No abstract

The genus Taraxacum Wigg. (Asteraceae) forms a polyploid complex within which there are strong links between the ploidy level and the mode of reproduction. Diploids are obligate sexual, whereas polyploids are usually apomictic. The paper reports on a comparative study of the ovary and especially the ovule anatomy in the diploid dandelion T. linearisquameum and the triploid T. gentile. Observations with light and electron microscopy revealed no essential differences in the anatomy of both the ovary and ovule in the examined species. Dandelion ovules are anatropous, unitegmic and tenuinucellate. In both sexual and apomictic species, a zonal differentiation of the integument is characteristic of the ovule. In the integumentary layers situated next to the endothelium, the cell walls are extremely thick and PAS positive. Data obtained from TEM indicate that these special walls have an open spongy structure and their cytoplasm shows evidence of gradual degeneration. Increased deposition of wall material in the integumentary cells surrounding the endothelium takes place especially around the chalazal pole of the embryo sac as well as around the central cell. In contrast, the integumentary cells surrounding the micropylar region have thin walls and exhibit a high metabolic activity. The role of the thick-walled integumentary layers in the dandelion ovule is discussed. We also consider whether this may be a feature of taxonomic importance.

Female-biased sex ratio is an interesting phenomenon observed in Rumex acetosa, a dioecious plant with an XX/XY 1 Y 2 sex chromosome system. Previous authors have suggested that the biased sex ratio in this species is conditioned not only postzygotically (sex-differential sporophytic mortality) but also prezygotically, because the sex ratio of seeds is also female-biased, although to a lesser extent than the sex ratio of flowering plants. The mechanisms underlying female bias in Rumex seeds are only poorly understood. To gain more knowledge of them, we analysed male gametophyte development and used flow cytometry to determine the frequency of femaledetermining (n = 7, A + X) and male-determining (n = 8, A + Y 1 Y 2 ) pollen grains in anthers. Embryological studies showed a regular course of male gametophyte development in R. acetosa. There were no signs of degeneration of microspores or disturbances in pollen divisions (irregular nuclei, micronuclei, delayed chromosomes and anaphase bridges). The Alexander test revealed only 1.6% nonviable pollen grains within anthers. All mature pollen grains were uniformly equipped with starch granules. The two sexes were shown to substantially differ in nuclear 2C DNA amount in somatic tissues (7.00 pg in 2A + XX females and 7.50 pg in 2A + XY 1 Y 2 males), and two clearly different DNA classes of mature pollen grains, with lower and with higher DNA amounts (16.8% difference) were found. Most probably the grains with the lower DNA amount possess seven chromosomes, and grains with the higher DNA amount eight of them. The quantitative ratio of these grains in anthers at anthesis was 1:1.2, very close to the sex ratio of seeds observed by the majority of previous authors. All these observations support the opinion that the sex-ratio bias in Rumex is determined prezygotically to some extent.

Although callose occurs during megasporogenesis in most flowering plants, the knowledge about its general function and the mechanisms by which the callose layer is formed in particular places is still not sufficient. The results of previous studies suggest a total lack of callose in the ovules of diplosporous plants in which meiosis is omitted or disturbed. This report is the first documentation of callose events in dandelions ovules. We demonstrated the pattern of callose deposition during the formation of megaspores through diplospory of Taraxacum type and during normal meiotic megasporogenesis in apomictic triploid Taraxacum atricapillum and amphimictic diploid Taraxacum linearisquameum. We found the presence of callose in the megasporocyte wall of both diplosporous and sexual dandelions. However, in a diplosporous dandelion, callose predominated at the micropylar pole of megaspore mother cell (MMC) which may be correlated with abnormal asynaptic meiosis and may indicate diplospory of the Taraxacum type. After meiotic division, callose is mainly deposited in the walls between megaspores in tetrads and in diplodyads. In subsequent stages, callose gradually disappears around the chalazal functional megaspore. However, some variations in the pattern of callose deposition within tetrad may reflect variable positioning of the functional megaspore (FM) observed in the ovules of T. linearisquameum.

No abstract