Expression of the apomictic potential and selection for apomixis in sorghum line AS-1a

Expression of elements of apomixis was studied for ten seasons in sorghum line AS-1a and its backcross hybrids in the 9E and A3 sterile cytoplasms. Cytoembryological analysis revealed aposporous embryo sacks (apo-ESs), their initial cells, and, rare, parthenogeneic proembryos in ovules of line AS-1a and its BC₂ and BC₃ hybrids on the 9E cytoplasm. The A3 sterile cytoplasm suppressed the development of parthenogenetic proembryos, but did not affect the apo-ES formation. The frequency of apomictic elements increased in seasons with high daily temperatures and total precipitation deficiency in the period of ovule and megagametophyte development (r = −0.805, p <.01). Selection was used to isolate the families where the frequency of ovules with apo-ESs was 28% and the frequency of parthenogenetic proembryos was 14%. Emasculated panicles of line AS-1a were pollinated with pollen of line Volzhskoe-4v, which carried the Rs marker dominant gene, responsible for the anthocyan color of coleoptyles and leaves in seedlings. Plants of the maternal type were found in the progenies of these crosses at a frequency of 1.4–28.1%. The genetic structure of the endosperm in grains with maternal-type seedlings was inferred from the electrophoretic patterns of storage proteins (kafirins). The kafirin spectra of grains producing maternal-type seedlings was similar to the spectrum of line AS-1a and differed from the spectra of grains producing hybrid seedlings, indicating that the endosperm developed autonomously when apomictic grains formed in line AS-1a. The results showed that lines with facultative apomixis can be constructed in functionally diploid plants.     

A novel approach to differentiated embryos in the absence of endosperm

Summary In all of the Poaceae tested (Bromus, Festuca, Hordeum, Lolium, Poa, Triticum) the formation of grains without endosperm was induced from unpollinated ovules by treatment with the following synthetic auxins: DIG; 2,4-D; 2,4,5-T; or CPAA. Cytokinins (BAP, ZTN) as well as adenine or gibberellic acid (GA₃) alone were ineffective. In parthenogenetic lines auxin treatment resulted in grains with mature embryos without endosperm. Differences in embryo differentiation were found, which were dependent on the synthetic auxins used, their concentrations, and the developmental stages of the treated spikes or panicles. Thus, the regulation of embryogenesis by the endosperm can be replaced by exogenous auxin application. The developing proembryos of grasses did not need nutritive support from the endosperm.Auxin treatment to give mature embryos without endosperm enables the screening of apomictic species for sexual plants and sexual species for parthenogenetic individuals. It opens ways for inducing haploid parthenogenesis and improves methods for overcoming interspecific crossing barriers.     

The of gamma-radiation for induction of apomixis in sea-buckthorn (Hippophaë rhamnoides L.)

The results of studying of the induced apomixis in pollen of sea-buckthorn irradiated by the 60Co gamma-Radiation are considered. Was established that the most effective dose for pollination of the experimental plants is 50 k Gy. In total, from 46 seedlings 19 cases of apomictic origin were revealed, 7 individuals were found to be haploid (n = = 12) and 19 ones were diploid (2n = 24) of maternal origin. Was supposed that apomictic plants (19 seedlings) have parthenogenetic origin. The reason for such conclusion is that the irradiated anomalous pollen tubes despite not having spermia, are entering embryo sac and stimulate the development of apomictic embryo from non-fertilized female gametes. Apparently, pollen tubes cause the induction of DNA replication in the ovules and the development of parthenogenetic plants. Consequently, the described method can be used for the regulation of parthenogenesis in sea-buckthorn to change natural ratio (1 : 1) of male to female plants in desirable quantity.     

Pseudogamous Apomixis in Maize and Sorghum in Diploid-Tetraploid Crosses

Apomictic seed development is a complex process including formation of unreduced embryo sac, parthenogenetic embryo development from the egg cell, and endosperm formation either autonomously, or due to fertilization of polar nuclei by the sperm (under pseudogamous form of apomixis). In the latter case, an obstacle to the normal endosperm development is disturbance of maternal (m) -to-paternal (p) genomic ratio 2m: 1p that occurs in the cases of pollination of unreduced embryo sac with haploid sperms. Usage of tetraploid pollinators can overcome this problem because in such crosses maternal-to-paternal genomic ratio is 4m: 2p that provides formation of kernels with plump endosperm. Using tetraploid lines as pollen parents we observed formation of plump kernels on the ears and panicles of diploid maize and sorghum accessions. These kernels had hybrid endosperm and diploid maternaltype embryo or hybrid embryo with different ploidy level (2n, 3n, 4n). The frequencies of plump kernels on the ear ranged from 0.2-0.3% to 5.7-6.2% counting from the number of ovaries. Maternal-type plants were found in two maize lines, their frequency varying from 10.7 to 37.5% of the progeny plants. In CMS-lines of sorghum pollinated with tetraploid sorghum accessions, the frequency of plump kernels ranged from 0.6 to 14.0% counting from the number of ovaries; the frequency of maternal-type plants varied from 33.0 up to 96.1%. The hybrid nature of endosperm of the kernels that gave rise to maternal-type plants has been proved by marker gene expression and by SDS-electrophoresis of endosperm proteins. These data testify to variable modes of seed formation under diploid × tetraploid crosses in maize and sorghum both by amphi- and by apomixis. Therefore, usage of tetraploid pollinators might be a promising approach for isolation of apomixis in maize and sorghum accessions.     

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.     

Comparative cyto-embryological investigations of sexual and apomictic dandelions (Taraxacum) and their apomictic hybrids

. In the autonomous apomictic Taraxacum officinale (common dandelion), parthenogenetic egg cells develop into embryos and central cells into endosperm without prior fertilisation. Unreduced (2n) megaspores are formed via meiotic diplospory, a nonreductional type of meiosis. In this paper, we describe the normal developmental pathways of sexual and apomictic reproduction and compare these with the development observed in the apomictic hybrids. In sexual diploids, a standard type of megasporogenesis and embryo sac development is synchronised between florets in individual capitula. In contrast, we observed that megasporogenesis and gametogenesis proceeded asynchronously between florets within a single capitulum of natural triploid apomicts. In addition, autonomous endosperm and embryo development initiated independently within individual florets. Parthenogenetic initiation of embryo development in outdoor apomicts was found to be temperature-dependent. Egg cells produced in natural apomicts were not fertilised after pollination with haploid pollen grains although pollen tubes were observed to grow into their embryo sacs. Both reductional and diplosporous megasporogenesis were observed in individual inflorescences of triploid apomictic hybrids. Embryo and endosperm development initiated independently in natural and hybrid apomicts.     

Reconstruction of reproductive diversity in Hypericum perforatum L. opens novel strategies to manage apomixis

The mode of reproduction was characterized for 113 accessions of the tetraploid facultative apomictic species Hypericum perforatum using bulked or single mature seeds in the flow cytometric seed screen (FCSS). This screen discriminates several processes of sexual or asexual reproduction based on DNA contents of embryo and endosperm nuclei. Seed formation in H. perforatum proved to be highly polymorphic. Eleven different routes of reproduction were determined. For the first time, individual seeds were identified that originated from two embryo sacs: the endosperm from an aposporous and the embryo from the legitimate meiotic embryo sac. Moreover, diploid plants were discovered, which apparently reproduce by a hitherto unknown route of seed formation, that is chromosome doubling within aposporous initial cells followed by double fertilization. Although most plants were tetraploid and facultative sexual/apomictic, diploid obligate sexuals and tetraploid obligate apomicts could be selected. Additionally, genotypes were detected which at a high frequency produced embryos either from reduced parthenogenetic or unreduced fertilized egg cells. The endosperm developed most frequently after fertilization of the central cell in aposporous embryo sacs (pseudogamy) but in few cases also autonomously. The genetic control of apomixis appears to be complex in H. perforatum. Basic material was developed for breeding H. perforatum, and strategies are suggested for elucidation of inheritance as well as evolution of apomixis and for molecular approaches of apomixis engineering.     

Apomixis via recombination of genome regions for apomeiosis (diplospory) and parthenogenesis in Erigeron (daisy fleabane, Asteraceae)

Apomixis in daisy fleabanes (Erigeron annuus and E. strigosus) is controlled by two genetically unlinked loci that regulate, independently, the formation of unreduced female gametophytes (apomeiosis, diplospory) and autonomous seed formation (parthenogenesis). In this work, fully apomictic F2s were regenerated by crossing F1s bearing, separately, these two functional regions. Two triploid (3x = 2n = 27) highly diplosporous F1s served as seed parents to an aneuploid (2x + 1 = 2n = 19) meiotic pollen donor bearing four AFLP markers linked to parthenogenetic seed formation but producing only abortive embryos and endosperm. Of 408 hybrids, 21 (5.1%) produced seed. Nine of these putative apomicts were tetraploids (4x), likely combining an unreduced egg from the diplosporous seed parent and a haploid gamete from the pollen parent (3x + x). The other 12 hybrid apomicts were pentaploid, interpreted as arising from the fusion of an unreduced diplosporous egg with an unreduced sperm cell (3x + 2x). Analysis indicated that all but three of the 21 synthetic apomicts recombined markers linked to diplospory and parthenogenesis. In addition, three additional hybrids combined markers linked to the two functional regions but produced only aborted embryos. The apomicts varied in percentage of diplosporous ovules (4.7–95.3% of all ovules produced) and in percentage of ovules that developed into seed (3.8–58.0%). These results support the hypothesis that apomeiosis and autonomous seed formation are genetically distinct, and that the traits can be separated and recombined to create hybrids exhibiting apomixis at near wildtype levels.     

Characterization of mode of reproduction in Commiphora wightii [(Arnot) Bhandari] reveals novel pollen–pistil interaction and occurrence of obligate sexual female plants

Guggul [Commiphora wightii (Arnot) Bhandari], a polygamous woody tree valued for its medicinal oleoresin gum rich in guggulsterone, is reported to reproduce via sporophytic apomixis. Details about its natural diversity, and mode and extent of sexual reproduction are, however, scanty. Therefore, a comprehensive investigation of guggul reproduction was made employing histology, controlled pollination, flow cytometry and RAPD analyses of progeny to assess the occurrence and extent of sexual reproduction. We report the discovery of obligate sexual female plants of guggul through these studies. Also, we document a unique pollen–pistil incompatibility that prevents all but one pollen tube growth into the style to effect fertilization. Consequently, obligate sexual female plants produced single-seeded fruit although each flower contains four ovules. In apomictic plants bearing more than one seed per fruit, at most only one seed was of sexual origin. Further, flow cytometric analysis conclusively demonstrated that endosperm development occurs either autonomously or following triple fusion. Autonomous endosperm development was invariably associated with endoreduplication, a unique feature of apomixis in guggul. Despite predominance of apomixis, a low frequency of sexual reproduction was found to persist in apomictic plants yielding new genetic variation. RAPD analysis clearly distinguished accessions and was useful in identifying sexual progenies. The implications of the novel pollen–pistil interaction on establishment and spread of apomixis in guggul are discussed. The study has not only revealed novel features of guggul reproduction but also opened new opportunities for molecular genetic analysis of sporophytic apomixis and breeding improvement of guggul.     

Tripsacum dactyloides (Poaceae): a natural model system to study parthenogenesis

Diplosporous apomeiosis, formation of unreduced embryo sacs primarily of the Antennaria type, followed by parthenogenetic embryo development and pseudogamy (fertilization of the central cell) describe gametophytic apomixis within the Tripsacum agamic complex. Tripsacum dactyloides (Eastern gamagrass) is a close relative of domesticated maize and was chosen as a natural model system to investigate gene expression patterns associated with parthenogenesis. The genome size of diploid sexual and polyploid apomictic T. dactyloides was estimated by flow cytometry to be 7.37 pg (2C), 14.74 pg (4C) and 22.39 pg (6C), respectively. The diploid genome size is thus approximately 1.352 larger than that of maize. The apomeiotic-pseudogamous pathway of seed formation was demonstrated at a rate of 92% by the flow cytometric seed screen (FCSS) with single mature seeds in tetraploid accessions. This number includes twin embryos which were detected in 13% of the seeds analyzed. Fertilization of unreduced egg cells (B_III hybrids) was measured in 10% of apomictic seeds. Autonomous (fertilization-independent) embryo development and fertilization-dependent endosperm formation were confirmed by pollination of tetraploid T. dactyloides with a diploid transgenic maize line carrying an actin::#-glucuronidase (GUS) reporter construct. GUS expression was detected after pollination in the developing endosperm, but not in the embryo. In similar intraspecific crossing experiments with maize, GUS expression was detected in both the embryo and endosperm. A protocol was established for microdissection of embryo sacs and early parthenogenetic embryos of T. dactyloides. Together, these techniques provide new tools for future studies aimed at comparing gene expression patterns between sexual maize and sexual or apomictic T. dactyloides.     

Genetic diversity and reproductive biology in ecotypes of the facultative apomict Hypericum perforatum L

Apomixis is a mode of asexual reproduction through seed. Progeny produced by apomixis are clonal replicas of a mother plant. The essential feature of apomixis is that embryo sacs and embryos are produced in ovules without meiotic reduction or egg cell fertilisation. Thus, apomixis fixes successful gene combinations and propagates high fitness genotypes across generations. A more profound knowledge of the mechanisms that regulate reproductive events in plants would contribute fundamentally to understanding the evolution and genetic control of apomixis. Molecular markers were used to determine levels of genetic variation within and relationship among ecotypes of the facultative apomict Hypericum perforatum L. (2n = 4x = 32). All ecotypes were polyclonal, being not dominated by a single genotype, and characterised by different levels of differentiation among multilocus genotypes. Flow cytometric analysis of seeds indicated that all ecotypes were facultatively apomictic, with varying degrees of apomixis and sexuality. Seeds set by haploid parthenogenesis and/or by fertilisation of aposporic egg cells were detected in most populations. The occurrence of both dihaploids and hexaploids indicates that apospory and parthenogenesis may be developmentally uncoupled and supports two distinct genetic factors controlling apospory and parthenogenesis in this species. Cyto-embryological analysis showed that meiotic and aposporic processes do initiate within the same ovule: the aposporic initial often appeared evident at the time of megaspore mother cell differentiation. Our observations suggest that the egg cell exists in an active metabolic state before pollination, and that its parthenogenetic activation leading to embryo formation may occur before fertilisation and endosperm initiation.     

Apomixis and ploidy barrier suppress pollen-mediated gene flow in field grown transgenic turf and forage grass (Paspalum notatum Flüggé)

Bahiagrass (Paspalum notatum Flüggé) is the predominant forage grass in the southeastern US. The commercially important bahiagrass cultivar ‘Argentine’ is preferred for genetic transformation over sexual diploid cytotypes, since it produces uniform seed progeny through apomixis. Pseudogamous apomictic seed production in Argentine bahiagrass may contribute to transgene confinement. It is characterized by embryo development which is independent of fertilization of the egg cell, but requires fertilization with compatible pollen to produce the endosperm. Pollen-mediated gene transfer from transgenic, glufosinate-resistant apomictic bahiagrass as pollen donor at close proximity (0.5–3.5 m) with non-transgenic sexual or apomictic bahiagrass cultivars as pollen receptors was evaluated under field conditions. Hybridization frequency was evaluated by glufosinate herbicide resistance in >23,300 seedlings derived from open-pollinated (OP) pollen receptor plants. Average gene transfer between transgenic apomictic, tetraploid and sexual diploid bahiagrass was 0.03%. Herbicide-resistant hybrids confirmed by immuno-chromatographic detection of the PAT protein displayed a single copy bar gene identical to the pollen parent. Hybrids resulting from diploid pollen receptors were confirmed as triploids or aneu-triploids with significantly reduced vigor and seed set as compared to the parents. Transmission of transgenes to sexual bahiagrass is severely restricted by the ploidy difference between tetraploid apomicts and diploid sexual bahiagrass. Average gene transfer between transgenic apomictic tetraploid and non-transgenic, apomictic tetraploid bahiagrass was 0.17%, confirming a very low frequency of amphimixis in apomictic bahiagrass cultivars. While not providing complete transgene containment, gene transfer between transgenic apomictic and non-transgenic bahiagrass occurs at a much lower frequency than reported for other cross-pollinating or facultative apomictic grasses.     

Sexual and apomictic reproduction in Hieracium subgenus pilosella are closely interrelated developmental pathways

Seed formation in flowering plants requires meiosis of the megaspore mother cell (MMC) inside the ovule, selection of a megaspore that undergoes mitosis to form an embryo sac, and double fertilization to initiate embryo and endosperm formation. During apomixis, or asexual seed formation, in Hieracium ovules, a somatic aposporous initial (AI) cell divides to form a structurally variable aposporous embryo sac and embryo. This entire process, including endosperm development, is fertilization independent. Introduction of reproductive tissue marker genes into sexual and apomictic Hieracium showed that AI cells do not express a MMC marker. Spatial and temporal gene expression patterns of other introduced genes were conserved commencing with the first nuclear division of the AI cell in apomicts and the mitotic initiation of embryo sac formation in sexual plants. Conservation in expression patterns also occurred during embryo and endosperm development, indicating that sexuality and apomixis are interrelated pathways that share regulatory components. The induction of a modified sexual reproduction program in AI cells may enable the manifestation of apomixis in HIERACIUM:     

Sexual and apomictic development in Hieracium

 Most members of the genus Hieracium are apomictic and set seed without fertilization, but sexual forms also exist. A cytological study was conducted on an apomictic accession of H. aurantiacum (A3.4) and also H. piloselloides (D3) to precisely define the cellular basis for apomixis. The apomictic events were compared with the sexual events in a self-incompatible isolate of H. pilosella (P4). All plants were maintained as vegetatively propagated lines each derived from a single plant. Sexual P4 exhibited characteristic events of polygonum-type embryo sac formation, showed no latent apomitic tendencies, and depended upon fertilization to set seed. In contrast, D3 and A3.4 were autonomous aposporous apomicts, forming both embryo and endosperm spontaneously inside an unreduced embryo sac. The two apomicts exhibited distinct mechanisms, but variation was also observed within each apomictic line. Seeds from apomicts often contained more than one embryo. A degree of developmental instability was also observed amongst germinated seedlings and included variation in meristem and cotyledon number, altered phyllotaxis, callus formation, and seedling fusion. In most cases abnormal seedlings developed into normal plants. Such phenomena were not observed following germination of hybrid seeds derived from crosses between sexual P4 and the apomictic plants. The three plants can now be used in inheritance studies and also to investigate the molecular mechanisms controlling apomixis. Received: 11 February 1998 / Revision accepted: 23 July 1998     

Expression of rolB in apomictic Hieracium piloselloides Vill. causes ectopic meristems in planta and changes in ovule formation, where apomixis initiates at higher frequency

The effects of the Agrobacterium rhizogenes rolB oncogene on apomixis were examined in the facultative apomictic plant Hieracium piloselloides because the oncogene has been shown to alter plant growth, morphogenesis and cellular sensitivity to auxin. Introduction of rolB under the control of either its own promoter or the CaMV35S promoter induced ectopic meristem formation from the inflorescence, confirming in planta a meristem-inducing role for this oncogene previously observed only in tissue culture. These ectopic meristems formed vegetative rosettes and floral plant organs. Upon immersion in water these meristems generated roots, suggesting that meristem commitment towards the generation of a specific organ type is a separate and later event that is dependent upon the developmental context. Ovule identity and form was altered in ectopically induced florets in plants expressing the CaMV35S::rolB construct. In contrast to the ovules of untransformed apomictic plants, the sexual process ceased earlier, prior to meiosis, yet surprisingly, apomixis initiated from a greater number of cells, and embryos and endosperm continued to develop in the structurally altered ovules. The alternative possibilities that the effects on reproduction might result from rolB influencing cellular response to auxin, or from alterations in cell signaling caused by changes in ovule morphology that are induced because of the expression of the oncogene are discussed.     

Discovery of highly apomictic and highly amphimictic dihaploids in Allium tuberosum

 To discover highly apomictic and amphimictic Allium tuberosum diploids, we evaluated the degree of apomixis in three dihaploids (2n=16, 2x), KaD2, TeD1 and GMD1, derived from highly apomictic tetraploids. The degree of apomixis, calculated as the percentage of diploid seedlings in the progeny obtained after cross-pollination with tetraploid cultivars, was 96% in KaD2, 7% in TeD1 and 39% in GMD1. In addition to this general index of apomictic nature, two analytical indices were evaluated in KaD2 and TeD1. The degree of diplospory, calculated as the percentage of endoreduplicated embryo-sac mother cells, was 96% in KaD2 and 2% in TeD1. The degree of parthenogenesis, calculated as the percentage of ovules with the egg cell developing parthenogenetically, was 98% in KaD2 and 10% in TeD1. Among angiosperms with gametophytic apomixis, KaD2 is the first diploid apomict whose reproductive mode has been fully described by these three quantitative indices of apomictic nature. And TeD1 is the first highly amphimictic plant found in the A. tuberosum complex. Although TeD1 is poorly fertile, the present results encourage further screening trials for highly fertile, highly amphimictic dihaploids, which may be effective counterparts to KaD2 in diploid-level cross experiments to genetically analyze apomixis in A. tuberosum. Received: 4 December 1995 / Revision accepted: 8 May 1996     

Inheritance of parthenogenesis in Poa pratensis L.: auxin test and AFLP linkage analyses support monogenic control

 Gametophytic apomixis in Kentucky bluegrass (Poa pratensis L.) involves the parthenogenetic development of unreduced eggs from aposporic embryo sacs. Attempts to transfer the apomictic trait beyond natural sexual barriers require further elucidation of its inheritance. Controlled crosses were made between sexual clones and apomictic genotypes, and the parthenogenetic capacity of (poly)diploid hybrids was ascertained by the auxin test. A bulked segregant analysis with RAPD and AFLP markers was then used to identify a genetic linkage group related to the apomictic mode of reproduction. This approach enabled us to detect both an AFLP marker located 6.6 cM from the gene that putatively controls parthenogenesis and a 15.4-cM genomic window surrounding the target locus. A map of the P. pratensis chromosome region carrying the gene of interest was constructed using additional RAPD and AFLP markers that co-segregated with the parthenogenesis locus. Highly significant linkage between parthenogenesis and a number of AFLP markers that also appeared to belong to a tight linkage block strengthens the hypothesis of monogenic inheritance of this trait. If a single gene is assumed, apomictic polyploid types of P. pratensis would be simplex for a dominant allele that confers parthenogenesis, and this genetic model would be further supported by the bimodal distribution of the degree of parthenogenesis exhibited in the (poly)diploid progenies from sexual x apomictic matings. The molecular tagging of apomixis in P. pratensis is an essential step towards marker-assisted breeding and map-based cloning strategies aimed at investigating and manipulating its mode of reproduction. Received: 13 January 1998 / Accepted: 19 January 1998     

Apomixis in flowering plants: an overview

Apomixis is a common feature of perennial plants, which occurs in ca. 60% of the British flora, but has been largely ignored by reproductive theoreticians. Successful individuals may cover huge areas, and live to great ages, favoured by 'symmetrical' selection. Apomixis is favoured by colonizing modes, for instance post-glacially. Despite its theoretical advantages, apomixis usually coexists with sexuality, suggesting 'hidden' disadvantages. Agamospermy (apomixis by seed) is relatively uncommon, but gains from the attributes of the seed. It pays agamospermy genes, which discourage recombination, to form co-adapted linkage groups, so that they become targets for disadvantageous recessive mutant accumulation. Consequently, agamospermy genes cannot succeed in diploids and agamosperms are hybrid and highly heterotic. Agamospermous endosperm may suffer from genomic imbalance, so that nutritious ovules, which can support embryos without endosperm, may be preadapted for agamospermy. When primary endosperm nucleus fertilization ('pseudogamy') continues as a requirement for many aposporous agamosperms, selfing sex becomes preadaptive and archesporial sex remains an option. Apomictic populations can be quite variable although apomictic families are much less variable than sexuals. Only in some diplosporous species does sex disappear completely, and in those species some release of variability may persist through somatic recombination. The search for an agamospermy gene suitable for genetic modification should target fertile sexuals with a single localized agamospermy (A) gene, which therefore lack a genetic load. The A gene should coexist alongside sexuality, so that it would be easy to select seedlings of sexual and asexual origins. Plants with sporophytic agamospermy provide all these attributes.