Apomixis technology development-virgin births in farmers' fields?

Apomixis is the process of asexual reproduction through seed, in the absence of meiosis and fertilization, generating clonal progeny of maternal origin. Major benefits to agriculture could result from harnessing apomixis in crop plants. Although >400 apomictic plant species are known, apomixis is rare among crop plants, and the transfer of apomixis to crop varieties by conventional breeding has been largely unsuccessful. Because apomictic and sexual pathways are closely related, de novo engineering of apomixis might be achieved in sexually reproducing crops. Early consideration of issues relating to biosafety and intellectual property (IP) management can facilitate the acceptance and deployment of apomixis technology in agriculture.     

Apomixis in agriculture: the quest for clonal seeds

Apomixis, or asexual reproduction through seeds, is a natural trait that could have an immense positive impact on crop production. Apomictic breeding strategies could allow the fixation and indefinite propagation of any desired genotype, however complex. Apomicts display a wide variety of developmental mechanisms, which can be viewed as a short-circuiting of sexual development. Gametophytic and sporophytic apomixis are distinguished by the developmental origin of apomictically derived embryos. Genetic studies suggest that individual elements of gametophytic apomixis, such as apomeiosis and parthenogenesis, are either controlled by one or two dominant Mendelian factors. As recombination around apomeiosis loci is suppressed, it is currently not known how complex these loci are. Much less is known regarding the genetic control of sporophytic apomixis but initial studies suggest a complex genetic control. Genetic analyses of sexual reproduction in plant model systems have identified genes that, when mutated, display elements of apomixis. Such studies help in the identification of candidate genes and promoters that can be used for the de novo engineering of apomixis through biotechnology. Molecular genetic studies in apomictic and sexual systems will generate the knowledge necessary for the engineering of conditional apomixis technology. Approaches encouraging collaboration and widespread dissemination of the acquired knowledge will constitute the most innovative route to the development, deployment and acceptance of apomixis technology in agriculture.     

Apomixis for crop improvement

Summary Apomixis is a genetically controlled reproductive process by which embryos and seeds develop in the ovule without female meiosis and egg cell fertilization. Apomixis produces seed progeny that are exact replicas of the mother plant. The major advantage of apomixis over sexual reproduction is the possibility to select individuals with desirable gene combinations and to propagate them as clones. In contrast to clonal propagation through somatic embryogenesis or in vitro shoot multiplication, apomixis avoids the need for costly processes, such as the production of artificial seeds and tissue culture. It simplifies the processes of commercial hybrid and cultivar production and enables a large-scale seed production economically in both seed- and vegetatively propagated crops. In vegetatively reproduced plants (e.g., potato), the main applications of apomixis are the avoidance of phytosanitary threats and the spanning of unfavorable seasons. Because of its potential for crop improvement and global agricultural production, apomixis is now receiving increasing attention from both scientific and industrial sectors. Harnessing apomixis is a major goal in applied plant genetic engineering. In this regard, efforts are focused on genetic and breeding strategies in various plant species, combined with molecular methods to analyze apomictic and sexual modes of reproduction and to identify key regulatory genes and mechanisms underlying these processes. Also, investigations on the components of apomixis, i.e., apomeiosis, parthenogenesis, and endosperm development without fertilization, genetic screens for apomictic mutants and transgenic approaches to modify sexual reproduction by using various regulatory genes are receiving a major effort. These can open new avenues for the transfer of the apomixis trait to important crop species and will have far-reaching potentials in crop improvement regarding agricultural production and the quality of the products.     

植物无融合生殖研究进展

植物无融合生殖是一种特殊的无性生殖方式 ,它不经过精卵融合即可繁殖后代 ,其二倍体子代基因型与母本精确相同 ,可以固定杂种优势 ,对于作物育种等工作具有巨大的经济意义。对无融合生殖的分类、遗传进化、发生机制、分子机理等方面进行了介绍。并对无融合生殖的一些最新的研究进展 :无孢子生殖专化基因组区、脱调节理论、基因组冲撞观点、表观遗传基因调节理论等进行了简要的评述。并简单介绍了无融合生殖甜菜单体附加系目前的研究进展 。     

Cloning plants by seeds: Inheritance models and candidate genes to increase fundamental knowledge for engineering apomixis in sexual crops

Apomixis is desirable in agriculture as a reproductive strategy for cloning plants by seeds. Because embryos derive from the parthenogenic development of apomeiotic egg cells, apomixis excludes fertilization in addition to meiotic segregation and recombination, resulting in offspring that are exact replicas of the parent. Introgression of apomixis from wild relatives to crop species and transformation of sexual genotypes into apomictically reproducing ones are long-held goals of plant breeding. In fact, it is generally accepted that the introduction of apomixis into agronomically important crops will have revolutionary implications for agriculture. This review deals with the current genetic and molecular findings that have been collected from model species to elucidate the mechanisms of apomeiosis, parthenogenesis and apomixis as a whole. Our goal is to critically determine whether biotechnology can combine key genes known to control the expression of the processes miming the main components of apomixis in plants. Two natural apomicts, as the eudicot Hypericum perforatum L. (St. John's wort) and the monocot Paspalum spp. (crowngrass), and the sexual model species Arabidopsis thaliana are ideally suited for such investigations at the genomic and biotechnological levels. Some novel views and original concepts have been faced on this review, including (i) the parallel between Y-chromosome and apomixis-bearing chromosome (e.g., comparative genomic analyses revealed common features as repression of recombination events, accumulation of transposable elements and degeneration of genes) from the most primitive (Hypericum-type) to the most advanced (Paspalum-type) in evolutionary terms, and (ii) the link between apomixis and gene-specific silencing mechanisms (i.e., likely based on chromatin remodelling factors), with merging lines of evidence regarding the role of auxin in cell fate specification of embryo sac and egg cell development in Arabidopsis. The production of engineered plants exhibiting apomictic-like phenotypes is critically reviewed and discussed.     

Developmental genetics of gametophytic apomixis

Some higher plants reproduce asexually by apomixis, a natural way of cloning through seeds. Apomictic plants produce progeny that are an exact genetic replica of the mother plant. The replication is achieved through changes in the female reproductive pathway such that female gametes develop without meiosis and embryos develop without fertilization. Although apomixis is a complex developmental process, genetic evidence suggests that it might be inherited as a simple mendelian trait - a paradox that could be explained by recent data derived from apomictic species and model sexual organisms. The data suggest that apomixis might rely more on a global deregulation of sexual reproductive development than on truly new functions, and molecular mechanisms for such a global deregulation can be proposed. This new understanding has direct consequences for the engineering of apomixis in sexual crop species, an application that could have an immense impact on agriculture.     

Harnessing apomictic reproduction in grasses: what we have learned from Paspalum

Background

Apomixis is an alternative route of plant reproduction that produces individuals genetically identical to the mother plant through seeds. Apomixis is desirable in agriculture, because it guarantees the perpetuation of superior genotypes (i.e. heterotic hybrid seeds) by self-seeding without loss of hybrid vigour. The Paspalum genus, an archetypal model system for mining apomixis gene(s), is composed of about 370 species that have extremely diverse reproductive systems, including self-incompatibility, self-fertility, full sexual reproduction, and facultative or obligate apomixis. Barriers to interspecific hybridization are relaxed in this genus, allowing the production of new hybrids from many different parental combinations. Paspalum is also tolerant to various parental genome contributions to the endosperm, allowing analyses of how sexually reproducing crop species might escape from dosage effects in the endosperm.

Scope

In this article, the available literature characterizing apomixis in Paspalum spp. and its use in breeding is critically reviewed. In particular, a comparison is made across species of the structure and function of the genomic region controlling apomixis in order to identify a common core region shared by all apomictic Paspalum species and where apomixis genes are likely to be localized. Candidate genes are discussed, either as possible genetic determinants (including homologs to signal transduction and RNA methylation genes) or as downstream factors (such as cell-to-cell signalling and auxin response genes) depending, respectively, on their co-segregation with apomixis or less. Strategies to validate the role of candidate genes in apomictic process are also discussed, with special emphasis on plant transformation in natural apomictic species.     

植物无融合生殖相关基因研究进展

无融合生殖是指不经过精卵融合即可形成胚从而进行种子繁殖后代的一种特殊的无性生殖方式, 无融合生殖胚的形成没有父本的参与, 其后代是母本基因型的完整克隆, 因此是植物杂种优势固定与利用的一种最理想的途径, 具有巨大的潜在利用价值, 被誉为“无性革命”。按其胚体发生的途径, 无融合生殖可分为二倍体孢子生殖、无配子生殖和不定胚生殖三种类型。本文介绍了植物胚发育、胚乳发育、减数分裂等涉及无融合生殖过程的相关基因的研究进展, 同时介绍了可能与植物无融合生殖途径调控相关的几个基因片段的研究情况。     

Apomixis and cassava

Apomixis means seed formation without fertilization. In cassava (Manihot esculenta) it is an alternative to reproduction by cuttings, which normally transmits pathogens and leads to an accumulation of viral and bacterial diseases. Apomixis also assures preservation of heterosis and avoids genetic segregation. It occurs in wild relatives of cassava and has been transferred successfully from Manihot glaziovii and M. neusana. It is facultative, and occurs at a low frequency, ranging from 1-2%, and apparently is genetically different from apomixis in other crops. With selection, the frequency can reach 13%. Apomixis in cassava is frequently associated with aneuploidy but it does occur in some diploid types. It is due to the formation of aposporic sacs, which can easily be detected by clearing tissue preparations. Apomixis appears to have played an important role in speciation during the evolution of Manihot, since it leads to the maintenance and perpetuation of sterile interspecific hybridization. The use of apomixis in cassava breeding could lead to a boom in line improvement and commercial production. In addition to preserving superior genotypes, avoiding contamination of new plants, it would enable international programs to export their germplasm to destination countries. This would allow the use of superior genotypes even if apomixis occurs at a low frequency. A scheme to maximize benefits is to use diploid apomictic clones as maternal parents, which can be crossed with pollinators of polyploid interspecific hybrids, followed by selection among the progeny of new apomictic types that combine the heteroses of both interspecific hybridization and polyploidy. In addition, they acquire favored genes that have been transferred from the wild to the commercial crop.     

MIME--有丝分裂替代减数分裂及其在作物无融合生殖中的应用

无融合生殖因其在杂种优势固定中的巨大潜力而受到广泛关注,人工创制无融合生殖是当前无融合生殖研究的重要方向,有丝分裂替代减数分裂(Mitosis instead of Meiosis,MIME)能产生与母本遗传组成完全一致的二倍体配子,是人工创制无融合生殖的关键步骤。文中对MIME的发生及其在作物无融合生殖中的应用以及MIME应用中的问题进行综述,以期为扩大MIME在作物无融合生殖中的应用提供参考。     

The Genetic Control of Apomixis: Asexual Seed Formation

Apomixis (asexual seed formation) is the result of a plant gaining the ability to bypass the most fundamental aspects of sexual reproduction: meiosis and fertilization. Without the need for male fertilization, the resulting seed germinates a plant that develops as a maternal clone. This dramatic shift in reproductive process has been documented in many flowering plant species, although no major seed crops have been shown to be capable of apomixis. The ability to generate maternal clones and therefore rapidly fix desirable genotypes in crop species could accelerate agricultural breeding strategies. The potential of apomixis as a next-generation breeding technology has contributed to increasing interest in the mechanisms controlling apomixis. In this review, we discuss the progress made toward understanding the genetic and molecular control of apomixis. Research is currently focused on two fronts. One aims to identify and characterize genes causing apomixis in apomictic species that have been developed as model species. The other aims to engineer or switch the sexual seed formation pathway in non-apomictic species, to one that mimics apomixis. Here we describe the major apomictic mechanisms and update knowledge concerning the loci that control them, in addition to presenting candidate genes that may be used as tools for switching the sexual pathway to an apomictic mode of reproduction in crops.     

单子叶植物无融合生殖的研究进展

植物的无融合生殖是指不经过雌雄配子融合而产生种子的一种特殊生殖方式。由于利用无融合生殖途径可以固定杂种优势,从而改良现有植物的育种策略,因此对无融合生殖的研究已成为生物学科的新生长点。本文主要从无融合生殖的概念和类型,无融合生殖在单子叶植物中的分布,无融合生殖的胚胎学,分子生物学和遗传学机制及创造新的无融合生殖种质资源的方法等6方面对单子叶植物的无融合生殖的研究进展进行了综述,并提出了今后开展无融合生殖研究的思路和设想。     

单子叶植物无融合生殖的研究进展

植物的无融合生殖是指不经过雌雄配子融合而产生种子的一种特殊生殖方式。由于利用无融合生殖途径可以固定杂种优势 ,从而改良现有植物的育种策略 ,因此对无融合生殖的研究已成为生物学科的新生长点。本文主要从无融合生殖的概念和类型 ,无融合生殖在单子叶植物中的分布 ,无融合生殖的胚胎学 ,分子生物学和遗传学机制及创造新的无融合生殖种质资源的方法等 6方面对单子叶植物的无融合生殖的研究进展进行了综述 ,并提出了今后开展无融合生殖研究的思路和设想     

Apomixis technology and the paradox of sex

Most plant species produce genetically variable seeds by the fusion of meiotically reduced egg cells and pollen grains. However, a small proportion of seed plants produces clonal, asexual seeds by the process of apomixis. The fixation of heterosis by apomixis is of great interest for plant breeding. The prospect of changing sexual crop species into apomictic crop species by genetic engineering--apomixis technology--has recently caused a boom in apomixis research. According to evolutionary biological theories, a dominant apomixis gene will rapidly become fixed in an outcrossing sexual population. Therefore, in theory, apomixis transgenes could have unconditional advantages that could result in the uncontrollable spread of the transgenes. By contrast, 'classic' transgenes might only have conditional advantages. Paradoxically, sexual reproduction and not apomixis is common in nature. However, this is no guarantee that apomixis transgenes will be ecologically safe because there could be essential differences between natural and transgenic apomicts.     

禾本科植物无融合生殖(综述)

禾本科植物包含了世界上最重要的农作物,也包含了最多的无融合生殖的种类,通过无融合生殖可将农作物的F1代杂种优势固定下来,这在固定农作物杂种优势的利用上具有巨大的潜力,然而禾本科植物无融合生殖作为其繁殖多样性的一种形式,在系统进化过程中的作用是非常复杂的,本文统计了禾本科无融合生殖的分布,概述了其无融合生殖的细胞学,遗传学和分子生物学研究进展。     

Altering sexual development inArabidopsis

The reproductive system determines the way in which gametes develop and interact to form a new organism. Therefore, it exerts the primary level of control of genotypic frequencies in plant populations, and plays a fundamental role in plant breeding. A basic understanding of plant reproductive development will completely transform current breeding strategies used for seed production. Apomixis is an asexual form of reproduction in which embryogenesis occurs in a cell lineage lacking both meiosis and fertilization, and that culminates in the formation of viable progeny genetically identical to the mother plant. The transfer of apomixis into sexual crops will allow the production of self-perpetuating improved hybrids, and the fixation of any desired heterozygous genotype. The initiation of apomictic development invariably takes place at early stages of ovule ontogeny, before the establishment of the megagametophytic phase. The developmental versatility associated with megagametophyte formation suggests that the genetic and molecular regulation of apomixis is intimately related to the regulation of sexuality. Differences between the initiation of sexual and apomictic development may be determined by regulatory genes that act during megasporogenesis, and that control events leading to the formation of unreduced female gametophytes. To test this hypothesis, we are isolating and characterizing genes that act during megasporogenesis inArabidopsis thaliana and investigating their potential role in the induction of apomixis. We are using a recently established transposon-based enhancer detection and gene trap insertional mutagenesis system that allows the identification of genes based on their expression patterns. An initial screen of transposants has yielded over 20 lines conferring restricted GUS expression during early ovule development. We have obtained the sequence of genomic fragments flanking the transposon insertion. Several have homology to genes playing important roles in plant and animal development. They include cell cycle regulators, enzymes involved in callose hydrolysis, leucine-rich repeat protein kinase receptors, and expressed sequence tags (ESTs) of unknown function. Independently, a genetic screen allows the identification of female sterile mutants defective in megasporogenesis. Results from these experiments will improve our basic understanding of reproductive development in plants, and will set the basis for a sustained effort in plant germ line biotechnology, a first step toward a flexible transfer of apomixis into a large variety of sexual crops.     

Sexual and apomictic plant reproduction in the genomics era: exploring the mechanisms potentially useful in crop plants

Arabidopsis, Mimulus and tomato have emerged as model plants in researching genetic and molecular basis of differences in mating systems. Variations in floral traits and loss of self-incompatibility have been associated with mating system differences in crops. Genomics research has advanced considerably, both in model and crop plants, which may provide opportunities to modify breeding systems as evidenced in Arabidopsis and tomato. Mating system, however, not recombination per se, has greater effect on the level of polymorphism. Generating targeted recombination remains one of the most important factors for crop genetic enhancement. Asexual reproduction through seeds or apomixis, by producing maternal clones, presents a tremendous potential for agriculture. Although believed to be under simple genetic control, recent research has revealed that apomixis results as a consequence of the deregulation of the timing of sexual events rather than being the product of specific apomixis genes. Further, forward genetic studies in Arabidopsis have permitted the isolation of novel genes reported to control meiosis I and II entry. Mutations in these genes trigger the production of unreduced or apomeiotic megagametes and are an important step toward understanding and engineering apomixis.     

Differential effects of polyploidy and diploidy on fitness of apomictic Boechera

The co-occurrence of apomixis (asexual reproduction) and polyploidy in plants has been the subject of debate in regard to the origin and evolution of asexuality. In recent years, polyploidy has been postulated as a maintenance and stabilization factor rather than as a source of apomixis origin. It is assumed polyploidy facilitates the compensation for mutation accumulation, and hence, the rare occurrence of diploid apomixis indirectly supports this finding. Nevertheless, diploid apomicts exist and are successful, especially in the genus Boechera. While comparing phenotypic traits, fitness-related traits and apomixis penetrance between both diploid and triploid apomicts in the genus Boechera, it was expected to find trait variance that can be attributed to ploidy. Surprisingly, little trait variation could be assigned to ploidy, but rather trait variations were mainly genotype-specific. Additionally, it is shown that paternal contribution is very important for trait success, even though all offspring are genetically identical to the mother plant. This harbors implications for the introduction of apomixis into crop plants, considering the effects of paternal contribution during asexual reproduction. Nevertheless, polyploidy is an efficient way to buffer deleterious mutations, but the flexibility of diploid apomicts of the genus Boechera for rare sexual events contributes to their success in nature.     

A model for linkage analysis with apomixis

Apomixis, or asexual reproduction through seeds, occurs in over 400 species of angiosperms. Although apomixis can favorably perpetuate desired genotypes through successive seed generation, it may also bring about some difficulty for linkage analysis and quantitative trait locus mapping. In this article, we explore the issue of how apomixis affects the precision and power of linkage analysis with molecular markers. We derive a statistical model for estimating the linkage between different markers when some progeny are derived from apomixis. The model was constructed within the maximum likelihood framework and implemented with the EM algorithm. A series of procedures are formulated to test the linkage of markers, the rate of apomixis, and the degree of genetic interference during meiosis. The model was examined and validated through simulation studies. The model will provide a tool for linkage mapping and evolutionary studies for plant species that undergo apomixis.