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Abstract

We used cytological and embryological methods to study reproductive cycle stages in Cerasus fruticosa Pall., Cerasus × eminens (Beck) Buia and Cerasus × mohacsyana (Kárpáti) Janchen from SW Slovakia, focusing on development of the male and female reproductive organs, fertilization processes and embryo formation. We found that reproductive potential was reduced by synergistic effects of negative biotic and abiotic factors. Despite the presence of degenerated, deformed pollen grains and their great variability of shape and size, a sufficient amount of normally developed viable pollen grains developed in anthers of C. fruticosa and C. × mohacsyana. Disturbed microsporogenesis in C. × eminens led to significantly lower production of viable pollen grains. We did not observe serious disturbances during megasporogenesis and megagametogenesis. Lower fruit set was caused by degeneration of ovules as a result of unsuccessful pollination, fertilization failure, or embryo degeneration during its initial development.
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Abstract

Miscanthus ×giganteus Greef et Deu. (Poaceae), a hybrid of Miscanthus sinensis and M. sacchariflorus native to Japan, is an ornamental and a highly lignocellulosic bioenergy crop, cultivated in the European Union as an alternative source of energy. This grass reproduces exclusively vegetatively, by rhizomes or via expensive in vitro micropropagation. The present study was aimed at finding the barriers that prevent sexual seed production, based on detailed embryological analyses of the whole generative cycle, including microsporogenesis, pollen viability, megasporogenesis, female gametophyte development, and embryo and endosperm formation. Sterility of M. ×giganteus results from abnormal development of both male and female gametophytes. Disturbed microsporogenesis (laggard chromosomes, univalents, micronuclei) was further highlighted by low pollen staining. The frequency of stainable pollen ranged from 13.9% to 55.3% depending on the pollen staining test, and no pollen germination was observed either in vitro or in planta. The wide range of pollen sizes (25.5-47.6 μm) clearly indicated unbalanced pollen grain cytology, which evidently affected pollen germination. Only 9.7% of the ovules developed normally. No zygotes nor embryos were found in any analyzed ovules. Sexual reproduction of M. ×giganteus is severely hampered by its allotriploid (2n=3x=57) nature. Hybrid sterility, a strong postzygotic barrier, prevents sexual reproduction and, therefore, seed formation in this taxon.
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Abstract

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.
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Abstract

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.
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