Apomictic Processes Research Articles

Pollen morphology and its relation with meiotic irregularities in ten species of Campuloclinium (Eupatorieae, Asteraceae)

Pollen grains of ten species of Campuloclinium (Eupatorieae, Asteraceae) are described and illustrated using light and scanning electron microscopy. The species included in this study are C. burchelli, C. campuloclinioides, C. chlorolepis, C. hirsutum, C. irwinii, C. macrocephalum, C. megacephalum, C. parvulum, C. purpurascens and C. riedelli. Pollen grains of Campuloclinium are typically radially symmetric, echinate, tectate, oblate-spheroidal to prolate-spheroidal (P/E ratio: 0.94–1.10). Pollen is mostly triaperturate, but tetraaperturate or biaperturate pollen also occurs in some species. The pollen diameter was highly variable ranging from 10.54 µm to 36.18 μm. The significance of the variation in type and size of pollen is discussed in relation to diverse meiotic abnormalities and possible reproductive apomictic processes within the polyploid species.

A reciprocal cross design to map the genetic architecture of complex traits in apomictic plants.

Many higher plants of economic and biological importance undergo apomixis in which the maternal tissue of the ovule forms a seed, without experiencing meiosis and fertilization. This feature of apomixis has made it difficult to perform linkage mapping which relies on meiotic recombination. Here, we describe a computational model for mapping quantitative trait loci (QTLs) that control complex traits in apomictic plants. The model is founded on the mixture model-based likelihood in which maternal genotypes are dissolved into two possible components generated by meiotic and apomictic processes, respectively. The EM algorithm was implemented to discern meiotic and apomictic genotypes and, therefore, allow the marker-QTL linkage relationship to be estimated. By capitalizing on reciprocal crosses, the model is renovated to estimate and test imprinting effects of QTLs, providing a better gateway to characterize the genetic architecture of complex traits. The model was validated through computer simulation and further demonstrated for its usefulness by analyzing a real data for an apomictic woody plant. The model has for the first time provided a unique tool for genetic mapping in apomictic plants.

Elementary apomictic processes in Rubus L.

Two elementary apomictic processe were found in the examined <em>Rubus</em> species: <em>R. saxatilis</em> developed diplosporous and aposporous embryo sacs, <em>R. caesius</em> - displosporous, apart from meiotic ones. These result; confirmed the wide occurrence of facultative apomixis and intraspecific differentiation of the mode of reproduction within the taxa of the investigated genus.

Asynchronous male/female gametophyte development in facultative apomictic plants of Cenchrus ciliaris (Poaceae)

Apomixis has been suggested to result from the asynchronous gene expression of duplicated genes governing plant reproduction. The similarity of embryological stages and gene expression patterns observed during sexual and apomictic processes, and appearance of apomictic stages (e.g., aposporous initials) at different stages of ovule development support this hypothesis. We evaluated this hypothesis by assessing temporal variation during microsporogenesis, microgametogenesis, megasporogenesis and megagametogenesis in Cenchrus ciliaris L. This study was conducted using individuals of C. ciliaris with two distinct modes of reproduction viz., facultative apomictic or sexual. Inflorescences were classified into five stages based on morphological indicators. Variation in the configuration and developmental timing of gametophytes of facultative sexual and apomictic plants was studied at these five stages using the high-throughput technique of whole-mount confocal microscopy. Asynchrony in development of early reproductive stages in apomictic plants, as observed by presence of greater number of embryological stages in florets of the same inflorescence, is prominent in comparison to sexual plants. Such inconsistency was greater in female than in male gametophyte development. Stages like 3-nucleated sexual embryo sac and aposporous proembryo could also be observed. This supports the hypothesis that apomixis could be the result of de-regulated sexual reproductive pathway.

Evaluation of different methods for assessing the reproductive mode of weeping lovegrass plants, Eragrostis curvula (Schrad.) Nees

Weeping lovegrass is a forage grass cultivated in semiarid regions of the world that reproduces mainly by apomixis (diplospory), a process that involves the formation of asexual seeds and bypasses the processes of meiosis and fertilisation. The aim of this work was to evaluate and compare different techniques (cytoembryology, callose deposition, flow cytometry and progeny tests) to determine the reproductive mode of weeping lovegrass. Typical sexual and apomictic processes were clearly differentiated using cytoembryology, and different callose deposition patterns were observed in sexual and apomictic genotypes. Previous studies indicated that presence of callose on the cell wall of the megaspore mother cell is associated only with sexual processes. Nevertheless, our results also found callose deposition in apomictic genotypes, although clearly different from the pattern found in sexual processes, allowing discrimination between sexual and apomictic plants. Flow cytometry seed screening using individual seeds did not differentiate between sexual and apomictic plants as the embryo : endosperm DNA content ratio was similar in sexual and apomictic plants. Progeny tests using molecular markers showed uniform patterns in offspring from apomictic plants and variable patterns among the progeny of sexual plants. The results obtained from cytological studies and progeny tests were similar, indicating that both methods provide useful tools for determination of reproductive mode. However, the callose test with aniline blue was faster and easier to use than other techniques.

A MULTICOPY SUPPRESSOR OF IRA1 (MSI1) homologue is not associated with the switch to autonomous seed development in apomictic (asexual) Hieracium plants

In Hieracium, the formation of seeds through sexual and apomictic processes differ in two aspects: meiosis is avoided during apomictic female gametophyte formation (apomeiosis), which is controlled by the loss of apomeiosis ( LOA) locus; and embryo and endosperm develop autonomously, a process controlled by an independent locus, loss of parthenogenesis ( LOP). In sexual Arabidopsis, seed initiation is inhibited by the Fertilization Independent Seed (FIS) protein complex, which includes MULTICOPY SUPPRESSOR OF IRA1 (MSI1) and a distinct complex involving MSI1 and RETINOBLASTOMA-RELATED PROTEIN (RBR). msi1 mutants initiate autonomous egg cell proliferation forming a non-viable parthenogenetic embryo. Here we examined the association of HMSI1 orthologue with the initiation of seed development in Hieracium. HMSI1 is expressed in apomictic and sexual female gametes and in ovaries containing developing seeds. HMSI1 interacted with HRBR in yeast two-hybrid analyses as found in Arabidopsis. HMSI1 was not commonly altered in the genomes of gamma radiation lop mutants with lesions at the LOP locus and HMSI1 expression was down-regulated relative to the wild type in an unpollinated lop mutant that requires fertilization to develop functional embryos. The data suggest that HMSI1 is not linked to LOP and is thus unlikely to trigger the switch to autonomous seed development in apomictic Hieracium but more likely to be involved in seed formation events downstream of LOP activity.

Proteomic analysis of sugar beet apomictic monosomic addition line M14

Apomixis in plants holds great promise for agriculture because of its vigor associated with heterozygosity and superior genotype. Despite the significance of apomictic reproductive process, our knowledge of proteins and their functions in apomictic development is limited. Here we report a comparative proteomic and transcriptomic analysis of sexual and apomictic processes in sugar beet. A total of 71 differentially expressed protein spots were successfully identified in the course of apomictic reproductive development using high-resolution 2-DE and MS analysis. The differentially expressed proteins were involved in several processes that might work cooperatively to lead to apomictic reproduction. This study has generated potential protein markers important for apomictic development.

Sexual and asexual (apomictic) seed development in flowering plants: molecular, morphological and evolutionary relationships.

Reproduction in the flowering plants (angiosperms) is a dynamic process that relies upon the formation of inflorescences, flowers and eventually seed. Most angiosperms reproduce sexually by generating gametes via meiosis that fuse during fertilisation to initiate embryo and seed development, thereby perpetuating the processes of adaptation and evolution. Despite this, sex is not a ubiquitous reproductive strategy. Some angiosperms have evolved an alternate form of reproduction termed apomixis, which avoids meiosis during gamete formation and leads to the production of embryos without paternal contribution. Therefore, apomixis results in the production of clonal progeny through seed. The molecular nature and evolutionary origin of apomixis remain unclear, but recent studies suggest that apomixis evolved from the same molecular framework supporting sex. In this review, we consider physical and molecular relationships between the two pathways, with a particular focus on the initial stages of female reproduction where apomixis deviates from the sexual pathway. We also consider theories that explain the origin of apomictic processes from sexual progenitors. Detailed characterisation of the relationship between sex and apomixis in an evolutionary and developmental sense is an important step towards understanding how apomixis might be successfully integrated into agriculturally important, but currently sexual crops.

Understanding apomixis: recent advances and remaining conundrums.

It has been 10 years since the last review on apomixis, or asexual seed formation, in this journal ([Koltunow, 1993][1]). In that article, emphasis was given to the commonalties known among apomictic processes relative to the events of sexual reproduction. The inheritance of apomixis had been

Polyembryony in angiospermous trees of the Brazilian Cerrado and Caatinga vegetation

The occurrence of polyembryony was investigated in 75 woody species of the Cerrado in central Brazil and the xerophilous Caatinga vegetation in northeastern Brazil. Fourteen species showed polyembryony, a type of anomalous angiospermous reproduction. Polyembryony is reported for the first time for nine genera, Astronium, Byrsonima, Cariniana, Copaifera, Hancornia, Magonia, Myracrodruon, Tabebuia, and Tapirira. The positive correlation found between polyembryony, sexual reproduction, and apomictic processes suggests that a number of angiospermous species may make regular use of multiple breeding systems.

Dynamics of callose deposition and beta-1,3-glucanase expression during reproductive events in sexual and apomictic Hieracium.

Callose accumulates in the walls of cells undergoing megasporogenesis during embryo sac formation in angiosperm ovules. Deficiencies in callose deposition have been observed in apomictic plants and causal linkages between altered callose deposition and apomictic initiation proposed. In apomictic Hieracium, embryo sacs initiate by sexual and apomictic processes within an ovule, but sexual development terminates in successful apomicts. Callose deposition and the events that lead to sexual termination were examined in different Hieracium apomicts that form initials pre- and post-meiosis. In apomictic plants, callose was not detected in initial cell walls and deficiencies in callose deposition were not observed in cells undergoing megasporogenesis. Multiple initial formation pre-meiosis resulted in physical distortion of cells undergoing megasporogenesis, persistence of callose and termination of the sexual pathway. In apomictic plants, callose persistence did not correlate with altered spatial or temporal expression of a beta-1,3-glucanase gene (HpGluc) encoding a putative callose-degrading enzyme. Expression analysis indicated HpGluc might function during ovule growth and embryo sac expansion in addition to callose dissolution in sexual and apomictic plants. Initial formation pre-meiosis might therefore limit the access of HpGluc protein to callose substrate while the expansion of aposporous embryo sacs is promoted. Callose deposition and dissolution during megasporogenesis were unaffected when initials formed post-meiosis, indicating other events cause sexual termination. Apomixis in Hieracium is not caused by changes in callose distribution but by events that lead to initial cell formation. The timing of initial formation can in turn influence callose dissolution.

Non-radioactive mRNA fingerprinting to visualise gene expression in mature ovaries of Brachiaria hybrids derived from B. brizantha, an apomictic tropical forage

Apomixis is an asexual mode of reproduction by seed, which is found in a number of families of higher plants. During apomictic processes, both meiosis and egg cell fertilisation are omitted making offspring exact genetic replicas of the mother plant. To gain an insight into the molecular basis of apomictic reproduction, gene expression in mature ovaries containing reduced and unreduced embryo sacs was compared in Brachiaria, an apomictic tropical forage grass. Experimental systems for the molecular study of apomixis and sexuality have limitations mainly resulting from numerous differences in the genetic make-up of the plant material. When unrelated to the reproductive behaviour, these may dramatically affect the results if a conventional differential display strategy is applied (comparison made on a one-to-one plant basis). To overcome these limitations, we examined mRNA populations extracted from ovaries collected from pooled samples of various apomictic and sexual plants using the polymerase chain reaction (PCR) differential display and non radioactive cDNA detection procedures. Using this approach, two mRNAs whose expression appeared to be related to the late stages of apomictic reproduction were identified. These results indicate that gene expression in pooled mRNA analysis is a promising tool to elucidate the molecular basis of apomixis.

Cytological and embryological studies inSetaria cordobensisHerrmann andSetaria leianthaHackel (Poaceae)

SUMMARYCytogenetics and embryology of Setaria cordobensis Herrmann and S. leiantha Hackel were investigated in four populations collected from different environments in Argentina. A somatic chromosome number of 2n = 36 was confirmed for S. leiantha and reported for the first time for S. cordobensis. Bivalent pairing was regular in pollen mother cell meiosis, but abnormalities such as dyads, triads and tetrads with micronuclei or non-conventional arrangement were described with different frequencies. A high degree of similarity between the two species was found investigating megasporangium, megasporogenesis and megagametogenesis. There were major differences in ovule length and width at anthesis. Both species follow a typical Polygonum development and no indicator of apomictic processes was noted.

Apomixis: Embryo Sacs and Embryos Formed without Meiosis or Fertilization in Ovules.

The term apomixis has, in the past, been used as a general term for any form of asexual reproduction in plants, including vegetative propagation. This original definition has become more restricted and now covers only those asexual reproductive processes that, paradoxically, occur in the ovule of flowering plants-the structure that has evolved to carry out female sexual reproductive functions in angiosperms (Nogler, 1984; Asker and Jerling, 1992). Apomictic processes mimic many of the events of sexual reproduction and give rise to fertile seeds. An important difference is that the apomictic embryo is derived solely from cells in the maternal ovule tissues rather than from the fusion of male and female gametes. The fertile seeds that result from apomictic reproduction contain embryos that have, barring mutation, a genetic constitution identical to that of the female parent. At least three developmental differences also serve to distinguish apomictic embryo formation from somatic embryogenesis. First, apomictic embryo formation occurs within a differentiated structure. Second, apomictic embryos form directly from a cell located in, or close to, a gametophytic structure, without entering an intervening callus phase, which is often necessary for somatic embryogenesis (Nomura and Komamine, 1985; see also Zimmerman, 1993, this issue). Third, the pattern of embryo formation in apomictic species is often indistinguishable from that which occurs in the nearest sexual relative (Nogler, 1984), which is not always the case for somatic embryogenesis. Apomictic processes have been ObServed in at least 300 plant species spanning 35 different families and are most common in the Gramineae, Compositae, Rosaceae, and Rutaceae (Richards, 1986; Hanna and Bashaw, 1987). With the exception of apple and Citrus, apomixis is not very common in agriculturally important crops. Because apomictically produced embryos are genetically identical to the female parent plant, they are of obvious benefit to agriculture. If apomixis can be introduced into agricultural crops, it can be an inexpensive way to perpetuate a given genotype, preserving even such characters as heterosis through successive generations via seed (Hanna and Bashaw, 1987). Apomictic plants could potentially provide a constant source of renewable seed capable of producing high-yielding food crops, an agricultural trait of great value, particularly for developing countries. Programs aimed at using conventional breeding methods to transfer apomixis to most agricultural crops have been largely unsuccessful, although pearl millet has been an exception (Dujardin and Hanna, 1989). The continuation of such organized breeding programs is important, and a greater understanding of the processes controlling apomixis at the molecular level should facilitate the transfer of apomixis into crops of economic importance. In this review, the different forms of apomixis that occur in ovules will be discussed and compared at a developmental level with sexual reproductive processes. Current mechanistic and genetic knowledge about apomictic processes will be presented. An understanding of apomixis at the molecular level should result in the characterization of genes that control female gametophyte formation and the early events of embryo development in both apomictic and sexually reproducing plants.