The evolution of apomixis in angiosperms: A reappraisal

Abstract

Apomixis, the asexual reproduction via seed, has long been regarded a blind alley of evolution. This hypothesis was based on the assumption that apomixis is an irreversible, phylogenetically derived trait that would rapidly lead to extinction of the respective lineages. However, recent updates of the taxonomic distribution of apomixis in angiosperms suggest an alternative evolutionary scenario. Apomixis is taxonomically scattered and occurs in both early and late branching lineages, with several reversals from apomixis to obligate sex along phylogeny. Genetic control of apomixis is based on altered expression patterns of the same genes that control sexual development; epigenetic changes following polyploidization and/or hybridization may trigger shifts from sexuality to apomixis. Mendelian inheritance confirms the facultative nature and possible reversibility of apomixis to sexual reproduction. Apomixis, therefore, could represent a transition period in the evolution of polyploid complexes, with polyspory in paleopolyploids being a remnant of lost apomixis. In neopolyploids, apomixis helps to overcome sterility and allows for geographical range expansions of agamic polyploid complexes. The facultative nature of apomixis allows for reversal to sexuality and further speciation of paleopolyploid lineages. Thus, apomixis may facilitate diversification of polyploid complexes and evolution in angiosperms.     

Apomixis: Developmental Characteristics and Genetics

Asexual reproduction through seeds, or apomixis, is widespread in angiosperms, although does not happen frequently. It occurs in no major crop plant, but its deployment in major crops would afford advantages for breeding and maintenance of hybrid genotypes. Deployment is still a long-term goal, however, since the genetic mechanisms underlying apomixis in nature have not been determined nor has the isolation of apomictic mutants in sexual plants been achieved. Nevertheless, an increasing intensity of research toward these goals over the last decade has greatly expanded our knowledge of genome structure and gene expression in naturally occurring apomicts and female gametophyte development in sexual plants. A common working hypothesis is that apomixis is a “deregulation” of sexual processes and is increasingly supported by gene expression data. Nevertheless, the search for a unique trigger that initiates apomictic development still cannot be disqualified. Further characterization of female gametophyte-related genes and genomes of apomicts and model sexual plants will be fruitful for identifying overlaps in developmental networks.

     

Apomixis,hybridization, and taxonomic complexity in eastern north AmericanAmelanchier (Rosaceae)

Apomixis and hybridization together contribute to taxonomic complexity inAmelanchier. Hybridization combines genetically divergent genomes and spawns new forms that apomixis perpetuates. Apomixis is aposporous, facultative, and pseudogamous in the genus, and apomicts are generally polyploid, pollen fertile, and pollinated by generalists. That gene flow actually occurs is empirically evident. As apomixis is genetically dominant over sexuality, hybrids involving at least one apomictic parent are apomictic. Clonal reproduction may thus perpetuate F₁ individuals and generate agamospecies. Alternatively hybrids may interbreed and backcross to create hybrid swarms or cross with species other than the parents. In eastern North America, the abundance of published names and general taxonomic confusion in the genus doubtless result at least in part from this interplay of apomixis and hybridization. The roles of apomixis and hybridization in diversification withinAmelanchier are examined in light of new data about breeding system of an apomictic, hybrid microspecies, informally namedA. “erecta” and its formation of a hybrid swarm with anotherAmelanchier apomict,A. laevis.     

Asynchronous expression of duplicate genes in angiosperms may cause apomixis, bispory, tetraspory, and polyembryony

Apomicts that produce unreduced parthenogenetic eggs are generally polyploid and occur in at least 33 of 460 families of angiosperms. Embryo sacs of these apomicts form precociously from ameiotic megaspore mother cells (diplospory) or adjacent somatic cells (apospory). Polysporic species (bisporic and tetrasporic) are sexual and occur in at least 88 families. Their embryo sacs also form precociously, but only non-critical portions of meiosis are affected. It is hypothesized that (i) the partial to complete replacement of meiosis by embryo sac formation in apomictic and polysporic species results from asynchronously-expressed duplicate genes that control female development, (ii) duplicate genes result from polyploidy or paleopolyploidy (diploidized polyploidy with chromatin from multiple genomes), (iii) apomixis results from competition between nearly complete sets of asynchronously-expressed duplicate genes, and (iv) polyspory and polyembryony result from competition between incomplete sets of asynchronously-expressed duplicate genes. Phylogenetic and genomic studies were conducted to evaluate this hypothesis. Apomictic, polysporic, and polyembryonic species tended to occur together in cosmopolitan families in which temporal variation in female development is expected, apomicts were generally polyploid with few chromosomes per genome (X = 9.6pL0.4 SE), and polysporic and polyembryonic species were paleopolyploid with many chromosomes per genome (x= 15.7pL0.6 and 13.2pL0.4, respectively). These findings support the proposed duplicate-gene asynchrony hypothesis and further suggest asexual reproduction in apomicts preserves primary genomes, sexual reproduction in polysporic and polyembryonic polyploids accelerates paleopolyploidization, and pa-leopolyploidization may sometimes eliminate gene duplications required for apomixis while retaining duplications required for polyspory or polyembryony. Hence, apomixis, with its long-term reproductive stability, may occasionally serve as an evolutionary springboard in the evolution of normal and developmentally-novel paleopolyploid sexual species and genera.     

Heterochronic expression of sexual reproductive programs during apomictic development in Tripsacum

Some angiosperms reproduce by apomixis, a natural way of cloning through seeds. Apomictic plants bypass both meiosis and egg cell fertilization, producing progeny that are genetic replicas of the mother plant. In this report, we analyze reproductive development in Tripsacum dactyloides, an apomictic relative of maize, and in experimental apomictic hybrids between maize and Tripsacum. We show that apomictic reproduction is characterized by an alteration of developmental timing of both sporogenesis and early embryo development. The absence of female meiosis in apomictic Tripsacum results from an early termination of female meiosis. Similarly, parthenogenetic development of a maternal embryo in apomicts results from precocious induction of early embryogenesis events. We also show that male meiosis in apomicts is characterized by comparable asynchronous expression of developmental stages. Apomixis thus results in an array of possible phenotypes, including wild-type sexual development. Overall, our observations suggest that apomixis in Tripsacum is a heterochronic phenotype; i.e., it relies on a deregulation of the timing of reproductive events, rather than on the alteration of a specific component of the reproductive pathway.     

In search of the genetic footprints of Sumerians: a survey of Y-chromosome and mtDNA variation in the Marsh Arabs of Iraq

Background

Apomixis is an intriguing trait in plants that results in maternal clones through seed reproduction. Apomixis is an elusive, but potentially revolutionary, trait for plant breeding and hybrid seed production. Recent studies arguing that apomicts are not evolutionary dead ends have generated further interest in the evolution of asexual flowering plants.

Results

In the present study, we investigate karyotypic variation in a single chromosome responsible for transmitting apomixis, the Apospory-Specific Genomic Region carrier chromosome, in relation to species phylogeny in the genera Pennisetum and Cenchrus. A 1 kb region from the 3' end of the ndhF gene and a 900 bp region from trnL-F were sequenced from 12 apomictic and eight sexual species in the genus Pennisetum and allied genus Cenchrus. An 800 bp region from the Apospory-Specific Genomic Region also was sequenced from the 12 apomicts. Molecular cytological analysis was conducted in sixteen Pennisetum and two Cenchrus species. Our results indicate that the Apospory-Specific Genomic Region is shared by all apomictic species while it is absent from all sexual species or cytotypes. Contrary to our previous observations in Pennisetum squamulatum and Cenchrus ciliaris, retrotransposon sequences of the Opie-2-like family were not closely associated with the Apospory-Specific Genomic Region in all apomictic species, suggesting that they may have been accumulated after the Apospory-Specific Genomic Region originated.

Conclusions

Given that phylogenetic analysis merged Cenchrus and newly investigated Pennisetum species into a single clade containing a terminal cluster of Cenchrus apomicts, the presumed monophyletic origin of Cenchrus is supported. The Apospory-Specific Genomic Region likely preceded speciation in Cenchrus and its lateral transfer through hybridization and subsequent chromosome repatterning may have contributed to further speciation in the two genera.     

Diversity of higher plants in China

The Species Catalogue of China: Volume 1: Plants (SCCP) is a new, comprehensive, hardcopy inventory of Chinese higher plants that combines several datasets and references to recent taxonomic treatments. The database, with all attached additional information, is freely accessible via the internet (http://www.sp2000.org.cn/) and on CD-ROM, and will be updated yearly. It includes bryophytes (157 families, 599 genera, and 3167 specific and infraspecific taxa), lycophytes and ferns (41 families, 181 genera, and 2336 specific and infraspecific taxa), gymnosperms (10 families, 45 genera, and 311 specific and infraspecific taxa), and angiosperms (270 families, 3227 genera, and 35 873 specific and infraspecific taxa); in total 478 families, 4052 genera, and 41 687 specific and infraspecific taxa. Several other important statistics can also be drawn from the database, such as the distribution pattern of the four major groups of higher plants, as well as number of endemic and naturalized or cultivated genera/taxa. Entries in SCCP are also compared with Flora of China, and Flora reipublicae popularis Sinicae at the genus level. The SCCP will not only be a useful reference for floristic or biodiversity studies in China, but will also serve as a key resource to direct action and monitor progress. It is intended to be a useful resource for achieving Target 1 of the Global Strategy for Plant Conservation (GSPC).     

The complex causality of geographical parthenogenesis

Asexual organisms usually have larger and more northern distributions than their sexual relatives. This phenomenon, called geographical parthenogenesis, has been controversially attributed to predispositions in certain taxa; advantages of polyploidy and/or hybrid origin; better colonizing abilities because of uniparental reproduction; introgression of apomixis into sexuals; niche differentiation of clones; or biotic interactions. This review on apomictic plants demonstrates that each of these factors alone has not been able to explain the observed distributions. Establishment of the complex regulatory system of apomixis requires taxonomic and geographical predispositions; hybridization and/or polyploidization do create diversity, but they do not necessarily result in large distributions; colonizing abilities depend on clonal diversity and are outweighed by sexuals by self-compatibility and higher potentials for speciation; niche differentiation, ploidy levels and selfing keep sympatric sexuals and apomicts separated; and the impact of biotic interactions on distributions is uncertain. In conclusion, the distributional success of apomicts has a complex causality and depends on certain circumstances and combinations of factors. The rare establishment of apomixis may help to explain the predominance of sexuality on the large scale.     

Evolution of apomixis loci in Pilosella and Hieracium (Asteraceae) inferred from the conservation of apomixis-linked markers in natural and experimental populations

The Hieracium and Pilosella (Lactuceae, Asteraceae) genera of closely related hawkweeds contain species with two different modes of gametophytic apomixis (asexual seed formation). Both genera contain polyploid species, and in wild populations, sexual and apomictic species co-exist. Apomixis is known to co-exist with sexuality in apomictic Pilosella individuals, however, apomictic Hieracium have been regarded as obligate apomicts. Here, a developmental analysis of apomixis within 16 Hieracium species revealed meiosis and megaspore tetrad formation in 1 to 7% of ovules, for the first time indicating residual sexuality in this genus. Molecular markers linked to the two independent, dominant loci LOSS OF APOMEIOSIS (LOA) and LOSS OF PARTHENOGENESIS (LOP) controlling apomixis in Pilosella piloselloides subsp. praealta were screened across 20 phenotyped Hieracium individuals from natural populations, and 65 phenotyped Pilosella individuals from natural and experimental cross populations, to examine their conservation, inheritance and association with reproductive modes. All of the tested LOA and LOP-linked markers were absent in the 20 Hieracium samples irrespective of their reproductive mode. Within Pilosella, LOA and LOP-linked markers were essentially absent within the sexual plants, although they were not conserved in all apomictic individuals. Both loci appeared to be inherited independently, and evidence for additional genetic factors influencing quantitative expression of LOA and LOP was obtained. Collectively, these data suggest independent evolution of apomixis in Hieracium and Pilosella and are discussed with respect to current knowledge of the evolution of apomixis.     

Ecological and evolutionary opportunities of apomixis: insights from Taraxacum and Chondrilla

The ecological and evolutionary opportunities of apomixis in the short and the long term are considered, based on two closely related apomictic genera: Taraxacum (dandelion) and Chondrilla (skeleton weed). In both genera apomicts have a wider geographical distribution than sexuals, illustrating the short-term ecological success of apomixis. Allozymes and DNA markers indicate that apomictic populations are highly polyclonal. In Taraxacum, clonal diversity can be generated by rare hybridization between sexuals and apomicts, the latter acting as pollen donors. Less extensive clonal diversity is generated by mutations within clonal lineages. Clonal diversity may be maintained by frequency-dependent selection, caused by biological interactions (e.g. competitors and pathogens). Some clones are geographically widespread and probably represent phenotypically plastic 'general-purpose genotypes'. The long-term evolutionary success of apomictic clones may be limited by lack of adaptive potential and the accumulation of deleterious mutations. Although apomictic clones may be considered as 'evolutionary dead ends', the genes controlling apomixis can escape from degeneration and extinction via pollen in crosses between sexuals and apomicts. In this way, apomixis genes are transferred to a new genetic background, potentially adaptive and cleansed from linked deleterious mutations. Consequently, apomixis genes can be much older than the clones they are currently contained in. The close phylogenetic relationship between Taraxacum and Chondrilla and the similarity of their apomixis mechanisms suggest that apomixis in these two genera could be of common ancestry.     

Parallel origins of apomixis in two diverged evolutionary lineages in tribe Potentilleae (Rosaceae)

We synthesized the results from a flow cytometric seed screen and the literature to infer the phylogenetic origin and the geographical and taxonomic distribution of apomixis in tribe Potentilleae (Rosaceae). We distinguished between regular sexuality and apomixis, the zygotic and parthenogenetic origin of the embryo, and the pseudogamous (i.e. sexual) versus autonomous origin of the endosperm. The combined evidence provides information on reproductive modes for 11 genera and 120 species. For the first time records on reproductive mode are provided for the genus Farinopsis, 29 species (from five genera), and seven series of Potentilla. Regular sexuality was observed in Aphanes, Argentina, Comarum, Dasiphora, Drymocallis, Farinopsis, Fragaria, Horkeliella, Potentilla, and Sibbaldia. Reliable evidence for apomixis is restricted to two evolutionary lineages of Potentilleae: the Potentilla core group and Alchemilla/Aphanes. Early evolutionary divergence of these lineages (approximately 50 Mya), characterized by pseudogamous and autonomous apomictic seed formation, respectively, suggests parallel origins of apomixis. Apomixis is shown to be taxonomically widespread in the whole Northern Hemisphere distribution range of Potentilla, a pattern that is explained by hybrid transfer and repeated intercontinental dispersals. Taxonomic and geographical coverage is discussed with reference to species diversity centres of genera. © 2014 The Linnean Society of London, Botanical Journal of the Linnean Society, 2015, 177 , 214–229.     

Allelic ratios and sterility in the agamic complex of the Maximae (Panicoideae): evolutionary role of the residual sexuality

The agamic complex of the tribe Maximae is constituted by two pools characterized by their own breeding system (sexuality on the one hand, facultative apomixis on the other hand). Facultative apomicts shows two types of embryo-sacs (aposporic or meiotic). Presence of aposporic embryo-sacs is genetically controlled by the dominant allele A (all apomictic plants are Aaaa). Meiotic embryo-sacs, constituting residual sexuality, can allow crosses between apomicts and theoretically the generation of other genotypes than Aaaa. Additionally, some cases of sterile apomicts appears in a breeding system known to bypass it. This led us to study these two paradoxes. In fact, the sterility would exist when the ratio A/a is greater than 0.25. This would prevent the total overrunning of an apomictic population by AAAA genotypes. The role of residual sexuality is then to allow generation of true sexuals (aaaa) from apomicts (Aaaa).     

Embryological and genetic evidence of amphimixis and apomixis in <Emphasis Type="Italic">Boehmeria tricuspis</Emphasis>

Apomixis is a widespread alternative mode of sexual reproduction resulting in offspring that are genetically identical to the maternal plant. Boehmeria tricuspis (Hance) Makino is a perennial, wind-pollinated, herbaceous plant in the nettle family Urticaceae. The diploid B. tricuspis is monoecious but the triploid B. tricuspis is gynoecious, bearing female inflorescences only. Apomixis in B. tricuspis was first reported 50 years ago, but the mode of apomixis in the species has not been described yet. Here, we provide embryological observations of the embryo sac formation proving that triploid B. tricuspis reproduced apomictically following the Antennaria type of diplospory, and that the diploid individuals were the sexual genotype with the classical Polygonum-type maturation pattern of embryo sac development. A subsequent flow cytometry seed screen (FCSS) showed that the triploids were obligate apomicts with autonomous endosperm development, and the diploids reproduced sexually. In addition, a progeny test by molecular marker assays further demonstrated the above results.     

Apomictic Parthenogenesis and the Pattern of the Environment

Advantages of obligatory apomixis are modeled, and curculionidweevils are used as examples. It is suggested that the allegedtwofold number of female offspring in relation to sexual parentalsshould not be exaggerated. Instead, specific circumstances facilitatingthe origin of especially heterotic genotypes capable of apomixisshould be sought; crosses between differentiated (long-isolated)populations (preferably followed by a population flush) seempromising. Optimal strategies in temporally heterogeneous environmentsare studied, and situations favoringsexual reproduction bothin fine-grained and coarse-gained environments seem common (apomictsare capable only for individual adaptability, but sexual populationsmay track even one-generation changes by evolving). In apomicts,diapause determination and other seasonal constraints need specificattention, as they may interfere with geographical dispersal,and only life-cycles of one or more years seem possible. Inrich biotic communities, which evolve continuously, evolutionaryrigidity of apomicts may lead to their extinction. This showsthat the dichotomy between immediate and long-term advantagesof apomixis needs revision; if the number of generations neededfor competitive exclusion between asexual and sexual organismsis relatively high, the greater evolutionary potential of sexualpopulations must be considered (there are directional changesboth in the physical and biotic environments). Apomixis thenseems most advantageous in low-competitive, early successionhabitats which are recurrent and change relatively slowly (increasedcolonization ability of apomicts is here advantageous). Also,apomixis may succeed in spatially coarse-grained environmentin stable patches small enough to exclude high-competitive communityat the site. Then reduced vagility (loss of flight) may facilitatethe origin of apomixi     

Is supernumerary chromatin involved in gametophytic apomixis of polyploid plants?

Gametophytic apomixis, or unreduced embryo sac development that results in asexual reproduction through seeds, occurs in several families of angiosperms and must be polyphyletic in origin. The molecular mechanisms underlying gametophytic apomixis have not been discovered and are the subject of intense investigation. A common feature of almost all apomicts is their polyploid nature. From genetic mapping studies in both monocots and dicots, there is low genetic recombination associated with a single (rarely two), dominant locus for either aposporous or diplosporous embryo sac formation. In Pennisetum squamulatum and Cenchrus ciliaris, some DNA sequences mapping to the apospory locus are unique to apomictic genotypes and apparently hemizygous. This sequence divergence at the apomixis locus could be a consequence of genome rearrangements and isolation from genetic recombination, both of which may have contributed to the definition of a chromosomal region as supernumerary. The possible involvement of supernumerary chromatin, formed as a result of interspecific hybridization, in the origin of apomixis, is explored here. Received: 26 October 2000 / Revision accepted: 5 April 2001     

Fish-otoliths from the early Miocene of the Castillo Formation,Venezuela: a view into the proto-Caribbean teleostean assemblages

We describe the otolith assemblage from the early Miocene (Burdigalian) exposures of the Castillo Formation at the Cerro La Cruz locality (Falcón Basin, Lara State, northwestern Venezuela). The samples were obtained through surface and bulk sampling at distinct stratigraphic levels of the study area. The otolith assemblage from Cerro La Cruz includes 14 families, 25 genera and 30 taxa, of which 20 are reported for first time from the Castillo Formation. These include two new sciaenid taxa, Ctenosciaena angusticaudata nov. sp. and Pareques laraensis nov. sp. The Castillo Formation otolith assemblage represent fishes typical of shallow and near-shore and coral reef associated environments as characterized primarily by a high taxonomic diversity and abundance of Sciaenidae, and the lack of mesopelagic families (Myctophidae). The faunal composition of Cerro La Cruz is similar to other chronological equivalent sedimentary units along the proto-Caribbean area (Pirabas and Cantaure formations; Brazil and Venezuela respectively), though the Castillo Formation fish fauna shows lower taxonomic diversity, suggesting some heterogeneity of the coastal and near shore marine environments along the southern portion of the proto-Caribbean.

http://www.zoobank.org/urn:lsid:zoobank.org:pub:B79F51B8-BC35-4119-8957-84544286868D     

Problems of apomictic reproduction in the FamiliesCompositae andRosaceae

For many years several taxa belonging to the familiesCompositae andRosaceae have been subjected to an intensive research on apomixis. These two families, together with theGramineae have headed the list of the most thoroughly examined taxonomic groups in this field. At present, new methods are being elaborated and new problems solved within the following genera known to be apomictic, viz.Antennaria, Hieracium, Taraxacum, Alchemilla, Potentilla andRubus. Many general problems still require more attention in future studies. Seven of them are considered in the present article: 1. frequency of apomixis; 2. occurrence of apospory and diplospory within one taxon; 3. nonfunctional apospory; 4. facultativeness of autonomous apomixis; 5. origin of endosperm; 6. timing of embryo and endosperm development in apomicts; 7. instability of the endosperm type. BothCompositae andRosaceae produce suitable material for such research projects.     

Molecular characterization of the genomic region linked with apomixis in<Emphasis Type="Italic"> Pennisetum/Cenchrus</Emphasis>

Apomixis is defined as asexual reproduction through seeds, although this outcome can be achieved by multiple pathways. Since little is known about the molecular control of these pathways, how they might intersect is also a mystery. Two of these pathways in the grass family, diplospory and apospory, are receiving attention from molecular biologists. Apospory in Pennisetum/Cenchrus, two genera of panicoid grasses, results in the formation of four-nucleate embryo sacs that lack antipodals. Sexual reproduction frequently aborts so that the resulting seed is composed of (1) a parthenogenetically derived embryo that is genetically identical to the mother and (2) endosperm formed through pseudogamy. The transmission of apomixis is associated with the transfer of a linkage block on a single chromosome. This linkage block contains repetitive sequences as well as hemizygous, low-copy DNA sequences. Fluorescence in situ hybridization has demonstrated that these DNA regions occur on only a single chromosome, but not its homologs, in the polyploid apomicts studied. Features of the apomixis-associated region resemble those of other chromosomal segments isolated from recombination and replete with "selfish" DNAs.     

Methods for induction and utilization of variability in the improvement of an apomictic grass,Dichanthium annulatum complex

Summary Many problems and difficulties are encountered in making genetic improvements in plants where both apomixis and polyploidy occur together. From biosystematic studies on an agamic species complex, Dichanthium annulatum, information is presented on: (A) Mechanisms which create variability in apomicts — (i) genome building and reduction, (ii) hybridization between ecotypes of facultative apomicts, (iii) fertilization of unreduced gametes, (iv) introgressive hybridization, (v) preferential pairing and genotypic control of bivalent formation and (vi) induced mutation; (B) Embryo-sac variations, vis-a-vis sexual/apomictic sacs — (i) production of sexual embryo-sac in apomicts, (ii) balance between apomixis and sexual process, (iii) effect of environment and experimental manipulation of the type of embryo-sac; and (C) Heterosis and fixation of apomixis.The utilization and exploitation of these mechanisms and phenomena for accelerating the genetic improvement of apomictic plants is discussed.Mating systems impose certain restrictions on the breeding methodology to be used in the genetic improvement of crop plants. Allogamous species have built-in mechanisms for self-improvement and, for them, the breeding techniques are well worked out. Little information is, however, available on the procedures to be followed for the genetic improvement of apomicts. Recently gathered information on the causal mechanisms of apomixis and its mode of inheritance, the genetic systems which regulate the balance between apomixis and sexuality, the physical and chemical agents for artificial induction of sexuality in apomicts, and the processes which promote variability and adaptive polymorphism in apomicts show a way for the creation, exploitation and fixation of superior genotypes. Such information, based on biosystematic studies on an agamic species complex, Dichanthium annulatum, at the Oklahoma State University, Stillwater, Oklahoma, U.S.A., is presented here.Breeding procedures commonly followed for the genetic improvement of apomicts are outlined below:1. Collection of varieties, strains or ecotypes from diverse sources; 2. Evaluation of the germ plasm for the presence of desirable characters; 3. Building up of selection indices and estimation of genetic parameters; 4. Determination of mode of reproduction and isolation of sexual types or clones; 5. Hybridization using the sexual types; 6. Progeny testing, comparisons, multiplication and release of superior types.Thus, the success of the breeding programme would depend on the range of variability already present in the germ plasm collections, the relative proportion of sexual/apomictic seed produced and the exploitation of variability from the crossbred progenies. Since large collections of plants with different genotypes are not often available, one would like to look for the mechanisms which can create variability in the apomicts. Such mechanisms are as follows.