Polyembryony in Melastomataceae from Brazilian Cerrado: multiple embryos in a small world

Polyembryony has been commonly associated with apomixis in the angiosperms and seems to be more common than expected, even in biomes where sexual reproduction processes are predominant. Recent studies in Cerrado, the Neotropical savannas of Central Brazil, showed high frequencies of apomixis and polyembryony and indicated these processes as reproductive and evolutionary alternatives for plants in these areas. In this sense, we investigated the occurrence of polyembryony and its relationships with ecological (season and type of dispersal, ploidy, species distribution and breeding system) and taxonomic (tribe) factors in the Melastomataceae, a mostly tropical family already known for its high frequency of apomixis and very common in Cerrado. We collected seeds from 69 populations of 53 species, which were sown in germination chambers. After seed germination, the presence and number of seedlings per seed were evaluated as a method to estimate polyembryony. We encountered 18 species (33.96%) with polyembryony (more than one seedling, or gemellar seedlings, originated per seed) concentrated in species of the tribe Miconieae (64%) and Microlicieae (16.67%), but absent in Melastomeae. Monoembryony was present only in sexual species, while all apomictic species were polyembryonic. In Miconia, the polyembryony was correlated with polyploidy, and monoembryony with diploid species. Polyembryony was more common among species with wide distribution in the Cerrado region, which indicates that the presence of gemellar seedlings is important for establishment and survival of the group in the Cerrado biome.     

Diplospory and obligate apomixis in Miconia albicans (Miconieae, Melastomataceae) and an embryological comparison with its sexual congener M. chamissois

Apomixis, or asexual reproduction through seeds, has been reported for species of the tribe Miconieae, Melastomataceae, but details of the process have yet to be described. We analyzed and compared sporogenesis and gametogenesis in the apomictic Miconia albicans and the sexual M. chamissois. The results point to some differences between species, which were related to the apomictic process. In M. albicans microsporogenesis, problems during meiosis and degeneration of its products led to total pollen sterility, while M. chamissois presented normal bicellular pollen grains in the mature anther. The absence or abnormality of meiosis in M. albicans megasporogenesis led to the formation of an unreduced embryo sac and also to egg cell parthenogenesis, which gave rise to the apomictic embryo. Embryo and endosperm development were autonomous, resulting in seeds and fruits independent of pollination and fertilization. Thus, in this species, apomixis can be classified as diplosporic and obligate. In contrast, meiosis was as expected in the sexual M. chamissois, and led to the development of a reduced embryo sac. Despite the divergent pathways, many embryological characteristics were similar between the studied species and other Melastomataceae and they seem to be conservative character states for the family.     

Apomixis and Polyembryony in the Guggul Plant, Commiphora wightii

The present report is a part of our study on the reproductivebiology of a traditional Indian medicinal plant,Commiphora wightii,a source of guggul steroids. Field examination showed a predominantlylarge number of isolated and groups of female individuals. Onlyone andromonoecious and two exclusively male plants were recorded.Female plants set seed irrespective of the presence or absenceof pollen. Hand-pollination experiments and embryological studieshave confirmed the occurrence of non-pseudogamous apomixis,nucellar polyembryony and autonomous endosperm formation forthe first time in this plant, which is presently threatenedby over-exploitation. Commiphora wightii ; autonomous endosperm; guggul; medicinal plant; non-pseudogamous apomixis; nucellar polyembryony     

Evolution of apomixis as a strategy of colonization in the dioecious species <Emphasis Type="Italic">Lindera glauca</Emphasis> (Lauraceae)

I have investigated the reproductive biology of four dioecious species of Lindera in Japan: Lindera obtusiloba Bl., L. umbellata Thunb., L. erythrocarpa Makino, and L. glauca Bl. The sex ratios in populations of the first three species are close to equality, but in L. glauca only female individuals are found in Japan, although males are known from continental Asia. The persistence of this dioecious species in the absence of males is surprising, and prompts the question: What mechanisms operate to ameliorate problems of colonization in species of Lindera? I carried out bagging experiments in order to test for apomixis (asexual reproduction by seeds) and to establish the importance of pollination and fertilization, and potential pollen vectors. Only L. glauca reproduced by apomixis. Level of fruit set was high, and was not affected by pollination. In the other three species, seed set was entirely dependent on pollen transfer, which could be accomplished by small Coleoptera and Diptera. The evolution of apomixis in L. glauca appears to have been a strategy to overcome sterility and establish a population in the Japanese islands even in the absence of males. However, this change in breeding system has not occurred in congeneric, co-occurring species, which do not seem to have experienced a lack of male plants.     

Sporophytic apomixis in polyploid Anemopaegma species (Bignoniaceae) from central Brazil

Apomixis and polyploidy have been important in the evolution of the angiosperms, and sporophytic apomixis has been associated with polyembryony and polyploidy in tropical floras. We studied the occurrence of polyembryony in populations of tetraploid Anemopaegma acutifolium, A. arvense and A. glaucum from the Brazilian cerrados, and histological features of sexual and apomictic processes were investigated in A. acutifolium. All populations and species were polyembryonic (68.9–98.4% of seeds). Normal double fertilization occurred in most ovules, with exceptions being that 3% of ovules were penetrated but not fertilized and in 4% of ovules both synergids were penetrated. The penetration of both synergids suggests a continuous attraction of pollen tubes and polyspermy. Adventitious embryo precursor cells (AEPs) arose from nucellar and integumental cells of the ovule in pollinated and unpollinated A. acutifolium, indicating sporophytic apomixis. However, further embryo and endosperm development required pollination and fertilization. This pseudogamy also allows concurrent sexual embryo development. Similar polyembryony rates and polyploidy indicated that A. arvense and A. glaucum are also apomictic, forming an agamic complex similar to that observed for some species of confamilial, but not closely related Handroanthus. The co‐occurrence of apomixis and polyploidy in different groups of Bignoniaceae indicates homoplasious origin of these agamic complexes. © 2013 The Linnean Society of London, Botanical Journal of the Linnean Society, 2013, 173 , 77–91.     

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.

     

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.     

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.     

Apomixis technology and the paradox of sex

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

Role of antioxidant defense system and biochemical adaptation on stress tolerance of high mountain and steppe plants

Plants growing in small fragmented populations under stressful environmental conditions may have reduced sexual reproduction. This can cause low gene flow between populations and eventually extinction. Here we report on a pollination experiment with Sorbus torminalis, a rare fleshy-fruited tree with a submediterranean distribution in Europe. At the northern limit of its range in SE-Denmark two relatively small and isolated populations were studied for effects of seven pollination treatments on fruit production and on the timing of fruit abortion. There was evidence that lack of pollination and spontaneous self-pollination caused particularly high fruit abortion, which indicates that apomixis is unlikely and spontaneous self-pollination not efficient. Fruit abortion was delayed after hand pollination, which suggests limitation by pollen quantity. Self-pollination caused earlier abortion than experimental cross-pollination within or between populations indicating inbreeding depression. There was no evidence for outbreeding depression as measured by fruit abortion. We conclude that generative reproduction of S. torminalis is reduced on its northern distribution limit and that it might be negatively affected by pollen limitation and inbreeding effects, which have not been compensated for by increased self-compatibility or apomixis.     

Beyond promiscuity: From sexuality to apomixis in flowering plants

Summary Little is known about the genetic basis and molecular mechanisms regulating female gametogenesis in flowering plants. In many species sexuality is replaced by apomixis, a method of asexual reproduction that circumvents female meiosis and fertilization, and culminates in the formation of clonal seeds. Using a new generation of transposon based insertional mutagenesis strategies and their resulting molecular tools, we are investigating how female meiotically derived cells (megaspores) acquire their identity. We are also determining their function and interactions, and attempting the induction of apomixis initiation in the ovule of Arabidopsis. This basic knowledge will contribute to establish the transfer of apomixis into sexual crops, a major challenge faced by plant biotechnology. The introduction of apomixis as a reproductive alternative could represent a unique opportunity to simplify breeding schemes and genetically perpetuate any desired heterozygous genotype, including hybrids.     

Phenomenon of polyembryony. Genetic heterogeneity of seeds

Different concepts of polyembryony and genetic heterogeneity of seeds in flower plants have been reviewed. Different types, ways, and forms of plant reproduction appeared in the course of evolution as a consequences of the attached mode of life and autotrophy. This is ascribed to totipotency, “stemminess” of plant cells, and presence of constantly functioning meristems, which determined to a great extent the system of plant safety. There are two ways of formation of a new individual: sexual process → gamospermy involving meiosis and gamete fusion and asexual process → agamospermy without meiosis and gamete fusion and two types of reproduction: seed and vegetative. Both processes may take place simultaneously in one seed, as a result of which many embryos of different origins are formed: uniparental and biparental inheritance. Traditionally, this phenomenon is called polyembryony. It comprises embryoidogeny (a new category of vegetative reproduction): formation of somatic embryos (= embryoids) in the flower, seed, and on vegetative organs. Genetic heterogeneity is one of the most important characteristics of seeds, which is based on different phenomena, such as embryogeny, embryoidogeny, and gametophytic and sporophytic apomixis. When describing two types of polyembryony, sporophytic (nucellar, integumental, cleavage) and gametophytic (synergidal, antipodal), a great attention is paid to characterization of initial cells of the sexual and adventive embryos. A new concept of apogamety is developed from new positions (totipotency and “stemminess”), which is based on different genesis of cells of the egg and antipodal systems. Five possible pathways of formation of the adventive embryos have been proposed from cells of the egg apparatus. Specific features of the formation of adventive embryos in the case of gametophytic apomixis, such as androgenesis and semigamy, are discussed. Morphogenesis of the sexual and adventive embryos proceeds in the mother organism and is determined by the origin and formation of their initials, types of ovule and embryo sac, and specific features of developmental biology. This determines parallelism in their development. The main difference consists in the way of reproduction: heterophasic and homophasic. The phenomenon of polyembryony and genetic heterogeneity of seeds is essential for development of the theory of reproduction and applied research related to seed productivity of plants.     

Cryptic self‐incompatibility and distyly in Hedyotis acutangula Champ. (Rubiaceae)

Distyly, floral polymorphism frequently associated with reciprocal herkogamy, self‐ and intramorph incompatibility and secondary dimorphism, constitutes an important sexual system in the Rubiaceae. Here we report an unusual kind of distyly associated with self‐ and/or intramorph compatibility in a perennial herb, Hedyotis acutangula. Floral morphology, ancillary dimorphisms and compatibility of the two morphs were studied. H. acutangula did not exhibit precise reciprocal herkogamy, but this did not affect the equality of floral morphs in the population, as usually found in distylous plants. Both pin and thrum pollen retained relatively high viability for 8 h. The pollen to ovule ratio was 72.5 in pin flowers and 54.4 in thrum flowers. Pistils of pin flowers remained receptive for longer than those of thrum flowers. No apparent difference in the germination rate of pin and thrum pollen grains was observed when cultured in vitro, although growth of thrum pollen tubes was much faster than that of pin pollen tubes. Artificial pollination revealed that pollen tube growth in legitimate intermorph crosses was faster than in either intramorph crosses or self‐pollination, suggesting the occurrence of cryptic self‐incompatibility in this species. Cryptic self‐incompatibility functioned differently in the two morphs, with pollen tube growth rates after legitimate and illegitimate pollination much more highly differentiated in pin flowers than in thrum flowers. No fruit was produced in emasculated netted flowers, suggesting the absence of apomixis. Our results indicate that H. acutangula is distylous, with a cryptic self‐incompatibility breeding system.     

Transmission of the apomictic mode of reproduction in Pennisetum: co-inheritance of the trait and molecular markers

Summary Apomixis, asexual reproduction through seed, is an obligate mode of reproduction in several species from the genus Pennisetum. Transfer of apomixis to sexual, cultivated pearl millet (P. glaucum) from a wild species P. squamulatum has resulted in an obligate apomictic backcross line with a low, but unknown number, of chromosomes from the wild species. Molecular markers (restriction fragment length polymorphisms and random amplified polymorphic DNAs) have been identified that unequivocally demonstrate the presence of P. squamulatum DNA in BC₃. Three of the informative RFLP clones have been sequenced and converted to sequence-tagged sites that can be amplified by the polymerase chain reaction. Molecular genetic analysis of more advanced back-cross individuals, using the two types of polymerase chain reaction-based molecular markers, has demonstrated co-inheritance of apomictic reproduction and two of the molecular markers. The remaining five molecular markers generally co-segregate with each other but are not linked with the mode of reproduction. These results suggest that genes for apomixis apparently can be transmitted by a single chromosome. Chromosome-specific markers will provide a starting point for the mapping of this genetically intractable reproductive trait.     

Assessment of reproductive characteristics of Jatropha curcas L. in south Florida

Jatropha curcas L. (jatropha) is a species identified for biofuel production because of the high quality of the oil produced by its seeds. However, jatropha is undomesticated and little information is available about its reproductive characteristics. Breeding and genetic improvement programs are much needed for the jatropha development as a bioenergy crop. Information about floral display and mode of reproduction are considered essential for breeding programs. In this study, the total number of female flowers, male to female flower ratio, fruit set, in vitro pollen germination, and the formation of fruits by apomixis, self‐pollination, and natural pollination were evaluated in 17 jatropha accessions planted in South Florida. The total number of female flowers per inflorescence and male to female flower ration ranged from 2.8 to 9.1 and 9.9:1 to 55.4 : 1, respectively. During summer 2011, high fruit setting average was observed (75.5%). In vitro pollen germination varied from 64.6% during spring 2011 to 51.6% during fall 2011. The fruit set observed was from 10.1% to 64.0% through natural pollination and apomixes, respectively. Characteristics such as fruit fresh weight, number of seeds per fruit, seed dry weight, and oil content were influenced by mode of reproduction.     

Exposure to water increased pollen longevity of pondweed (Potamogeton spp.) indicates different mechanisms ensuring pollination success of angiosperms in aquatic habitat

Pollen longevity in seven Potamogeton species representing different pollination systems (anemophily, epihydrophily and hydroautogamy) was assessed both under aerial condition and in contact with water to investigate how water impacts the sexual reproduction in these aquatic taxa. Stainability of pollen with MTT was considered as an indicator of pollen viability. The half-life of pollen longevity was calculated using exponential decay regression. Overall, pollen viability decreased relatively rapidly with time. Pollen grains of obligate anemophilic species had lower initial viability and shorter half-lives than those of facultative anemophilic species. Pollen in these latter species may take more time to reach the stigma. The pollen of Potamogeton may be categorized as partially hydrated pollen owing to its generally spherical shape and lack of furrows, rapid loss of viability, and fast pollen tube initiation. The half-life is positively correlated with pollen size. Smaller-sized grains are at greater risk of desiccation than larger grains. In contrast with the situation observed in most terrestrial angiosperms, contact with water increases pollen longevity in Potamogeton species. In our present study the half-lives of pollen longevity of Potamogeton species in which the pollen had come into contact with water (mean of 10.65 h) were markedly higher than those under aerial conditions (mean of 5.79 h, t = 2.622, P = 0.039). The results of our study contradict a widely held belief that water is detrimental to pollen viability in angiosperms and furthermore indicate that close proximity to water results in selection for wettability. The transition to a hydrated status together with its morphology, make Potamogeton pollen more adapted to the aquatic environment and thus serves to ensure reproductive process. Results of our present study may have direct implications for understanding the evolution of the sexual reproductive system in aquatic angiosperms.     

Phenomenon of polyembryony. Genetic heterogeneity of seeds

Different concepts of polyembryony and genetic heterogeneity of seeds in flower plants have been reviewed. Different types, ways, and forms of plant reproduction appeared in the course of evolution as a consequences of the attached mode of life and autotrophy. This is ascribed to totipotency, "stemminess" of plant cells, and presence of constantly functioning meristems, which determined to a great extent the system of plant safety. There are two ways of formation of a new individual: sexual process --> gamospermy involving meiosis and gamete fusion and asexual process --> agamospermy without meiosis and gamete fusion and two types of reproduction: seed and vegetative. Both processes may take place simultaneously in one seed, as a result of which many embryos of different origins are formed: uniparental and biparental inheritance. Traditionally, this phenomenon is called polyembryony. It comprises embryoidogeny (a new category of vegetative reproduction): formation of somatic embryos (= embryoids) in the flower, seed, and on vegetative organs. Genetic heterogeneity is one of the most important characteristics of seeds, which is based on different phenomena, such as embryogeny, embryoidogeny, and gametophytic and sporophytic apomixes. When describing two types of polyembryony, sporophytic (nucellar, integumental, cleavage) and gametophytic (synergidal, antipodal), a great attention is paid to characterization of initial cells of the sexual and adventive embryos. A new concept of apogamety is developed from new positions (totipotency and "stemminess"), which is based on different genesis of cells of the egg and antipodal systems. Five possible pathways of formation of the adventive embryos have been proposed from cells of the egg apparatus. Specific features of the formation of adventive embryos in the case of gametophytic apomixis, such as androgenesis and semigamy, are discussed. Morphogenesis of the sexual and adventive embryos proceeds in the mother organism and is determined by the origin and formation of their initials, types of ovule and germ sac, and specific features of developmental biology. This determines parallelism in their development. The main difference consists in the way of reproduction: heterophasic and homophasic, The phenomenon of polyembryony and genetic heterogeneity of seeds is essential for development of the theory of reproduction and applied research related to seed productivity of plants.     

Apomixis in <Emphasis Type="Italic">Eulaliopsis binata</Emphasis>: characterization of reproductive mode and endosperm development

Apomixis represents an alteration of classical sexual plant reproduction to produce seeds with essentially clonal embryos, stimulating wide interest from biologists and plant breeders for its ability to fix heterosis. Eulaliopsis binata (Poaceae), is identified here as a new apomictic species. Embryological investigation indicates that the developmental pattern of embryo sac formation in E. binata represents gametophytic apospory, the embryo originating from an unreduced cell, without fertilization and the mode of endosperm development was autonomous. Sexual embryo sacs were found with a frequency of 1–4% depending on the biotype. The DNA content of nuclei (C-value) in mature seeds was screened by flow cytometry (FCSS) and demonstrated that the endosperm was derived autonomously without fertilization and the three biotypes of E. binata showed varying degrees of apomixis. The Wide-leaf type showed obligate apomixis whereas the Slender-leaf and the Red-haulm type displayed facultative apomixis. In addition, adventitious embryos were observed on the wall of ovary, integument and nucellus cells, indicating that E. binata produces embryos via a mixture of apospory and adventitious embryony.     

The Genetic Control of Apomixis: Asexual Seed Formation

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

Quantification of progeny classes in two facultatively apomictic accessions of Hieracium

Hieracium is an established model system for studying the cytological and genetic basis of gametophytic apomixis. In common with most known apomicts, the formation of 'maternal seed' is not exclusive in Hieracium, as apomixis operates in conjunction with a low level of sexuality. When this occurs the form of apomixis is described as 'facultative'. The formation of maternal seed in these plants is characterised by the avoidance of meiosis followed by the parthenogenetic development of an unreduced egg cell. In some ovules, however, meiosis does proceed, and sometimes the fertilisation of an egg cell presages embryogenesis. As a result, this mechanism of facultative apomixis leads to the formation of several different types of progeny, each representing a unique combination of meiosis/apomeiosis and fertilisation/parthenogenesis. Furthermore, fertilisation may involve either self or non-self pollen, leading to the recognition of six progeny classes from each individual plant. To facilitate an understanding of these processes we have developed a method for identifying individuals from different progeny classes based on the inheritance of introduced heterologous marker genes. This technique permits the screening of many thousands of seedlings at germination, and the consequent isolation of individuals associated with rare classes. Progeny profiles were determined for two apomictic accessions of Hieracium. Both were found to develop approximately 2.5% of their seed from meiotically derived eggs under the experimental conditions used and to have a rate of hybridity of approximately 2%. Evidence was also found for the action of a self-incompatibility mechanism operating in these plants despite the autonomous nature of apomixis in Hieracinum. As a demonstration of the utility of this approach, a study was conducted of polyembryony in one accession. The results indicate that there was a 7 fold greater likelihood that a meiotically derived seedling would arise in a polyembryonic seed than in a single-embryo seed. This indicates that facultative apomixis in Hieracium not only results from the simultaneous occurrence of sexual and asexual seed formation in the same capitulum as previously demonstrated, but most often as parallel processes within the same ovule.