Issue Date : October Anyone you share the following link with will be able to read this content:. Sorry, a shareable link is not currently available for this article. Provided by the Springer Nature SharedIt content-sharing initiative. Skip to main content. Search SpringerLink Search. Summary Plant taxa that reproduce asexually display some distinct geographical and ecological patterns. References 1 Babcock, E. Google Scholar 3 Baker, H. Google Scholar 4 Barker, W. Google Scholar 5 Bayer, R.
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Geographical parthenogenesis: New Zealand. Springer, Dordrecht, pp. Nathan, R. Mechanisms of long-distance seed dispersal. Additional Supporting Information may be found online in Ozenda, P. Masson, Paris. In any case, factors that are involved in chromatin-remodeling and transcriptional regulation are candidates to play a role in the parthenogenetic pathway.
In egg cell arrest and embryo development, a role for signals from the surrounding tissue is indicated, particularly from the companion cells, the central cell in the mature gametophyte and endosperm in the developing seeds Grossniklaus, In sexual plant reproduction, the central cell also arrests until fertilization, but shows chromatin that is depleted from repressive marks and displays a more active transcriptional competence Baroux and Grossniklaus, This allows for the expression of maternal alleles and TEs, the latter thought to serve the production of 24 nucleotide siRNAs to reinforce silencing of TEs in the egg cell Ibarra et al.
In many species, the endosperm is, therefore, sensitive to a maternal to paternal dosage, and deviations from this lead to endosperm failure and embryo arrest Scott et al. Silencing of TEs in the egg cell by small RNAs from the surrounding nucellar tissue is also reported. Another study shows higher overall transcription levels in early embryos of parthenogenetic Tripsacum x maize hybrids as compared to embryos of sexual maize, supporting reduced silencing under parthenogenetic conditions Garcia-Aguilar et al.
Taken together, findings suggest that: 1 dedicated TE- silencing pathways, involving companion cells and surrounding ovular tissue, result in dynamic patterns of transcriptional suppression in the egg cell, and 2 the m : p balance in the endosperm is important for proper functioning of the endosperm, which in turn is essential for embryo survival.
They imply that changes in the accompanying cells and, as a result, in the communication to the egg cell, for example, changes in genes involved in DNA de- methylation or small RNA pathways, may have evolved in parthenogenesis. The PRC2 maintains the repressive state of its target genes by preserving the tri-methylation of the N-terminal tail of histone H3 on lysine 27 H3K27me3 , a mark of transcriptional silencing. Mutations in one of the FIS -class genes result in autonomous endosperm formation, showing diploid nuclei and development until cellularization Chaudhury et al.
Mutations in MSI 1 result in spontaneous embryo development in addition, although, with early embryo abortion up to the c.
These non-viable, haploid embryos express molecular markers and polarity similar to the diploid wild-type embryos produced by fertilization. Mutants of the FIS -class genes mea and fis 2 also rarely show embryo-like structures Chaudhury et al. Later studies showed that the functional requirement of the FIS -PRC2 could be bypassed by increasing the maternal genome dosage in the endosperm Kradolfer et al. This is particularly interesting in the context of apomixis, in which the ability to reproduce sexually is lost or modified and the maternal genome dosage in the endosperm is usually increased in pseudogamous apomicts and unique in autonomous apomicts.
Apomictic species may thus have become independent from the FIS -PRC2, either because it has a relatively modified expression or they evolved changed requirements for it. Unraveling this changes in more detail may give clues for parthenogenetic reproduction.
Despite the great discoveries discussed above, the precise molecular mechanism s by which the egg cell achieves its competence and is activated for embryogenesis is still unknown.
In animals, early embryogenesis mainly depends on maternal genetic information deposited in the egg cell before fertilization Tadros and Lipshitz, ; Eckersley-Maslin et al. In flowering plants, the large cytoplasm of the egg cell also allows for the deposition of maternally derived molecules. Several evidences suggest that embryogenesis in plants also mainly relies on maternal transcripts Autran et al.
If particularly or solely maternal transcripts are involved in the initiation of embryogenesis and MZT, parthenogenetic embryo development might be similar to that in sexual reproduction. However, if paternal factors are involved in addition, alternatives for their need should have been evolved in parthenogenesis, e. An interesting recent finding that highlights the importance of auxin in embryogenesis regulation is the identification of an auxin-response network that suppresses embryo development from the suspensor in Arabidopsis Radoeva et al.
In rice and maize egg cells, TFs are also over-represented, as are genes involved in transcriptional regulation and nucleic acid binding Anderson et al. A comparative transcriptome analysis between egg cells and zygotes in maize shows ZGA to involve c. Particularly genes that encode transcriptional regulators are activated in ZGA and chromatin assembly is modified, while the egg cell becomes primed to activate the translational machinery.
In summary, data show that a range of molecules known to play a role in development are stored in the egg cell and ready for use in embryogenesis.
They suggest that only a trigger is needed to release the repressive state and activate transcription and translation in order to initiate this. Subsequently, cell wall material is formed, likely representing a block to polyspermy. In some parthenogenetic organisms of other kingdoms, such as insects, stimuli imparted to the egg cell during ovulation or egg-laying, or non-sperm-based signals, e.
However, these external stimuli are not generally applicable, and despite being successful in some species and genotypes others can be completely recalcitrant to such triggers. Searching for factors that underlie such changes likely aid in defining the molecular basis of parthenogenesis.
In summary, data show that egg cells in sexually reproducing species undergo a period of arrest that goes together with condensed, repressive chromatin and silenced transcription, and an egg cell that is stocked with molecules ready for use in embryo development.
The molecular mechanism s or trigger by which the egg cell is activated and embryogenesis initiates is yet unknown, but results suggest the involvement of factors that release the chromatin repressive and transcriptionally silent state, e.
Parthenogenetic egg cells lack arrest or arrest for only a very short period, and it is unknown whether this implies that chromatin repression and transcriptional silencing are also omitted.
Since a quiescent state is hypothesized to be necessary to attain totipotency in the zygote, probably this state occurs also in parthenogenetic eggs, but only for a very short period. In any case, factors that are involved in chromatin-remodeling or transcriptional regulation are likely candidates in the parthenogenesis developmental pathway.
Parthenogenesis in angiosperms has long been thought to be a process that was initiated by apomeiosis and unknown to occur independently from it. Also the genetic control of parthenogenesis and apomeiosis was long assumed to rely on a single master gene or one locus with tightly linked genes Mogie, However, some of the former genetic models Richards, ; Nogler, ; Asker and Jerling, and more recent genetic mapping and other studies reviewed by Vijverberg and van Dijk, ; Hand and Koltunow, , showed that parthenogenesis is able to segregate from apomeiosis.
Among the first evidence for this came from artificial crosses in the common dandelion Taraxacum officinale , using diploid sexual female x triploid apomictic male crosses, resulting in small amounts of diploid, triploid, and tetraploid hybrid offspring, corresponding to fertilization of the haploid egg cell with a haploid, diploid, or triploid sperm, respectively Tas and van Dijk, ; van Dijk et al.
All tetraploid and some triploid hybrids showed spontaneous seed formation, thus displaying all apomixis elements. The other triploid hybrids produced near- diploid type-A; three plants , triploid type-B; four plants , or tetraploid type-C; two plants offspring after pollination with haploid pollen. Additional cytological investigations van Baarlen et al. Apart from demonstrating that apomeiosis, parthenogenesis, and also endosperm autonomy, were inherited independently from each other, these results established that parthenogenesis functions independently from endosperm autonomy and vice versa in dandelion.
Apparently, the usual precocious development of the embryo, as occurs in full apomicts, was disturbed, possibly as a result of the separation of parthenogenesis from apomeiosis or from modifiers or enhancers. All type-A triploids and the diploid hybrids gave rise to diploid offspring only after pollination with haploid pollen, suggesting that they were true sexuals. The absence of apomeiosis in diploid hybrids was later confirmed by the absence of a linked microsatellite marker in progeny from a similar cross van Dijk et al.
It was suggested to be the result of a genetic load associated to long-term asexual reproduction that becomes apparent and lethal in haploid gametes. This would then possibly also hold for parthenogenesis. Although presumed, the absence of parthenogenesis in the type-A triploids and diploid hybrids and the independent acting of parthenogenesis from apomeiosis were not explicitly demonstrated. Shortly afterward, the separate inheritance of apomeiosis and parthenogenesis was confirmed in another diplosporous Asteraceae, Erigeron annuus Noyes, ; Noyes and Rieseberg, In a follow-up, Noyes et al.
Parthenogenesis could thus act as an embryogenesis inducer in the absence of apomeiosis, whereas a possible role in embryo growth needed verification. All offspring investigated in these studies were triploid, implying imbalanced meiosis, resulting in aberrant chromosome numbers in most of the egg cells. Apparently, parthenogenesis can act in an aneuploid context , although with early embryo arrest. This confirmed that parthenogenesis is able to act and give rise to viable offspring independently from apomeiosis, at least, in a di-haploid context.
The study also mentioned rare, spontaneous development of seeds by some of the di-haploid parthenogenetic plants, that germinated normally and grew-out into haploid plants up to flowering Noyes and Wagner, This showed that parthenogenesis is able to function in a haploid context, although, with very low efficiency.
The low haploid survival rate is likely a result of recessive-lethal selection against the parthenogenesis locus, as was suggested in one of the previous studies in Erigeron for the absence of apomixis elements from diploid offspring Noyes and Rieseberg, Currently, the separate inheritance of apomeiosis and parthenogenesis is confirmed for most apomictic model species, including aposporous and diplosporous apomicts and monocots as well as dicots, as is implied by, e.
The independent inheritance of endosperm autonomy from parthenogenesis is also supported by other studies, e. This co-segregation likely evolved as a prerequisite for each of the apomixis elements alone to survive, since their separate occurrence will be untenable in the long-term due to their creation of plant lines with accumulating increasing or decreasing ploidy levels Asker and Jerling, This clearly demonstrated that parthenogenesis is fully functional also in the haploid context, and supports that the usual absence of parthenogenesis from haploidy is likely a result of recessive-lethal selection against associated flanking genomic regions.
In summary, studies in natural apomicts show that parthenogenesis usually co-segregates with apomeiosis, but is able to segregate and function independently from it. Parthenogenesis can also segregate and act independently from autonomous endosperm formation. It functions normally in di-haploid egg cells, and is able to induce embryogenesis in aneuploid eggs, although, with early embryo abortion. Parthenogenesis is virtually absent from reduced, haploid plant egg cells, but rare observations of its functioning in haploid eggs have been reported for di-haploid parthenogenetic Erigeron see above and a diploid apomictic Hieracium plant Bicknell, The isolation of the parthenogenesis inducing PsASGR-BBML gene, and the demonstration of its function as a transgene in haploid eggs from Pennisetum , rice, and maize, confirms that parthenogenesis is able to act independently from polyploidy.
It suggests that the absence of the trait from haploidy is likely explained by a genetic load in linked genomic regions. Finally, the results support the gametic presence and expression of parthenogenesis rather than non-cell-autonomous signaling from companion cells or surrounding, sporophytic tissue.
BBM genes were originally identified in Brassica napus Boutilier et al. This TF-family of almost members in Arabidopsis Sakuma et al. The one-domain ERF-like genes typically are involved in biotic or abiotic stress response whereas the two-domain AP2-like genes function in growth and development Floyd and Bowman, None of the AIL genes in Arabidopsis is expressed in pre-fertilization gametic cells during sexual reproduction Horstman et al.
On the other hand, after transformation to sexual P. Since tetraploid pearl millet was the transgenic background, parthenogenetic development of the reduced eggs gave rise to diploid progeny. A second cycle of parthenogenesis resulted in true haploids, which expectedly were sterile. Sterile haploids also were derived through parthenogenesis of reduced egg cells in rice and maize Conner et al.
Mature haploid seed formation was irregular possibly as a result of asynchronous embryo-endosperm development. This first demonstration of parthenogenesis gene function opens the door for synthesizing apomixis in cereal crops once the capacity to produce unreduced gametes at high frequency is installed. Interestingly, it was recently found that a wild-type rice BBM 1 Os-BBM 1 transgene under an Arabidopsis egg-cell-specific promoter DD45 was also able to initiate embryogenesis in rice egg cells without fertilization Khanday et al.
Most interestingly, it was shown that Os-BBM 1 lacks expression in the egg cells of rice, but is expressed in sperm cells, whereas only male BBM 1-transcripts are expressed in early zygotes. This suggests the requirement of fertilization in embryogenesis for the transmission of male-genomic factors that are maternally silenced. It would imply that in parthenogenesis, an essential, normally maternally imprinted gene, may have become maternally expressed. Tsunewaki and Mukai concluded that besides a cytoplasmic Restorer of fertility Rfv1 factor, two nuclear genes were involved in spontaneous embryo development: the inducer gene Parthenogenesis gain Ptg that is under sporophytic control, and the suppressor gene Suppressor of parthenogenesis Spg that is under gametophytic control.
These two genes were concomitantly exchanged with the chromosome 1 arm. To improve the system for in vivo investigation of parthenogenesis, three isogenic homozygous lines were produced, the male fertile sexual line Ae. Comparative protein analysis from ovary extracts of these three lines resolved one protein that was uniquely expressed in the two parthenogenetic lines from 3 days before and during anthesis.
This protein, P Further studies on isolated egg cells from the three isogenic lines and a common wheat line indicated that parthenogenetic development is independent from ovary-derived signals Kumlehn et al. This encouraged the researches to focus on the egg cells and construct cDNA libraries from them. Some eggs of the parthenogenetic lines showed a second nucleolus, a characteristic of zygotes isolated from sexual lines Naumova and Matzk, ; Kumlehn et al. Together, the results showed that parthenogenesis apparently is an inherent property of the egg cell and not the surrounding tissue and is able to establish zygotic competence in the absence of fertilization.
Encouraged by their identification in wheat egg cell cDNA libraries Kumlehn et al. At least two of them were preferentially expressed in the egg cell and, interestingly, their ectopic expression induced cell proliferation and activated an egg cell- like transcriptome. MpRKD showed wide expression in M. MpRKD mutants lacked effects on the overall morphology of reproductive organs, but showed striking defects in egg and sperm cell differentiation Koi et al. Together, these results indicate that RKD -TFs are evolutionary conserved regulators of germ cell differentiation in land plants and particularly act in the gametophyte-to-sporophyte transition by preventing the egg cell from entering mitosis in the absence of fertilization, i.
It was isolated via a mutant screen in the sexual model A. At that time, it became apparent that the modulation of chromatin structure played an important role in the regulatory decisions and gene expression during development, also in plants. As discussed above, the FIS -PRC2 is involved in gene suppression during seed development, particularly affecting the endosperm, whereas MSI 1, and to a lesser extent also other FIS -class genes, affects embryo initiation in addition Chaudhury et al.
Other studies showed a role for the FIS -PRC2 in balancing the maternal versus paternal gene dosage , by showing plants with an increased maternal dosage resembling FIS -mutant phenotypes Kradolfer et al. The results indicate that a release of gene suppression alone is insufficient to obtain viable seeds, but that this, particularly or solely, is a result of failure of the endosperm and maybe not embryo.
Investigations on the role of the endosperm showed that, indeed, endosperm cellularization impacts embryo growth, and FIS -mutant embryos could be rescued on appropriate medium in vitro Hehenberger et al. This indicates that release of the repressive state in the egg cell can be sufficient for the initiation of embryo development, however, finding clues for restoration of endosperm development is also necessary for successful parthenogenetic seed development.
Thus, PHE 1 is also imprinted, but expressed from the paternal allele only. A recent study asked the question what would happen with embryo growth in autonomous apomicts, where paternal alleles are absent Kirioukhova et al.
It was hypothesized that the silencing of maternal alleles might have become reduced or relieved during the evolution of apomixis, allowing maternally imprinted genes to be expressed from the maternal allele. In apomictic Boechera , the maternal PHE-like allele indeed was expressed, indicating a reversion of the imprinting status of this gene.
The authors proposed a model in which parthenogenesis in Boechera evolved via changes in epigenetic regulation of imprinted genes based on changes in DNA methylation see Figure 3 in Kirioukhova et al. This shows parallelisms to an artificially induced case of parthenogenesis in mice through the loss of distal DNA methylation, resulting in maternal activation of the paternally expressed Insulin-like growth factor 2 Igf 2 gene Kono et al.
Thus, a modified role in transcriptional regulation of maternal alleles is indicated and interesting to further investigate in the context of parthenogenesis. This gene is related to the tumor suppressor gene RB in mammals, which has a role in inhibiting cell cycle progression. RBR 1 in plants functions in cell cycle control during gametogenesis, with mutants showing supernumerary nuclei at the micropylar end and impaired cellularization Johnston et al. Polar nuclei do not fuse in rbr gametophytes and cell-type-specific markers usually lack expression.
Indeed, FIS 2 expression is reduced in rbr -gametophytes Johnston et al. They are mentioned here for completeness, but we refer to the other reviews for their listing and details, since a specific role in parthenogenesis is yet undetermined. Whether they are part of one or a few larger networks also needs further elucidation. Yet knowing the putative role of parental expression of Os-BBM 1 in embryogenesis, it is relevant to investigate this also for the other genes mentioned.
In summary, a PsASGR-BBML gene has been isolated and verified as the first parthenogenesis gene by demonstrating its functionality in related, sexual relative grasses such as pearl millet and rice, but not yet in eudicots.
Other candidates and studies support that the suppression of spontaneous embryo and endosperm development in sexual reproduction is under tight epigenetic control and release of this control allows for the initiation of spontaneous embryo and endosperm development. Although initiated, mutants of these genes show early embryo arrest and endosperm development up to cellularization, indicating that a release of transcriptional suppression alone is not enough to obtain viable seeds.
Functional endosperm is important in addition, either because of probable roles in the regulation of embryogenesis, but especially also to nourish the embryo. Restoring endosperm development is, therefore, necessary for successful seed development via parthenogenesis. Alternatively, the haploid embryos can be cultivated in vitro after embryo rescue, as is also done in some of the other haploid induction methods currently used in DH -production.
Interesting recent results show that Os-BBM 1 is paternally expressed, maternally silenced, and hypothesized to induce embryogenesis in rice egg cells after fertilization. Other recent results support a role for evolved changes in apomicts in this context, by showing the normally paternally expressed PHE-Like genes to be maternally expressed in apomictic Boechera. The results converge upon the importance of genes involved in the suppression of transcription and modifications thereof in apomicts at one hand and genes involved in the developmental process for which either transcription is allowed or artificially overexpressed on the other in parthenogenetic reproduction.
In Table 1 and Figure 2 this convergence is summarized. Table 1. Genes important in either the suppression or activation of embryogenesis in vivo or in vitro in plants and potentially relevant for parthenogenesis. Figure 2. This process is experimental and the keywords may be updated as the learning algorithm improves. This is a preview of subscription content, log in to check access. Agronomie 17 5 — Chase SS Monoploid frequencies in a commercial double cross hybrid maize, and in its component single cross hybrids and inbred lines.
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