Megasporogenesis and female gametophyte development were investigated in ovules of the everbearing strawberry Fragaria x ananassa Duch. cv. Selva. Observations of thin sections revealed that ovule development starts from the formation of a nucellus and coincides in time with the beginning of receptacle overgrowth. The most characteristic feature during nucellus differentiation is the formation of a multicellular archesporium, beginning from at least two cells. Analysis of female gametophyte development indicated that in addition to the meiotic mode, female gametophytes develop by an apomeiotic mode of Antennaria type. Asynchronous development of female gametophytes of different origin occurs. The mature, eight-nucleate, seven-celled female gametophyte of meiotic origin is cylindrical and slightly curved. It occupies the central part of the nucellus. The egg apparatus, consisting of an egg cell and two synergids, is formed in the micropylar part of the female gametophyte; the opposite chalazal pole is occupied by antipodal cells. Besides the ovule in which only one seven-celled female gametophyte finally develops, ovules with a different number of cells were observed to initiate female gametophyte development. Some ovules contain a nucellus with a tetrad of linearly arranged megaspores surrounded by enlarged cells, each of which has the potential to develop into an apomictic female gametophyte. After degeneration of some post-meiotic cells or developmentally advanced female gametophytes, some of the chalazal cells initiated female gametophyte development.
The paper reports a comparative study of the female gametophyte and especially synergid structure in sexual and apomictic dandelions. We analyzed diploid sexually reproducing Taraxacum linearisquameum (2n = 2x = 16) and two triploids, T. alatum and T. udum (2n = 3x =24), with autonomous embryo and endosperm development. There were no observed differences in the organization of the mature megagametophyte between the examined species. Both meiotically reduced and diplosporous embryo sacs showed typical polarity of the egg apparatus cells, together with development of a filiform apparatus in the synergids, but immunocytochemical analyses indicated that microtubules form longitudinal brush-like bundles adjacent to the filiform apparatus in the synergids of the sexual T. linearisquameum. This arrangement of cytoskeletal elements is similar to the configuration described in other amphimictic plants. The synergids of the apomictic T. alatum and T. udum show a uncharacteristic and relatively weak cytoskeleton with no brush-like bundles. We discuss the role of synergids in autonomous apomicts.
In flowering plants, seeds are produced both sexually (double fertilization is required) and asexually via apomixis (meiotic reduction and egg fertilization are omitted). An apomictic-like pattern of endosperm development in planta is followed by fis mutants of sexual Arabidopsis thaliana. In our experiments in planta, autonomous endosperm (AE) developed in met1 mutants. Furthermore we obtained autonomous endosperm formation in vitro not only in unfertilized ovules of fie mutants but also in wild genotypes (Col-0, MET1/MET1, FIE/FIE) and met1 mutants. AE induction and development occurred in all genotypes on the each of the media used and in every trial. The frequency of AE was relatively high (51.2% ovaries) and genotype-dependent. AE induced in vitro represents a more advanced stage of development than AE induced in fie mutants in planta. This was manifested by a high number of nuclei surrounded by cytoplasm and organized in nuclear cytoplasmic domains (NCDs), nodule formation, division into characteristic regions, and cellularization. The high frequency of AE observed in homozygous met1 (met1/met1) mutants probably is due to accumulation of hypomethylation as an effect of the met1 mutation and the in vitro conditions. AE development was most advanced in FIE/fie mutants. We suggest that changes in the methylation of one or several genes in the DNA of Arabidopsis genotypes caused by in vitro conditions resulted in AE induction and/or further AE development.