Apomictic development during different flower development stages in Crataegus tanacetifolia (Lam.) Pers., endemic to Turkey

In this study, aposporic apomictic development and its relation to the different flower development stages were investigated by light and fluorescence microscopy in Crataegus tanacetifolia (Lam.) Pers. (Rosaceae). At pre-anthesis stage, aposporic initial cell differentiated at the somatic nucellus tissue shortly after the megaspore mother cell formation. The volume of aposporic initial cell increased during the generation of dyad and megaspore tetrad respectively by regular meiosis. At this stage, linear megaspore tetrad and vacuolated aposporic initial cell were located side by side into the same ovule. At anthesis stage, before pollination, four nucleated aposporic embryo sac was formed while sexual development came to end by atrophy of megaspores completely. At this stage, atrophied megaspores and two nucleated aposporic embryo sac were located side by side into the same ovule. At post-anthesis stage, pollination still had not begun and ovule contained only eight nucleated aposporic embryo sac. Mature aposporic embryo sac was composed of two synergid cells and one egg cell on the micropylar side, three antipodal cells on the chalazal side and a central cell with two polar nuclei in the middle of the sac. The absence of filiform apparatus in the synergid cells was quite remarkable. No callose accumulation around the aposporic initial cell was observed in any development stage. Pollination started shortly after the proembryo formation. Embryo and endosperm developed without fertilization due to the problems encountered in reaching pollen tubes to the ovary.

Developmental, ultrastructural and cytochemical investigations of the female gametophyte in Sedum rupestre L. (Crassulaceae)

This article describes the development of female gametophyte in Sedum rupestre L. New embryological information about the processes of megasporogenesis and megagametogenesis provided in this paper expand the current knowledge about the embryology of the studied species. S. rupestre is characterized by monosporic megasporogenesis and the formation of Polygonum–type embryo sac. The process of megasporogenesis is initiated by one megaspore mother cell, resulting in the formation of a triad of cells after meiosis and cytokinesis. The functional megaspore, which is located chalazally, is a mononuclear cell present next to the megaspore in the centre of the triad. Only one of the two non-functional cells of the triad is binucleate, which occur at the micropylar pole. In this paper, we explain the functional ultrastructure of the female gametophytic cells in S. rupestre. Initially, the cytoplasm of the gametophytic cells does not differ from each other; however, during differentiation, the cells reveal different morphologies. The antipodals and the synergids gradually become organelle-rich and metabolically active. The antipodal cells participate in the absorption and transport of nutrients from the nucellar cells towards the megagametophyte. Their ultrastructure shows the presence of plasmodesmata with electron-dense material, which is characteristic of Crassulaceae, and wall ingrowths in the outer walls. The ultrastructure of synergid cells is characterized by the presence of filiform apparatus and cytoplasm with active dictyosomes, abundant profiles of endoplasmic reticulum and numerous vesicles, which agrees with their main function—the secretion of pollen tube attractants. Reported data can be used to resolve the current taxonomic problems within the genus Sedum ser. Rupestria.

TOP1α, UPF1, and TTG2 regulate seed size in a parental dosage–dependent manner

Cues of maternal and paternal origins interact to control seed development, and the underlying molecular mechanisms are still far from clear. Here, we show that TOPOISOMERASE Iα (TOP1α), UP-FRAMESHIFT SUPPRESSOR 1 (UPF1), and TRANSPARENT TESTA GLABRA2 (TTG2) gametophytically, biparentally regulate seed size in Arabidopsis. TOP1α and UPF1 are mainly expressed in antipodal cells, and loss of their function leads to ectopic TTG2 expression in these female gametophytic cells. We further demonstrate that TOP1α and UPF1 directly repress TTG2 expression through affecting its chromatin status and determine its relative expression in antipodal cells versus sperm cells, which controls seed size in a dosage-dependent and parent-of-origin-dependent manner. The molecular interplay among these three genes explains their biparental gametophytic effect during diploidy and interploidy reciprocal crosses. Taken together, our findings reveal a molecular framework of parental interaction for seed size control.

Ovule and megagametophyte development in selected species of Apeibeae and Grewieae (Malvaceae–Grewioideae) from South America and its systematic implications

This is the first embryological report of the Grewioideae subfamily, which is meant to contribute to the characterisation of the genera Corchorus, Luehea and Triumfetta. Ovule and female gametophyte development in C. argutus, L. divaricata and T. semitriloba was analysed. The ovules of all species are anatropous, bitegmic and of crassinucellate mixed type. The micropyle of the studied species is formed by the outer integument (exostome). The ovule of L. divaricata differs from those of the other two species because the chalazal tissue expands forming a cap, which gives rise to a wing in the seed. All species present one hypostase. The megaspore mother cell gives rise to a linear megaspore triad in C. argutus and L. divaricata, whereas in T. semitriloba, triads and diads can be observed in the same ovule. The chalazal megaspore develops a seven-celled and eight-nucleate female gametophyte corresponding to the Polygonum type. The synergids of L. divaricata have hooks and a conspicuous filiform apparatus. The antipodal cells in C. argutus are persistent, whereas in the other species, they are small and ephemeral. The embryological characters are compared with those of other taxa within the family and the megagametophyte formation in these species is discussed.

Embryological studies on Anthemis tinctoria L.

The chromosome number of <i>Anthemis tinctoria</i> from natural habitats in Poland is n=9. The development of pollen grains is normal. The embryo-sac is of tetrasporic type and the number of antipodal cells is seven, all derived from one-cellular archespore. The embryo development is of Aster type and the endosperm is of nuclear type.