Flowering plant reproduction.

Flowering plants, also known as angiosperms, emerged approximately 150 to 200 million years ago. Since then, they have undergone rapid and extensive expansion, now encompassing around 90% of all land plant species. The remarkable diversification of this group has been a subject of in-depth investigations, and several evolutionary innovations have been proposed to account for their success. In this primer, we will specifically focus on one such innovation: the advent of seeds containing endosperm.

A paternal signal induces endosperm proliferation upon fertilization in Arabidopsis.

In multicellular organisms, sexual reproduction relies on the formation of highly differentiated cells, the gametes, which await fertilization in a quiescent state. Upon fertilization, the cell cycle resumes. Successful development requires that male and female gametes are in the same phase of the cell cycle. The molecular mechanisms that reinstate cell division in a fertilization-dependent manner are poorly understood in both animals and plants. Using Arabidopsis, we show that a sperm-derived signal induces the proliferation of a female gamete, the central cell, precisely upon fertilization. The central cell is arrested in S phase by the activity of the RETINOBLASTOMA RELATED1 (RBR1) protein. Upon fertilization, delivery of the core cell cycle component CYCD7;1 causes RBR1 degradation and thus S phase progression, ensuring the formation of functional endosperm and, consequently, viable seeds.

Hydro-Electro Hybrid Priming Promotes Carrot (Daucus carota L.) Seed Germination by Activating Lipid Utilization and Respiratory Metabolism.

Carrot (Daucus carota L.) is widely cultivated as one of the most important root crops, and developing an effective presowing treatment method can promote the development of modern mechanized precision sowing. In the present study, a novel seed priming technology, named hydro-electro hybrid priming (HEHP), was used to promote the germination of carrot seeds. Seed germination experiments showed that HEHP was able to increase the germination index (GI) and vigor index (VI) by 3.1-fold and 6.8-fold, respectively, and the effect was significantly superior to that of hydro-priming (HYD) and electrostatic field treatment (EF). The consumption and utilization rate of seed storage reserves were also greatly improved. Meanwhile, both glyoxysomes and mitochondria were found to appear ahead of time in the endosperm cells of HEHP through observations of the subcellular structure of the endosperm. Activities of isocitrate lyase (ICL), NAD-dependent malate dehydrogenase (MDH), pyruvate kinase (PK), and alcohol dehydrogenase (ADH) were significantly increased by HEHP. From transcriptome results, Kyoto Encyclopedia of Genes and Genomes (KEGG) pathways related to the glyoxylate cycle, glycolysis, gluconeogenesis, and the citrate cycle were significantly enriched and real-time quantitative PCR (qRT-PCR) analysis confirmed the expression pattern of 15 critical differentially expressed genes (DEGs) in these pathways. All DEGs encoding MDH, phosphoenolpyruvate carboxykinase (PEPCK), and PK were upregulated in HEHP; thus, it is reasonable to infer that the transformation of malate, oxalacetate, phosphoenolpyruvate, and pyruvate in the cytoplasm may be pivotal for the energy supply during early germination. The results suggest that the optimal effect of HEHP is achieved by initiating stored lipid utilization and respiratory metabolism pathways related to germination.

RH17 restricts reproductive fate and represses autonomous seed coat development in sexual Arabidopsis.

Plant sexual and asexual reproduction through seeds (apomixis) is tightly controlled by complex gene regulatory programs, which are not yet fully understood. Recent findings suggest that RNA helicases are required for plant germline development. This resembles their crucial roles in animals, where they are involved in controlling gene activity and the maintenance of genome integrity. Here, we identified previously unknown roles of Arabidopsis RH17 during reproductive development. Interestingly, RH17 is involved in repression of reproductive fate and of elements of seed development in the absence of fertilization. In lines carrying a mutant rh17 allele, development of supernumerary reproductive cell lineages in the female flower tissues (ovules) was observed, occasionally leading to formation of two embryos per seed. Furthermore, seed coat, and putatively also endosperm development, frequently initiated autonomously. Such induction of several features phenocopying distinct elements of apomixis by a single mutation is unusual and suggests that RH17 acts in regulatory control of plant reproductive development. Furthermore, an in-depth understanding of its action might be of use for agricultural applications.

Molecular mechanisms and hormonal regulation underpinning morphological dormancy: a case study using Apium graveolens (Apiaceae).

Underdeveloped (small) embryos embedded in abundant endosperm tissue, and thus having morphological dormancy (MD) or morphophysiological dormancy (MPD), are considered to be the ancestral state in seed dormancy evolution. This trait is retained in the Apiaceae family, which provides excellent model systems for investigating the underpinning mechanisms. We investigated Apium graveolens (celery) MD by combined innovative imaging and embryo growth assays with the quantification of hormone metabolism, as well as the analysis of hormone and cell-wall related gene expression. The integrated experimental results demonstrated that embryo growth occurred inside imbibed celery fruits in association with endosperm degradation, and that a critical embryo size was required for radicle emergence. The regulation of these processes depends on gene expression leading to gibberellin and indole-3-acetic acid (IAA) production by the embryo and on crosstalk between the fruit compartments. ABA degradation associated with distinct spatiotemporal patterns in ABA sensitivity control embryo growth, endosperm breakdown and radicle emergence. This complex interaction between gibberellins, IAA and ABA metabolism, and changes in the tissue-specific sensitivities to these hormones is distinct from non-MD seeds. We conclude that the embryo growth to reach the critical size and the associated endosperm breakdown inside MD fruits constitute a unique germination programme.

Endosperm cell size reduction caused by osmotic adjustment during nighttime warming in rice.

High night temperature (HNT) often reduces yield in field crops. In rice, HNT during the ripening stage diminishes endosperm cell size, resulting in a considerable reduction in final kernel weight; however, little is known about the underlying mechanisms at cell level. In this study, we performed picolitre pressure-probe-electrospray-ionization mass spectrometry to directly determine metabolites in growing inner endosperm cells of intact seeds produced under HNT conditions, combining with 13C feeding and water status measurements including in situ turgor assay. Microscopic observation in the inner zone suggested that approximately 24.2% of decrease in cell expansion rate occurred under HNT at early ripening stage, leading to a reduction in cell volume. It has been shown that HNT-treated plants were subjected to mild shoot water deficit at night and endosperm cell turgor was sustained by a decline in osmotic potential. Cell metabolomics also suggests that active solute accumulation was caused by a partial inhibition of wall and starch biosynthesis under HNT conditions. Because metabolites were detected in the single cells, it is concluded that a partial arrest of cell expansion observed in the inner endosperms was caused by osmotic adjustment at mild water deficit during HNT conditions.

Genetic Dissection of Apomixis in Dandelions Identifies a Dominant Parthenogenesis Locus and Highlights the Complexity of Autonomous Endosperm Formation.

Apomixis in the common dandelion (Taraxacum officinale) consists of three developmental components: diplospory (apomeiosis), parthenogenesis, and autonomous endosperm development. The genetic basis of diplospory, which is inherited as a single dominant factor, has been previously elucidated. To uncover the genetic basis of the remaining components, a cross between a diploid sexual seed parent and a triploid apomictic pollen donor was made. The resulting 95 triploid progeny plants were genotyped with co-dominant simple-sequence repeat (SSR) markers and phenotyped for apomixis as a whole and for the individual apomixis components using Nomarski Differential Interference Contrast (DIC) microscopy of cleared ovules and seed flow cytometry. From this, a new SSR marker allele was discovered that was closely linked to parthenogenesis and unlinked to diplospory. The segregation of apomixis as a whole does not differ significantly from a three-locus model, with diplospory and parthenogenesis segregating as unlinked dominant loci. Autonomous endosperm is regularly present without parthenogenesis, suggesting that the parthenogenesis locus does not also control endosperm formation. However, the high recovery of autonomous endosperm is inconsistent with this phenotype segregating as the third dominant locus. These results highlight the genetic complexity underlying apomixis in the dandelion and underline the challenge of introducing autonomous apomixis into sexual crops.

Protein sorting into protein bodies during barley endosperm development is putatively regulated by cytoskeleton members, MVBs and the HvSNF7s.

Cereal endosperm is a short-lived tissue adapted for nutrient storage, containing specialized organelles, such as protein bodies (PBs) and protein storage vacuoles (PSVs), for the accumulation of storage proteins. During development, protein trafficking and storage require an extensive reorganization of the endomembrane system. Consequently, endomembrane-modifying proteins will influence the final grain quality and yield. However, little is known about the molecular mechanism underlying endomembrane system remodeling during barley grain development. By using label-free quantitative proteomics profiling, we quantified 1,822 proteins across developing barley grains. Based on proteome annotation and a homology search, 94 proteins associated with the endomembrane system were identified that exhibited significant changes in abundance during grain development. Clustering analysis allowed characterization of three different development phases; notably, integration of proteomics data with in situ subcellular microscopic analyses showed a high abundance of cytoskeleton proteins associated with acidified PBs at the early development stages. Moreover, endosomal sorting complex required for transport (ESCRT)-related proteins and their transcripts are most abundant at early and mid-development. Specifically, multivesicular bodies (MVBs), and the ESCRT-III HvSNF7 proteins are associated with PBs during barley endosperm development. Together our data identified promising targets to be genetically engineered to modulate seed storage protein accumulation that have a growing role in health and nutritional issues.

A two-way molecular dialogue between embryo and endosperm is required for seed development.

The plant embryonic cuticle is a hydrophobic barrier deposited de novo by the embryo during seed development. At germination, it protects the seedling from water loss and is, thus, critical for survival. Embryonic cuticle formation is controlled by a signaling pathway involving the ABNORMAL LEAF SHAPE1 subtilase and the two GASSHO receptor-like kinases. We show that a sulfated peptide, TWISTED SEED1 (TWS1), acts as a GASSHO ligand. Cuticle surveillance depends on the action of the subtilase, which, unlike the TWS1 precursor and the GASSHO receptors, is not produced in the embryo but in the neighboring endosperm. Subtilase-mediated processing of the embryo-derived TWS1 precursor releases the active peptide, triggering GASSHO-dependent cuticle reinforcement in the embryo. Thus, a bidirectional molecular dialogue between embryo and endosperm safeguards cuticle integrity before germination.

Fertilization-Coupled Sperm Nuclear Fusion Is Required for Normal Endosperm Nuclear Proliferation.

Angiosperms exhibit double fertilization, a process in which one of the sperm cells released from the pollen tube fertilizes the egg, while the other sperm cell fertilizes the central cell, giving rise to the embryo and endosperm, respectively. We have previously reported two polar nuclear fusion-defective double knockout mutants of Arabidopsis thaliana immunoglobulin binding protein (BiP), a molecular chaperone of the heat shock protein 70 (Hsp70) localized in the endoplasmic reticulum (ER), (bip1 bip2) and its partner ER-resident J-proteins, ERdj3A and P58IPK (erdj3a p58ipk). These mutants are defective in the fusion of outer nuclear membrane and exhibit characteristic seed developmental defects after fertilization with wild-type pollen, which are accompanied by aberrant endosperm nuclear proliferation. In this study, we used time-lapse live-cell imaging analysis to determine the cause of aberrant endosperm nuclear division in these mutant seeds. We found that the central cell of bip1 bip2 or erdj3a p58ipk double mutant female gametophytes was also defective in sperm nuclear fusion at fertilization. Sperm nuclear fusion was achieved after the onset of the first endosperm nuclear division. However, division of the condensed sperm nucleus resulted in aberrant endosperm nuclear divisions and delayed expression of paternally derived genes. By contrast, the other double knockout mutant, erdj3b p58ipk, which is defective in the fusion of inner membrane of polar nuclei but does not show aberrant endosperm nuclear proliferation, was not defective in sperm nuclear fusion at fertilization. We thus propose that premitotic sperm nuclear fusion in the central cell is critical for normal endosperm nuclear proliferation.

The ratio of phytosiderophores nicotianamine to deoxymugenic acid controls metal homeostasis in rice.

MAIN CONCLUSION: The ratio of nicotianamine to deoxymugenic acid controls tissue-specific metal homeostasis in rice and regulates metal delivery to the endosperm. The metal-chelating phytosiderophores nicotianamine (NA) and 2'deoxymugenic acid (DMA) are significant factors for the control of metal homeostasis in graminaceous plants. These compounds are thought to influence metal homeostasis, but their individual roles and the effect of altering the NA:DMA ratio are unknown. We purposely generated rice lines with high and low NA:DMA ratios (HND and LND lines, respectively). The HND lines accumulated more iron (Fe), zinc (Zn), manganese (Mn) and copper (Cu) in the endosperm through the mobilization of Fe, Zn and Mn from the seed husk to the endosperm. In contrast, Fe, Zn and Mn were mobilized to the husk in the LND lines, whereas Cu accumulated in the endosperm. Different groups of metals are, therefore, taken up, transported and sequestered in vegetative tissues in the HND and LND lines to achieve this metal distribution pattern in the seeds. We found that different sets of endogenous metal homeostasis genes were modulated in the HND and LND lines to achieve differences in metal homeostasis. Our findings demonstrate that the NA:DMA ratio is a key factor regulating metal homeostasis in graminaceous plants. These findings can help formulate refined strategies to improve nutrient composition and nutrient use efficiency in crop plants.

The Role of RNA-Binding Protein OsTudor-SN in Post-Transcriptional Regulation of Seed Storage Proteins and Endosperm Development.

Tudor-SN is involved in a myriad of transcriptional and post-transcriptional processes due to its modular structure consisting of 4 tandem SN domains (4SN module) and C-terminal Tsn module consisting of Tudor-partial SN domains. We had previously demonstrated that OsTudor-SN is a key player for transporting storage protein mRNAs to specific ER subdomains in developing rice endosperm. Here, we provide genetic evidence that this multifunctional RBP is required for storage protein expression, seed development and protein body formation. The rice EM1084 line, possessing a nonsynonymous mutation in the 4SN module (SN3 domain), exhibited a strong reduction in grain weight and storage protein accumulation, while a mutation in the Tudor domain (47M) or the loss of the Tsn module (43M) had much smaller effects. Immunoelectron microscopic analysis showed the presence of a new protein body type containing glutelin and prolamine inclusions in EM1084, while 43M and 47M exhibited structurally modified prolamine and glutelin protein bodies. Transcriptome analysis indicates that OsTudor-SN also functions in regulating gene expression of transcriptional factors and genes involved in developmental processes and stress responses as well as for storage proteins. Normal protein body formation, grain weight and expression of many genes were partially restored in EM1084 transgenic line complemented with wild-type OsTudor-SN gene. Overall, our study showed that OsTudor-SN possesses multiple functional properties in rice storage protein expression and seed development and that the 4SN and Tsn modules have unique roles in these processes.

Characterization of the nucellus-specific dehydrin MdoDHN11 demonstrates its involvement in the tolerance to water deficit.

KEY MESSAGE: MdoDHN11 acts in the nucellus layer to protect the embryo and the endosperm from limited water availability during apple seed development. Dehydrins (DHNs) are protective proteins related to several plant developmental responses that involve dehydration such as seed desiccation and abiotic stresses. In apple (Malus × domestica Borkh.), the seed-specific MdoDHN11 was suggested to play important roles against dehydration during seed development. However, this hypothesis has not yet been evaluated. Within this context, several experiments were performed to functionally characterize MdoDHN11. In situ hybridization analysis during apple seed development showed that MdoDHN11 expression is confined to a maternal tissue called nucellus, a central mass of parenchyma between the endosperm and the testa. The MdoDHN11 protein was localized in the cytosol and nucleus. Finally, transgenic Arabidopsis plants expressing MdoDHN11 were generated and exposed to a severe water-deficit stress, aiming to mimic a situation that can occurs during seed development. All transgenic lines showed increased tolerance to water deficit in relation to wild-type plants. Taken together, our results provide evidences that MdoDHN11 plays important roles during apple seed development by protecting the embryo and the endosperm from limited water availability, and the mechanism of action probably involves the interaction of MdoDHN11 with proteins and other components in the cell.

Family quarrels in seeds and rapid adaptive evolution in Arabidopsis.

Evolutionary conflict can drive rapid adaptive evolution, sometimes called an arms race, because each party needs to respond continually to the adaptations of the other. Evidence for such arms races can sometimes be seen in morphology, in behavior, or in the genes underlying sexual interactions of host-pathogen interactions, but is rarely predicted a priori. Kin selection theory predicts that conflicts of interest should usually be reduced but not eliminated among genetic relatives, but there is little evidence as to whether conflict within families can drive rapid adaptation. Here we test multiple predictions about how conflict over the amount of resources an offspring receives from its parent would drive rapid molecular evolution in seed tissues of the flowering plant Arabidopsis As predicted, there is more adaptive evolution in genes expressed in Arabidopsis seeds than in other specialized organs, more in endosperms and maternal tissues than in embryos, and more in the specific subtissues involved in nutrient transfer. In the absence of credible alternative hypotheses, these results suggest that kin selection and conflict are important in plants, that the conflict includes not just the mother and offspring but also the triploid endosperm, and that, despite the conflict-reducing role of kinship, family members can engage in slow but steady tortoise-like arms races.

Gamma irradiation protect the developing wheat endosperm from oxidative damage by balancing the trade-off between the defence network and grains quality.

Gamma irradiation has been reported to modulate the biochemical and molecular parameters associated with the tolerance of plant species under biotic/ abiotic stress. Wheat is highly sensitive to heat stress (HS), as evident from the decrease in the quantity and quality of the total grains. Here, we studied the effect of pre-treatment of wheat dry seeds with different doses of gamma irradiation (0.20, 0.25 and 0.30 kGy) on tolerance level and quality of developing wheat endospermic tissue under HS (38 °C, 1 h; continuously for three days). Expression analysis of genes associated with defence and starch metabolism in developing grains showed maximum transcripts of HSP17 (in response to 0.25 kGy + HS) and AGPase (under 0.30 kGy), as compared to control. Gamma irradiation was observed to balance the accumulation of H2O2 by enhancing the activities of SOD and GPx in both the cvs. under HS. Gamma irradiation was observed to stabilize the synthesis of starch and amylose by regulating the activities of AGPase, SSS and α-amylase under HS. The appearance of isoforms of gliadins (α, ß, γ, ω) were observed more in gamma irradiated seeds (0.20 kGy), as compared to control. Gamma irradiation (0.25 kGy in HD3118 & 0.20 kGy in HD3086) was observed to have positive effect on the width, length and test seed weight of the grains under HS. The information generated in present investigation provides easy, cheap and user-friendly technology to mitigate the effect of terminal HS on the grain-development process of wheat along with development of robust seeds with high nutrient density.

OsNF-YC10, a seed preferentially expressed gene regulates grain width by affecting cell proliferation in rice.

Grain size and shape are important factors in determining the grain yield. In this study, OsNF-YC10, a member of the NF-Y transcription factor family encoding a putative histone transcription factor, was cloned and characterized. qRT-PCR and mRNA in situ hybridization analysis revealed that OsNF-YC10 was highly expressed in endosperm and spikelet hull at late developmental stages. The results showed that OsNF-YC10 was a nuclear protein showing transcription activation activity. The osnf-yc10 lines, produced using CRISPR/Cas9 technology, showed narrow, thin and light grains. Cytological experiments revealed significantly reduced cell number of spikelet hull in osnf-yc10 lines compared with that in WT. Narrow, thin, and light grains were found consistently in OsNF-YC10 RNAi transgenic lines. Moreover, the number of cells decreased in the grain-width direction than WT. These results indicated that OsNF-YC10 plays an important role in determining grain size and shape. OsNF-YC10 was further revealed to influence the expression of GW8 (a positive regulator of grain width), GW7 (a negative regulator of grain width) and cell cycle-regulated genes CYCD4, CYCA2.1, CYCB2.1, CYCB2.2, E2F2. Taken together, it is suggested that OsNF-YC10 regulates the grains size and shape by influencing the cell proliferation of spikelet hulls.

Effects of long-term exposure to elevated temperature on Zea mays endosperm development during grain fill.

Cereal yields decrease when grain fill proceeds under conditions of prolonged, moderately elevated temperatures. Endosperm-endogenous processes alter both rate and duration of dry weight gain, but underlying mechanisms remain unclear. Heat effects could be mediated by either abnormal, premature cessation of storage compound deposition or accelerated implementation of normal development. This study used controlled environments to isolate temperature as the sole environmental variable during Zea mays kernel-fill, from 12 days after pollination to maturity. Plants subjected to elevated day, elevated night temperatures (38°C day, 28°C night (38/28°C])) or elevated day, normal night (38/17°C), were compared with those from controls grown under normal day and night conditions (28/17°C). Progression of change over time in endosperm tissue was followed to dissect contributions at multiple levels, including transcriptome, metabolome, enzyme activities, product accumulation, and tissue ultrastructure. Integrated analyses indicated that the normal developmental program of endosperm is fully executed under prolonged high-temperature conditions, but at a faster rate. Accelerated development was observed when both day and night temperatures were elevated, but not when daytime temperature alone was increased. Although transcripts for most components of glycolysis and respiration were either upregulated or minimally affected, elevated temperatures decreased abundance of mRNAs related to biosynthesis of starch and storage proteins. Further analysis of 20 central-metabolic enzymes revealed six activities that were reduced under high-temperature conditions, indicating candidate roles in the observed reduction of grain dry weight. Nonetheless, a striking overall resilience of grain filling in the face of elevated temperatures can be attributed to acceleration of normal endosperm development.

RNA Binding Motif Protein 48 Is Required for U12 Splicing and Maize Endosperm Differentiation.

The last eukaryotic common ancestor had two classes of introns that are still found in most eukaryotic lineages. Common U2-type and rare U12-type introns are spliced by the major and minor spliceosomes, respectively. Relatively few splicing factors have been shown to be specific to the minor spliceosome. We found that the maize (Zea mays) RNA binding motif protein 48 (RBM48) is a U12 splicing factor that functions to promote cell differentiation and repress cell proliferation. RBM48 is coselected with the U12 splicing factor, zinc finger CCCH-type, RNA binding motif, and Ser/Arg rich 2/Rough endosperm 3 (RGH3). Protein-protein interactions between RBM48, RGH3, and U2 Auxiliary Factor (U2AF) subunits suggest major and minor spliceosome factors required for intron recognition form complexes with RBM48. Human RBM48 interacts with armadillo repeat containing 7 (ARMC7). Maize RBM48 and ARMC7 have a conserved protein-protein interaction. These data predict that RBM48 is likely to function in U12 splicing throughout eukaryotes and that U12 splicing promotes endosperm cell differentiation in maize.

Integrated physiological and proteomic analysis of embryo and endosperm reveals central salt stress response proteins during seed germination of winter wheat cultivar Zhengmai 366.

BACKGROUND: Salinity is a major abiotic stressor that affects seed germination, plant growth, and crop production. Seed germination represents the beginning of plant growth and is closely linked with subsequent crop development and ultimate yield formation. This study attempted to extend findings regarding the potential proteomic dynamics during wheat seed germination under salt stress and to explore the mechanism of crop salt response. RESULTS: Salt stress significantly affected seed physiological activities during the germination process, resulting in significant decreases in phytohormone and α-amylase activity and significant increases in soluble sugar, starch, and ADP glucose pyrophosphorylase activity. A comparative proteomics approach was applied to analyze the dynamic proteome changes of embryo and endosperm during seed germination in Chinese winter wheat cultivar Zhengmai 366 under salt stress. Two-dimensional electrophoresis identified 92 and 61 differentially accumulated proteins (DAPs) in response to salt stress in embryo and endosperm, respectively. Both organs contained a high proportion of DAPs involved in stress defense, energy metabolism, and protein/amino acid metabolism. The endosperm had more DAPs related to storage proteins and starch metabolism than the embryo, and 2% of DAPs participating in lipid and sterol metabolism were specifically detected in the embryo. CONCLUSIONS: Seed physiological activities were significantly affected during the germination process when subjected to salt stress. The DAPs involved in stress defense and energy metabolism were upregulated whereas those related to reserve substance degradation and protein/amino acid metabolism were significantly downregulated, leading to delayed seed germination under salt stress. Our proteomic results revealed synergistic regulation of the response to salt stress during seed germination.

Can obligate apomixis and more stable reproductive assurance explain the distributional successes of asexual triploids in Hieracium alpinum (Asteraceae)?

Although reproductive assurance has been suggested to be one of the most important factors shaping the differential distributional patterns between sexuals and asexuals (geographic parthenogenesis), it has only rarely been studied in natural populations of vascular plants with autonomous apomixis. Moreover, there are almost no data concerning the putative relationship between the level of apomictic versus sexual plant reproduction on one hand, and reproductive assurance on the other. We assessed the level of sexual versus apomictic reproduction in diploid and triploid plants of Hieracium alpinum across its distributional range using flow cytometric analyses of seeds, and compared the level of potential and realized seed set, i.e. reproductive assurance, between the two cytotypes under field and greenhouse conditions. Flow cytometric screening of embryos and endosperms of more than 4,100 seeds showed that diploids produced solely diploid progeny sexually, while triploids produced triploid progeny by obligate apomixis. Potential fruit set was much the same in diploids and triploids from the field and the greenhouse experiment. While in the pollination-limited environment in the greenhouse apomictic triploids had considerably higher realized fruit set than sexual diploids, there was no significant difference between cytotypes under natural conditions. In addition, sexuals varied to a significantly larger extent in realized fruit set than asexuals under both natural and greenhouse conditions. Our results indicate that triploid plants reproduce by obligate apomixis, assuring more stable and predictable fruit reproduction when compared to sexual diploids. This advantage could provide apomictic triploids with a superior colonisation ability, mirrored in a strong geographic parthenogenesis pattern observed in this species.