to differentiate meiotic from apomeiotic processes in
(Brassicaceae). Genes associated with translation, cell division, epigenetic silencing, flowering, and meiosis characterized sexual
(meiotic). In contrast, genes associated with stress responses, abscisic acid signaling, reactive oxygen species production, and stress attenuation mechanisms characterized apomictic
(apomeiotic). We next tested whether these metabolic differences regulate reproductive mode. Apomeiosis switched to meiosis when premeiotic ovules of apomicts were cultured on media that increased oxidative stress. These treatments included drought, starvation, and H
O
applications. In contrast, meiosis switched to apomeiosis when premeiotic pistils of sexual plants were cultured on media that relieved oxidative stress. These treatments included antioxidants, glucose, abscisic acid, fluridone, and 5-azacytidine. High-frequency apomeiosis was initiated in all sexual species tested: Brassicaceae,
,
and
; Fabaceae,
; Asteraceae,
. Unreduced gametophytes formed from ameiotic female and male sporocytes, first division restitution dyads, and nucellar cells. These results are consistent with modes of reproduction and types of apomixis, in natural apomicts, being regulated metabolically.
Background Genetically unreduced (2n) embryo sacs (ES) form in ovules of gametophytic apomicts, the 2n eggs of which develop into embryos parthenogenetically. In many apomicts, 2n ES form ...precociously during ovule development. Whether meiosis and sexual ES formation also occur precociously in facultative apomicts (capable of apomictic and sexual reproduction) has not been studied. We determined onset timing of meiosis and sexual ES formation for 569 Sorghum bicolor genotypes, many of which produced 2n ES facultatively. Results Genotype differences for onset timing of meiosis and sexual ES formation, relative to ovule development, were highly significant. A major source of variation in timing of sexual germline development was presence or absence of apomictic ES, which formed from nucellar cells (apospory) in some genotypes. Genotypes that produced these aposporous ES underwent meiosis and sexual ES formation precociously. Aposporous ES formation was most prevalent in subsp. verticilliflorum and in breeding lines of subsp. bicolor. It was uncommon in land races. Conclusions The present study adds meiosis and sexual ES formation to floral induction, apomictic ES formation, and parthenogenesis as processes observed to occur precociously in apomictic plants. The temporally diverse nature of these events suggests that an epigenetic memory of the plants' apomixis status exists throughout its life cycle, which triggers, during multiple life cycle phases, temporally distinct processes that accelerate reproduction.
The development of key methodologies for the study of battlefields in the USA in the 1980s inspired a generation of British and European archaeologists to turn their attention to sites in their own ...countries. The end of the Cold War and key anniversaries of the World Wars inspired others, especially in the UK, to examine the material legacy of those conflicts before they disappeared. By 2000 the study of war was again firmly on the archaeological agenda. The overall purpose of the book is to encourage proponents and practitioners of Conflict Archaeology to consider what it is for and how to develop it in the future.The central argument is that, at present , Conflict Archaeology is effectively divided into closed communities who do not interact to any large extent. These separate communities are divided by period and by nationality, so that a truly international Conflict Archaeology has yet to emerge. These divisions prevent the exchange of information and ideas across boundaries and thereby limit the scope of the field. This book discusses these issues in detail, clearly outlining how they affect the development of Conflict Archaeology as a coherent branch of archaeology.
Pedagogy in Process Freire, Paulo; John Hunter, Carman
2016, 2016-02-25
eBook
Pedagogy in Process presents a first-hand account of the most comprehensive attempt yet to put into practice Paulo Freire’s theory of education within a real societal setting. When Guinea Bissau on ...the West African coast declared independence in 1973 the rate of illiteracy in its adult population was ninety percent. The new government faced the enormous task of educating its citizens. With Freire as collaborator and advisor the government launched a huge grass-roots literacy campaign and this book is Freire’s memoir of that campaign. Those familiar with Freire’s work will identify his ongoing insistence on the unity between theory and practice, mental and manual work, and past and present experience. This is essential reading for anyone interested Freire’s revolutionary ideas on education and the transformative power they hold when applied to society and the classroom.
Apomixis in crucifer (Brassicaceae) species is rare, reported in just four genera (Boechera, Draba, Erysimum, and Parrya), and one intergeneric hybrid (Raphanobrassica). It is well studied only in ...Boechera, where it is widespread among 100+ recognized species. However, its occurrence in related genera of the tribe Boechereae has not been documented previously. Here we analyzed genome evolution, mode of reproduction, and fertility of the monospecific Boechereae genus Phoenicaulis (P. cheiranthoides), endemic to the northwestern United States. We discovered that the species encompasses diploid (2n = 2x = 14), triploid (2n = 3x = 21), and tetraploid (2n = 4x = 28) populations. Comparative chromosome painting analyses revealed that the three karyotypes are essentially structurally identical, differing only in the presence of a largely heterochromatic chromosome (Het) in the triploid and tetraploid cytotypes. The genome structure of Phoenicaulis appeared identical to that of Boechera species previously analyzed, suggesting genomic stasis despite the morphological and molecular divergence of the two genera. This genome colinearity extended to the presence and structure of the Het chromosomes, which are closely associated with apomictic reproduction in Boechera. Interestingly, all three cytotypes of Phoenicaulis proved to be apomictic, regardless of the presence or absence of a Het chromosome, and sexual populations have yet to be identified.
PDF
